* [PATCH v3 01/30] kho: init new_physxa->phys_bits to fix lockdep
From: Pasha Tatashin @ 2025-08-07 1:44 UTC (permalink / raw)
To: pratyush, jasonmiu, graf, changyuanl, pasha.tatashin, rppt,
dmatlack, rientjes, corbet, rdunlap, ilpo.jarvinen, kanie, ojeda,
aliceryhl, masahiroy, akpm, tj, yoann.congal, mmaurer,
roman.gushchin, chenridong, axboe, mark.rutland, jannh,
vincent.guittot, hannes, dan.j.williams, david, joel.granados,
rostedt, anna.schumaker, song, zhangguopeng, linux, linux-kernel,
linux-doc, linux-mm, gregkh, tglx, mingo, bp, dave.hansen, x86,
hpa, rafael, dakr, bartosz.golaszewski, cw00.choi, myungjoo.ham,
yesanishhere, Jonathan.Cameron, quic_zijuhu, aleksander.lobakin,
ira.weiny, andriy.shevchenko, leon, lukas, bhelgaas, wagi,
djeffery, stuart.w.hayes, ptyadav, lennart, brauner, linux-api,
linux-fsdevel, saeedm, ajayachandra, jgg, parav, leonro, witu
In-Reply-To: <20250807014442.3829950-1-pasha.tatashin@soleen.com>
Lockdep shows the following warning:
INFO: trying to register non-static key.
The code is fine but needs lockdep annotation, or maybe
you didn't initialize this object before use?
turning off the locking correctness validator.
[<ffffffff810133a6>] dump_stack_lvl+0x66/0xa0
[<ffffffff8136012c>] assign_lock_key+0x10c/0x120
[<ffffffff81358bb4>] register_lock_class+0xf4/0x2f0
[<ffffffff813597ff>] __lock_acquire+0x7f/0x2c40
[<ffffffff81360cb0>] ? __pfx_hlock_conflict+0x10/0x10
[<ffffffff811707be>] ? native_flush_tlb_global+0x8e/0xa0
[<ffffffff8117096e>] ? __flush_tlb_all+0x4e/0xa0
[<ffffffff81172fc2>] ? __kernel_map_pages+0x112/0x140
[<ffffffff813ec327>] ? xa_load_or_alloc+0x67/0xe0
[<ffffffff81359556>] lock_acquire+0xe6/0x280
[<ffffffff813ec327>] ? xa_load_or_alloc+0x67/0xe0
[<ffffffff8100b9e0>] _raw_spin_lock+0x30/0x40
[<ffffffff813ec327>] ? xa_load_or_alloc+0x67/0xe0
[<ffffffff813ec327>] xa_load_or_alloc+0x67/0xe0
[<ffffffff813eb4c0>] kho_preserve_folio+0x90/0x100
[<ffffffff813ebb7f>] __kho_finalize+0xcf/0x400
[<ffffffff813ebef4>] kho_finalize+0x34/0x70
This is becase xa has its own lock, that is not initialized in
xa_load_or_alloc.
Modifiy __kho_preserve_order(), to properly call
xa_init(&new_physxa->phys_bits);
Fixes: fc33e4b44b27 ("kexec: enable KHO support for memory preservation")
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
kernel/kexec_handover.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)
diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
index e49743ae52c5..6240bc38305b 100644
--- a/kernel/kexec_handover.c
+++ b/kernel/kexec_handover.c
@@ -144,14 +144,35 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
unsigned int order)
{
struct kho_mem_phys_bits *bits;
- struct kho_mem_phys *physxa;
+ struct kho_mem_phys *physxa, *new_physxa;
const unsigned long pfn_high = pfn >> order;
might_sleep();
- physxa = xa_load_or_alloc(&track->orders, order, sizeof(*physxa));
- if (IS_ERR(physxa))
- return PTR_ERR(physxa);
+ physxa = xa_load(&track->orders, order);
+ if (!physxa) {
+ new_physxa = kzalloc(sizeof(*physxa), GFP_KERNEL);
+ if (!new_physxa)
+ return -ENOMEM;
+
+ xa_init(&new_physxa->phys_bits);
+ physxa = xa_cmpxchg(&track->orders, order, NULL, new_physxa,
+ GFP_KERNEL);
+ if (xa_is_err(physxa)) {
+ int err = xa_err(physxa);
+
+ xa_destroy(&new_physxa->phys_bits);
+ kfree(new_physxa);
+
+ return err;
+ }
+ if (physxa) {
+ xa_destroy(&new_physxa->phys_bits);
+ kfree(new_physxa);
+ } else {
+ physxa = new_physxa;
+ }
+ }
bits = xa_load_or_alloc(&physxa->phys_bits, pfn_high / PRESERVE_BITS,
sizeof(*bits));
--
2.50.1.565.gc32cd1483b-goog
^ permalink raw reply related
* [PATCH v3 00/30] Live Update Orchestrator
From: Pasha Tatashin @ 2025-08-07 1:44 UTC (permalink / raw)
To: pratyush, jasonmiu, graf, changyuanl, pasha.tatashin, rppt,
dmatlack, rientjes, corbet, rdunlap, ilpo.jarvinen, kanie, ojeda,
aliceryhl, masahiroy, akpm, tj, yoann.congal, mmaurer,
roman.gushchin, chenridong, axboe, mark.rutland, jannh,
vincent.guittot, hannes, dan.j.williams, david, joel.granados,
rostedt, anna.schumaker, song, zhangguopeng, linux, linux-kernel,
linux-doc, linux-mm, gregkh, tglx, mingo, bp, dave.hansen, x86,
hpa, rafael, dakr, bartosz.golaszewski, cw00.choi, myungjoo.ham,
yesanishhere, Jonathan.Cameron, quic_zijuhu, aleksander.lobakin,
ira.weiny, andriy.shevchenko, leon, lukas, bhelgaas, wagi,
djeffery, stuart.w.hayes, ptyadav, lennart, brauner, linux-api,
linux-fsdevel, saeedm, ajayachandra, jgg, parav, leonro, witu
This series introduces the LUO, a kernel subsystem designed to
facilitate live kernel updates with minimal downtime,
particularly in cloud delplyoments aiming to update without fully
disrupting running virtual machines.
This series builds upon KHO framework by adding programmatic
control over KHO's lifecycle and leveraging KHO for persisting LUO's
own metadata across the kexec boundary. The git branch for this series
can be found at:
https://github.com/googleprodkernel/linux-liveupdate/tree/luo/v3
Changelog from v2:
- Addressed comments from Mike Rapoport and Jason Gunthorpe
- Only one user agent (LiveupdateD) can open /dev/liveupdate
- Release all preserved resources if /dev/liveupdate closes
before reboot.
- With the above changes, sessions are not needed, and should be
maintained by the user-agent itself, so removed support for
sessions.
- Added support for changing per-FD state (i.e. some FDs can be
prepared or finished before the global transition.
- All IOCTLs now follow iommufd/fwctl extendable design.
- Replaced locks with guards
- Added a callback for registered subsystems to be notified
during boot: ops->boot().
- Removed args from callbacks, instead use container_of() to
carry context specific data (see luo_selftests.c for example).
- removed patches for luolib, they are going to be introduced in
a separate repository.
What is Live Update?
Live Update is a kexec based reboot process where selected kernel
resources (memory, file descriptors, and eventually devices) are kept
operational or their state preserved across a kernel transition. For
certain resources, DMA and interrupt activity might continue with
minimal interruption during the kernel reboot.
LUO provides a framework for coordinating live updates. It features:
State Machine: Manages the live update process through states:
NORMAL, PREPARED, FROZEN, UPDATED.
KHO Integration:
LUO programmatically drives KHO's finalization and abort sequences.
KHO's debugfs interface is now optional configured via
CONFIG_KEXEC_HANDOVER_DEBUG.
LUO preserves its own metadata via KHO's kho_add_subtree and
kho_preserve_phys() mechanisms.
Subsystem Participation: A callback API liveupdate_register_subsystem()
allows kernel subsystems (e.g., KVM, IOMMU, VFIO, PCI) to register
handlers for LUO events (PREPARE, FREEZE, FINISH, CANCEL) and persist a
u64 payload via the LUO FDT.
File Descriptor Preservation: Infrastructure
liveupdate_register_filesystem, luo_register_file, luo_retrieve_file to
allow specific types of file descriptors (e.g., memfd, vfio) to be
preserved and restored.
Handlers for specific file types can be registered to manage their
preservation and restoration, storing a u64 payload in the LUO FDT.
User-space Interface:
ioctl (/dev/liveupdate): The primary control interface for
triggering LUO state transitions (prepare, freeze, finish, cancel)
and managing the preservation/restoration of file descriptors.
Access requires CAP_SYS_ADMIN.
sysfs (/sys/kernel/liveupdate/state): A read-only interface for
monitoring the current LUO state. This allows userspace services to
track progress and coordinate actions.
Selftests: Includes kernel-side hooks and userspace selftests to
verify core LUO functionality, particularly subsystem registration and
basic state transitions.
LUO State Machine and Events:
NORMAL: Default operational state.
PREPARED: Initial preparation complete after LIVEUPDATE_PREPARE
event. Subsystems have saved initial state.
FROZEN: Final "blackout window" state after LIVEUPDATE_FREEZE
event, just before kexec. Workloads must be suspended.
UPDATED: Next kernel has booted via live update. Awaiting restoration
and LIVEUPDATE_FINISH.
Events:
LIVEUPDATE_PREPARE: Prepare for reboot, serialize state.
LIVEUPDATE_FREEZE: Final opportunity to save state before kexec.
LIVEUPDATE_FINISH: Post-reboot cleanup in the next kernel.
LIVEUPDATE_CANCEL: Abort prepare or freeze, revert changes.
v2: https://lore.kernel.org/all/20250723144649.1696299-1-pasha.tatashin@soleen.com
v1: https://lore.kernel.org/all/20250625231838.1897085-1-pasha.tatashin@soleen.com
RFC v2: https://lore.kernel.org/all/20250515182322.117840-1-pasha.tatashin@soleen.com
RFC v1: https://lore.kernel.org/all/20250320024011.2995837-1-pasha.tatashin@soleen.com
Changyuan Lyu (1):
kho: add interfaces to unpreserve folios and physical memory ranges
Mike Rapoport (Microsoft) (1):
kho: drop notifiers
Pasha Tatashin (23):
kho: init new_physxa->phys_bits to fix lockdep
kho: mm: Don't allow deferred struct page with KHO
kho: warn if KHO is disabled due to an error
kho: allow to drive kho from within kernel
kho: make debugfs interface optional
kho: don't unpreserve memory during abort
liveupdate: kho: move to kernel/liveupdate
liveupdate: luo_core: luo_ioctl: Live Update Orchestrator
liveupdate: luo_core: integrate with KHO
liveupdate: luo_subsystems: add subsystem registration
liveupdate: luo_subsystems: implement subsystem callbacks
liveupdate: luo_files: add infrastructure for FDs
liveupdate: luo_files: implement file systems callbacks
liveupdate: luo_ioctl: add userpsace interface
liveupdate: luo_files: luo_ioctl: Unregister all FDs on device close
liveupdate: luo_files: luo_ioctl: Add ioctls for per-file state
management
liveupdate: luo_sysfs: add sysfs state monitoring
reboot: call liveupdate_reboot() before kexec
kho: move kho debugfs directory to liveupdate
liveupdate: add selftests for subsystems un/registration
selftests/liveupdate: add subsystem/state tests
docs: add luo documentation
MAINTAINERS: add liveupdate entry
Pratyush Yadav (5):
mm: shmem: use SHMEM_F_* flags instead of VM_* flags
mm: shmem: allow freezing inode mapping
mm: shmem: export some functions to internal.h
luo: allow preserving memfd
docs: add documentation for memfd preservation via LUO
.../ABI/testing/sysfs-kernel-liveupdate | 51 +
Documentation/admin-guide/index.rst | 1 +
Documentation/admin-guide/liveupdate.rst | 16 +
Documentation/core-api/index.rst | 1 +
Documentation/core-api/kho/concepts.rst | 2 +-
Documentation/core-api/liveupdate.rst | 57 +
Documentation/mm/index.rst | 1 +
Documentation/mm/memfd_preservation.rst | 138 +++
Documentation/userspace-api/index.rst | 1 +
.../userspace-api/ioctl/ioctl-number.rst | 2 +
Documentation/userspace-api/liveupdate.rst | 25 +
MAINTAINERS | 19 +-
include/linux/kexec_handover.h | 53 +-
include/linux/liveupdate.h | 203 ++++
include/linux/shmem_fs.h | 23 +
include/uapi/linux/liveupdate.h | 399 +++++++
init/Kconfig | 2 +
kernel/Kconfig.kexec | 14 -
kernel/Makefile | 2 +-
kernel/liveupdate/Kconfig | 90 ++
kernel/liveupdate/Makefile | 17 +
kernel/{ => liveupdate}/kexec_handover.c | 554 ++++-----
kernel/liveupdate/kexec_handover_debug.c | 222 ++++
kernel/liveupdate/kexec_handover_internal.h | 45 +
kernel/liveupdate/luo_core.c | 517 +++++++++
kernel/liveupdate/luo_files.c | 1033 +++++++++++++++++
kernel/liveupdate/luo_internal.h | 60 +
kernel/liveupdate/luo_ioctl.c | 297 +++++
kernel/liveupdate/luo_selftests.c | 345 ++++++
kernel/liveupdate/luo_selftests.h | 84 ++
kernel/liveupdate/luo_subsystems.c | 452 ++++++++
kernel/liveupdate/luo_sysfs.c | 92 ++
kernel/reboot.c | 4 +
mm/Makefile | 1 +
mm/internal.h | 6 +
mm/memblock.c | 56 +-
mm/memfd_luo.c | 507 ++++++++
mm/shmem.c | 52 +-
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/liveupdate/.gitignore | 1 +
tools/testing/selftests/liveupdate/Makefile | 7 +
tools/testing/selftests/liveupdate/config | 6 +
.../testing/selftests/liveupdate/liveupdate.c | 406 +++++++
43 files changed, 5448 insertions(+), 417 deletions(-)
create mode 100644 Documentation/ABI/testing/sysfs-kernel-liveupdate
create mode 100644 Documentation/admin-guide/liveupdate.rst
create mode 100644 Documentation/core-api/liveupdate.rst
create mode 100644 Documentation/mm/memfd_preservation.rst
create mode 100644 Documentation/userspace-api/liveupdate.rst
create mode 100644 include/linux/liveupdate.h
create mode 100644 include/uapi/linux/liveupdate.h
create mode 100644 kernel/liveupdate/Kconfig
create mode 100644 kernel/liveupdate/Makefile
rename kernel/{ => liveupdate}/kexec_handover.c (74%)
create mode 100644 kernel/liveupdate/kexec_handover_debug.c
create mode 100644 kernel/liveupdate/kexec_handover_internal.h
create mode 100644 kernel/liveupdate/luo_core.c
create mode 100644 kernel/liveupdate/luo_files.c
create mode 100644 kernel/liveupdate/luo_internal.h
create mode 100644 kernel/liveupdate/luo_ioctl.c
create mode 100644 kernel/liveupdate/luo_selftests.c
create mode 100644 kernel/liveupdate/luo_selftests.h
create mode 100644 kernel/liveupdate/luo_subsystems.c
create mode 100644 kernel/liveupdate/luo_sysfs.c
create mode 100644 mm/memfd_luo.c
create mode 100644 tools/testing/selftests/liveupdate/.gitignore
create mode 100644 tools/testing/selftests/liveupdate/Makefile
create mode 100644 tools/testing/selftests/liveupdate/config
create mode 100644 tools/testing/selftests/liveupdate/liveupdate.c
--
2.50.1.565.gc32cd1483b-goog
^ permalink raw reply
* Re: [PATCH v2 10/32] liveupdate: luo_core: Live Update Orchestrator
From: Pasha Tatashin @ 2025-08-06 22:28 UTC (permalink / raw)
To: Jason Gunthorpe
Cc: pratyush, jasonmiu, graf, changyuanl, rppt, dmatlack, rientjes,
corbet, rdunlap, ilpo.jarvinen, kanie, ojeda, aliceryhl,
masahiroy, akpm, tj, yoann.congal, mmaurer, roman.gushchin,
chenridong, axboe, mark.rutland, jannh, vincent.guittot, hannes,
dan.j.williams, david, joel.granados, rostedt, anna.schumaker,
song, zhangguopeng, linux, linux-kernel, linux-doc, linux-mm,
gregkh, tglx, mingo, bp, dave.hansen, x86, hpa, rafael, dakr,
bartosz.golaszewski, cw00.choi, myungjoo.ham, yesanishhere,
Jonathan.Cameron, quic_zijuhu, aleksander.lobakin, ira.weiny,
andriy.shevchenko, leon, lukas, bhelgaas, wagi, djeffery,
stuart.w.hayes, ptyadav, lennart, brauner, linux-api,
linux-fsdevel, saeedm, ajayachandra, parav, leonro, witu
In-Reply-To: <20250805123103.GH184255@nvidia.com>
On Tue, Aug 5, 2025 at 12:31 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> On Sun, Aug 03, 2025 at 09:11:20PM -0400, Pasha Tatashin wrote:
>
> > Having a global state is necessary for performance optimizations. This
> > is similar to why we export the state to userspace via sysfs: it
> > allows other subsystems to behave differently during a
> > performance-optimized live update versus a normal boot.
>
> > For example, in our code base we have a driver that doesn't
> > participate in the live update itself (it has no state to preserve).
> > However, during boot, it checks this global state. If it's a live
> > update boot, the driver skips certain steps, like loading firmware, to
> > accelerate the overall boot time.
>
> TBH, I'm against this. Give the driver a 0 byte state if it wants to
> behave differently during live update. We should not be making
> implicit things like this.
>
> Plus the usual complaining about building core kernel infrastructure
> around weird out of tree drivers.
>
> If userspace wants a device to participate in live update, even just
> "optimizations", then it has to opt in.
>
> Frankly, this driver has no idea what the prior kernel did, and by
> "optimizing" I think you are actually assuming that the prior kernel
> had it bound to a normal kernel driver that left it in some
> predictable configuration.
Fair enough, that subsystem / driver should simply participate in the
live update process normally.
> Vs say bound to VFIO and completely messed up.
>
> So this should be represented by a LUO serialization that says "the
> prior kernel left this device in well defined state X" even if it
> takes 0 bytes to describe that state.
>
> So no globals, there should be a way for a driver to tell if it is
> participating in LUO, but not some global 'is luo' boot fla.g
>
> > > + ret = liveupdate_register_subsystem(&luo_file_subsys);
> > > + if (ret) {
> > > + pr_warn("Failed to register luo_file subsystem [%d]\n", ret);
> > > + return ret;
> > > + }
> > > +
> > > + if (liveupdate_state_updated()) {
> > >
> > > Thats going to be a standard pattern - I would expect that
> > > liveupdate_register_subsystem() would do the check for updated and
> > > then arrange to call back something like
> > > liveupdate_subsystem.ops.post_update()
I added another callback liveupdate_subsystem.ops.post_update(), which
gets called on live update, just when a subsystem registers with LUO,
because that is when we know that it is ready. I will send the new
patch version soon.
Thank you,
Pasha
^ permalink raw reply
* Re: [PATCH v2 31/32] libluo: introduce luoctl
From: Pasha Tatashin @ 2025-08-06 20:14 UTC (permalink / raw)
To: Pratyush Yadav
Cc: Steven Rostedt, Jason Gunthorpe, Thomas Gleixner, jasonmiu, graf,
changyuanl, rppt, dmatlack, rientjes, corbet, rdunlap,
ilpo.jarvinen, kanie, ojeda, aliceryhl, masahiroy, akpm, tj,
yoann.congal, mmaurer, roman.gushchin, chenridong, axboe,
mark.rutland, jannh, vincent.guittot, hannes, dan.j.williams,
david, joel.granados, anna.schumaker, song, zhangguopeng, linux,
linux-kernel, linux-doc, linux-mm, gregkh, mingo, bp, dave.hansen,
x86, hpa, rafael, dakr, bartosz.golaszewski, cw00.choi,
myungjoo.ham, yesanishhere, Jonathan.Cameron, quic_zijuhu,
aleksander.lobakin, ira.weiny, andriy.shevchenko, leon, lukas,
bhelgaas, wagi, djeffery, stuart.w.hayes, lennart, brauner,
linux-api, linux-fsdevel, saeedm, ajayachandra, parav, leonro,
witu
In-Reply-To: <mafs0ectod5eb.fsf@kernel.org>
> I didn't mean this for the memfd patches, only for libluo.
Makes, sense, memfd patches are going to be submitted together with
the rest of the series as PATCH, sorry for confusion.
Pasha
^ permalink raw reply
* Re: [PATCH v4 3/4] procfs: add PROCFS_GET_PID_NAMESPACE ioctl
From: Randy Dunlap @ 2025-08-06 18:57 UTC (permalink / raw)
To: Aleksa Sarai
Cc: Alexander Viro, Christian Brauner, Jan Kara, Jonathan Corbet,
Shuah Khan, Andy Lutomirski, linux-kernel, linux-fsdevel,
linux-api, linux-doc, linux-kselftest
In-Reply-To: <2025-08-06.1754503216-vulgar-pinch-more-tasks-meager-grader-93KeQn@cyphar.com>
On 8/6/25 11:02 AM, Aleksa Sarai wrote:
> On 2025-08-05, Randy Dunlap <rdunlap@infradead.org> wrote:
>>
>>
>> On 8/4/25 10:45 PM, Aleksa Sarai wrote:
>>> /proc has historically had very opaque semantics about PID namespaces,
>>> which is a little unfortunate for container runtimes and other programs
>>> that deal with switching namespaces very often. One common issue is that
>>> of converting between PIDs in the process's namespace and PIDs in the
>>> namespace of /proc.
>>>
>>> In principle, it is possible to do this today by opening a pidfd with
>>> pidfd_open(2) and then looking at /proc/self/fdinfo/$n (which will
>>> contain a PID value translated to the pid namespace associated with that
>>> procfs superblock). However, allocating a new file for each PID to be
>>> converted is less than ideal for programs that may need to scan procfs,
>>> and it is generally useful for userspace to be able to finally get this
>>> information from procfs.
>>>
>>> So, add a new API to get the pid namespace of a procfs instance, in the
>>> form of an ioctl(2) you can call on the root directory of said procfs.
>>> The returned file descriptor will have O_CLOEXEC set. This acts as a
>>> sister feature to the new "pidns" mount option, finally allowing
>>> userspace full control of the pid namespaces associated with procfs
>>> instances.
>>>
>>> The permission model for this is a bit looser than that of the "pidns"
>>> mount option (and also setns(2)) because /proc/1/ns/pid provides the
>>> same information, so as long as you have access to that magic-link (or
>>> something equivalently reasonable such as being in an ancestor pid
>>> namespace) it makes sense to allow userspace to grab a handle. Ideally
>>> we would check for ptrace-read access against all processes in the pidns
>>> (which is very likely to be true for at least one process, as
>>> SUID_DUMP_DISABLE is cleared on exec(2) and is rarely set by most
>>> programs), but this would obviously not scale.
>>>
>>> setns(2) will still have their own permission checks, so being able to
>>> open a pidns handle doesn't really provide too many other capabilities.
>>>
>>> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
>>> ---
>>> Documentation/filesystems/proc.rst | 4 +++
>>> fs/proc/root.c | 68 ++++++++++++++++++++++++++++++++++++--
>>> include/uapi/linux/fs.h | 4 +++
>>> 3 files changed, 74 insertions(+), 2 deletions(-)
>>>
>>
>>
>>> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
>>> index 0bd678a4a10e..68e65e6d7d6b 100644
>>> --- a/include/uapi/linux/fs.h
>>> +++ b/include/uapi/linux/fs.h
>>> @@ -435,8 +435,12 @@ typedef int __bitwise __kernel_rwf_t;
>>> RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC |\
>>> RWF_DONTCACHE)
>>>
>>> +/* This matches XSDFEC_MAGIC, so we need to allocate subvalues carefully. */
>>> #define PROCFS_IOCTL_MAGIC 'f'
>>>
>>> +/* procfs root ioctls */
>>> +#define PROCFS_GET_PID_NAMESPACE _IO(PROCFS_IOCTL_MAGIC, 32)
>>
>> Since the _IO() nr here is 32, Documentation/userspace-api/ioctl/ioctl-number.rst
>> should be updated like:
>>
>> -'f' 00-0F linux/fs.h conflict!
>> +'f' 00-1F linux/fs.h conflict!
>
> Should this be 00-20 (or 00-2F) instead?
Oops, yes, it should be one of those. Thanks.
> Also, is there a better value to use for this new ioctl? I'm not quite
> sure what is the best practice to handle these kinds of conflicts...
I wouldn't worry about it. We have *many* conflicts.
(unless Al or Christian are concerned)
>> (17 is already used for PROCFS_IOCTL_MAGIC somewhere else, so that probably should
>> have update the Doc/rst file.)
>>
>>> +
>>> /* Pagemap ioctl */
>>> #define PAGEMAP_SCAN _IOWR(PROCFS_IOCTL_MAGIC, 16, struct pm_scan_arg)
--
~Randy
^ permalink raw reply
* Re: [PATCH v4 3/4] procfs: add PROCFS_GET_PID_NAMESPACE ioctl
From: Aleksa Sarai @ 2025-08-06 18:02 UTC (permalink / raw)
To: Randy Dunlap
Cc: Alexander Viro, Christian Brauner, Jan Kara, Jonathan Corbet,
Shuah Khan, Andy Lutomirski, linux-kernel, linux-fsdevel,
linux-api, linux-doc, linux-kselftest
In-Reply-To: <9027aa89-b3b2-46c8-8338-6c37f1c5b97a@infradead.org>
[-- Attachment #1: Type: text/plain, Size: 3871 bytes --]
On 2025-08-05, Randy Dunlap <rdunlap@infradead.org> wrote:
>
>
> On 8/4/25 10:45 PM, Aleksa Sarai wrote:
> > /proc has historically had very opaque semantics about PID namespaces,
> > which is a little unfortunate for container runtimes and other programs
> > that deal with switching namespaces very often. One common issue is that
> > of converting between PIDs in the process's namespace and PIDs in the
> > namespace of /proc.
> >
> > In principle, it is possible to do this today by opening a pidfd with
> > pidfd_open(2) and then looking at /proc/self/fdinfo/$n (which will
> > contain a PID value translated to the pid namespace associated with that
> > procfs superblock). However, allocating a new file for each PID to be
> > converted is less than ideal for programs that may need to scan procfs,
> > and it is generally useful for userspace to be able to finally get this
> > information from procfs.
> >
> > So, add a new API to get the pid namespace of a procfs instance, in the
> > form of an ioctl(2) you can call on the root directory of said procfs.
> > The returned file descriptor will have O_CLOEXEC set. This acts as a
> > sister feature to the new "pidns" mount option, finally allowing
> > userspace full control of the pid namespaces associated with procfs
> > instances.
> >
> > The permission model for this is a bit looser than that of the "pidns"
> > mount option (and also setns(2)) because /proc/1/ns/pid provides the
> > same information, so as long as you have access to that magic-link (or
> > something equivalently reasonable such as being in an ancestor pid
> > namespace) it makes sense to allow userspace to grab a handle. Ideally
> > we would check for ptrace-read access against all processes in the pidns
> > (which is very likely to be true for at least one process, as
> > SUID_DUMP_DISABLE is cleared on exec(2) and is rarely set by most
> > programs), but this would obviously not scale.
> >
> > setns(2) will still have their own permission checks, so being able to
> > open a pidns handle doesn't really provide too many other capabilities.
> >
> > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > ---
> > Documentation/filesystems/proc.rst | 4 +++
> > fs/proc/root.c | 68 ++++++++++++++++++++++++++++++++++++--
> > include/uapi/linux/fs.h | 4 +++
> > 3 files changed, 74 insertions(+), 2 deletions(-)
> >
>
>
> > diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> > index 0bd678a4a10e..68e65e6d7d6b 100644
> > --- a/include/uapi/linux/fs.h
> > +++ b/include/uapi/linux/fs.h
> > @@ -435,8 +435,12 @@ typedef int __bitwise __kernel_rwf_t;
> > RWF_APPEND | RWF_NOAPPEND | RWF_ATOMIC |\
> > RWF_DONTCACHE)
> >
> > +/* This matches XSDFEC_MAGIC, so we need to allocate subvalues carefully. */
> > #define PROCFS_IOCTL_MAGIC 'f'
> >
> > +/* procfs root ioctls */
> > +#define PROCFS_GET_PID_NAMESPACE _IO(PROCFS_IOCTL_MAGIC, 32)
>
> Since the _IO() nr here is 32, Documentation/userspace-api/ioctl/ioctl-number.rst
> should be updated like:
>
> -'f' 00-0F linux/fs.h conflict!
> +'f' 00-1F linux/fs.h conflict!
Should this be 00-20 (or 00-2F) instead?
Also, is there a better value to use for this new ioctl? I'm not quite
sure what is the best practice to handle these kinds of conflicts...
> (17 is already used for PROCFS_IOCTL_MAGIC somewhere else, so that probably should
> have update the Doc/rst file.)
>
> > +
> > /* Pagemap ioctl */
> > #define PAGEMAP_SCAN _IOWR(PROCFS_IOCTL_MAGIC, 16, struct pm_scan_arg)
> >
> >
> Thanks.
> --
> ~Randy
>
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]
^ permalink raw reply
* [PATCH v2 11/11] fsconfig.2, mount_setattr.2: add note about attribute-parameter distinction
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This was not particularly well documented in mount(8) nor mount(2), and
since this is a fairly notable aspect of the new mount API, we should
probably add some words about it.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/fsconfig.2 | 7 +++++++
man/man2/mount_setattr.2 | 37 +++++++++++++++++++++++++++++++++++++
2 files changed, 44 insertions(+)
diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
index e2121b7a6b68..9e0e25acff3b 100644
--- a/man/man2/fsconfig.2
+++ b/man/man2/fsconfig.2
@@ -448,6 +448,13 @@ .SH HISTORY
Linux 5.2.
.\" commit ecdab150fddb42fe6a739335257949220033b782
glibc 2.36.
+.SH NOTES
+.SS Mount attributes and filesystem parameters
+For a description of the distinction between
+mount attributes and filesystem parameters,
+see the "Mount attributes and filesystem paramers" subsection
+of
+.BR mount_setattr (2).
.SH EXAMPLES
To illustrate the different kinds of flags that can be configured with
.BR fsconfig (),
diff --git a/man/man2/mount_setattr.2 b/man/man2/mount_setattr.2
index b9afc21035b8..3e6b59e5b57a 100644
--- a/man/man2/mount_setattr.2
+++ b/man/man2/mount_setattr.2
@@ -790,6 +790,43 @@ .SS ID-mapped mounts
.BR chown (2)
system call changes the ownership globally and permanently.
.\"
+.SS Mount attributes and filesystem parameters
+Some mount attributes
+(traditionally associated with
+.BR mount (8)-style
+options)
+are also filesystem parameters.
+For example, the
+.I -o ro
+option to
+.BR mount (8)
+can refer to the
+"read-only" filesystem parameter,
+or the "read-only" mount attribute.
+.P
+The distinction between these two kinds of option is that
+mount object attributes are applied per-mount-object
+(allowing different mount objects
+derived from a given filesystem instance
+to have different attributes),
+while filesystem instance parameters
+("superblock flags" in kernel developer parlance)
+apply to all mount objects
+derived from the same filesystem instance.
+.P
+When using
+.BR mount (2),
+the line between these two types of mount options was blurred.
+However, with
+.BR mount_setattr ()
+and
+.BR fsconfig (2),
+the distinction is made much clearer.
+Mount attributes are configured with
+.BR mount_setattr (),
+while filesystem parameters can be configured using
+.BR fsconfig (2).
+.\"
.SS Extensibility
In order to allow for future extensibility,
.BR mount_setattr ()
--
2.50.1
^ permalink raw reply related
* [PATCH v2 10/11] open_tree_attr.2, open_tree.2: document new open_tree_attr() api
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This is a new API added in Linux 6.15, and is effectively just a minor
expansion of open_tree(2) in order to allow for MOUNT_ATTR_IDMAP to be
changed for an existing ID-mapped mount. Glibc does not yet have a
wrapper for this.
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/open_tree.2 | 74 +++++++++++++++++++++++++++++++++++++++++++++++
man/man2/open_tree_attr.2 | 1 +
2 files changed, 75 insertions(+)
diff --git a/man/man2/open_tree.2 b/man/man2/open_tree.2
index 3d38e27b5254..6e7ec4998d42 100644
--- a/man/man2/open_tree.2
+++ b/man/man2/open_tree.2
@@ -15,7 +15,19 @@ .SH SYNOPSIS
.BR "#include <sys/mount.h>"
.P
.BI "int open_tree(int " dirfd ", const char *" path ", unsigned int " flags ");"
+.P
+.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */"
+.P
+.BI "int syscall(SYS_open_tree_attr, int " dirfd ", const char *" path ","
+.BI " unsigned int " flags ", struct mount_attr *" attr ", \
+size_t " size ");"
.fi
+.P
+.IR Note :
+glibc provides no wrapper for
+.BR open_tree_attr (),
+necessitating the use of
+.BR syscall (2).
.SH DESCRIPTION
The
.BR open_tree ()
@@ -222,6 +234,64 @@ .SH DESCRIPTION
and attach it to the file descriptor.
This flag is only permitted in conjunction with
.BR \%OPEN_TREE_CLONE .
+.SS open_tree_attr()
+The
+.BR open_tree_attr ()
+system call operates in exactly the same way as
+.BR open_tree (),
+except for the differences described here.
+.P
+After performing the same operation as with
+.BR open_tree (),
+(before returning the resulting file descriptor)
+.BR open_tree_attr ()
+will apply the mount attributes requested in
+.I attr
+to the mount object.
+(See
+.BR mount_attr (2type)
+for a description of the
+.I mount_attr
+structure.
+As described in
+.BR mount_setattr (2),
+.I size
+must be set to
+.I sizeof(struct mount_attr)
+in order to support future extensions.)
+.P
+For the most part, the application of
+.I attr
+has identical semantics to
+.BR mount_setattr (2),
+except that it is possible to change the
+.B \%MOUNT_ATTR_IDMAP
+attribute for a mount object
+that is already configured as an ID-mapped mount.
+This is usually forbidden by
+.BR mount_setattr (2)
+and thus
+.BR open_tree_attr ()
+is currently the only permitted mechanism to change this attribute.
+Changing an ID-mapped mount is only permitted
+if a new detached mount object is being created with
+.I flags
+including
+.BR \%OPEN_TREE_CLONE .
+.P
+If
+.I flags
+contains
+.BR \%AT_RECURSIVE ,
+then the attributes are applied recursively
+(just as when
+.BR mount_setattr (2)
+is called with
+.BR \%AT_RECURSIVE ).
+This applies in addition to the
+.BR open_tree ()-specific
+behaviour regarding
+.BR \%AT_RECURSIVE .
.SH RETURN VALUE
On success, a new file descriptor is returned.
On error, \-1 is returned, and
@@ -316,9 +386,13 @@ .SH ERRORS
.SH STANDARDS
Linux.
.SH HISTORY
+.SS open_tree()
Linux 5.2.
.\" commit a07b20004793d8926f78d63eb5980559f7813404
glibc 2.36.
+.SS open_tree_attr()
+Linux 6.15.
+.\" commit c4a16820d90199409c9bf01c4f794e1e9e8d8fd8
.SH NOTES
.SS Anonymous mount namespaces
The bind-mount mount objects created by
diff --git a/man/man2/open_tree_attr.2 b/man/man2/open_tree_attr.2
new file mode 100644
index 000000000000..e57269bbd269
--- /dev/null
+++ b/man/man2/open_tree_attr.2
@@ -0,0 +1 @@
+.so man2/open_tree.2
--
2.50.1
^ permalink raw reply related
* [PATCH v2 09/11] mount_setattr.2: mirror opening sentence from fsopen(2)
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
All of the other new mount API docs have this lead-in sentence in order
to make this set of APIs feel a little bit more cohesive. Despite being
a bit of a latecomer, mount_setattr(2) is definitely part of this family
of APIs and so deserves the same treatment.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/mount_setattr.2 | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/man/man2/mount_setattr.2 b/man/man2/mount_setattr.2
index d44fafc93a20..b9afc21035b8 100644
--- a/man/man2/mount_setattr.2
+++ b/man/man2/mount_setattr.2
@@ -19,7 +19,11 @@ .SH SYNOPSIS
.SH DESCRIPTION
The
.BR mount_setattr ()
-system call changes the mount properties of a mount or an entire mount tree.
+system call is part of the suite of file descriptor based mount facilities
+in Linux.
+.P
+.BR mount_setattr ()
+changes the mount properties of a mount or an entire mount tree.
If
.I path
is relative,
--
2.50.1
^ permalink raw reply related
* [PATCH v2 08/11] open_tree.2: document 'new' mount api
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).
Co-developed-by: David Howells <dhowells@redhat.com>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/open_tree.2 | 405 +++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 405 insertions(+)
diff --git a/man/man2/open_tree.2 b/man/man2/open_tree.2
new file mode 100644
index 000000000000..3d38e27b5254
--- /dev/null
+++ b/man/man2/open_tree.2
@@ -0,0 +1,405 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH open_tree 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+open_tree \- open path or create detached mount object and attach to fd
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <fcntl.h>" \
+" /* Definition of " AT_* " constants */"
+.BR "#include <sys/mount.h>"
+.P
+.BI "int open_tree(int " dirfd ", const char *" path ", unsigned int " flags ");"
+.fi
+.SH DESCRIPTION
+The
+.BR open_tree ()
+system call is part of the suite of file descriptor based mount facilities
+in Linux.
+.P
+If
+.I flags
+contains
+.BR \%OPEN_TREE_CLONE ,
+.BR open_tree ()
+creates a detached mount object
+consisting of a bind-mount of the path
+specified by the
+.IR path ,
+and attaches it to a new file descriptor,
+which is then returned.
+The mount object is equivalent to a bind-mount
+that would be created by
+.BR mount (2)
+called with
+.BR MS_BIND ,
+except that it is tied to a file descriptor
+and is not mounted onto the filesystem.
+.P
+As with file descriptors returned from
+.BR fsmount (2),
+the resultant file descriptor can then be used with
+.BR move_mount (2),
+.BR mount_setattr (2),
+or other such system calls
+to do further mount operations.
+This mount object will be unmounted and destroyed
+when the file descriptor is closed
+if it was not otherwise attached to a mount point
+by calling
+.BR move_mount (2).
+.P
+If
+.I flags
+does not contain
+.BR \%OPEN_TREE_CLONE ,
+.BR open_tree ()
+returns a file descriptor
+that is exactly equivalent to
+one produced by
+.BR open (2).
+.P
+In either case, the resultant file descriptor
+acts the same as one produced by
+.BR open (2)
+with
+.BR O_PATH ,
+meaning it can also be used as a
+.I dirfd
+argument to
+"*at()" system calls.
+.P
+As with "*at()" system calls,
+.BR fspick ()
+uses the
+.I dirfd
+argument in conjunction with the
+.I path
+argument to determine the path to operate on, as follows:
+.IP \[bu] 3
+If the pathname given in
+.I path
+is absolute, then
+.I dirfd
+is ignored.
+.IP \[bu]
+If the pathname given in
+.I path
+is relative and
+.I dirfd
+is the special value
+.BR AT_FDCWD ,
+then
+.I path
+is interpreted relative to
+the current working directory
+of the calling process (like
+.BR open (2)).
+.IP \[bu]
+If the pathname given in
+.I path
+is relative,
+then it is interpreted relative to
+the directory referred to by the file descriptor
+.I dirfd
+(rather than relative to
+the current working directory
+of the calling process,
+as is done by
+.BR open (2)
+for a relative pathname).
+In this case,
+.I dirfd
+must be a directory
+that was opened for reading
+.RB ( O_RDONLY )
+or using the
+.B O_PATH
+flag.
+.IP \[bu]
+If
+.I path
+is an empty string,
+and
+.I flags
+contains
+.BR \%AT_EMPTY_PATH ,
+then the file descriptor referenced by
+.I dirfd
+is operated on directly.
+In this case,
+.I dirfd
+can refer to any type of file,
+not just a directory.
+.P
+.I flags
+can be used to control aspects of the path lookup
+and properties of the returned file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B AT_EMPTY_PATH
+If
+.I path
+is an empty string, operate on the file referred to by
+.I dirfd
+(which may have been obtained from
+.BR open (2),
+.BR fsmount(2),
+or from another
+.BR open_tree ()
+call).
+In this case,
+.I dirfd
+can refer to any type of file, not just a directory.
+If
+.I dirfd
+is
+.BR AT_FDCWD ,
+the call operates on the current working directory
+of the calling process.
+This flag is Linux-specific; define
+.B \%_GNU_SOURCE
+to obtain its definition.
+.TP
+.B AT_NO_AUTOMOUNT
+Don't automount the terminal ("basename") component of
+.I path
+if it is a directory that is an automount point.
+This allows the caller to gather attributes of an automount point
+(rather than the location it would mount).
+This flag has no effect if the mount point has already been mounted over.
+This flag is Linux-specific; define
+.B \%_GNU_SOURCE
+to obtain its definition.
+.TP
+.B AT_SYMLINK_NOFOLLOW
+If
+.I path
+is a symbolic link, do not dereference it; instead,
+create either a handle to the link itself
+or a bind-mount of it.
+The resultant file descriptor is indistinguishable from one produced by
+.BR openat (2)
+with
+.BR \%O_PATH | O_NOFOLLLOW .
+.TP
+.B OPEN_TREE_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.TP
+.B OPEN_TREE_CLONE
+Rather than opening the path as a regular file
+(a-la
+.BR openat (2)),
+create a detached bind-mount mount object
+and attach it to the file descriptor.
+In order to do this operation,
+the calling process must have the
+.BR \%CAP_SYS_ADMIN
+capability.
+.TP
+.B AT_RECURSIVE
+Create a recursive bind-mount of the path
+(a-la
+.BR mount (2)
+with
+.BR MS_BIND | MS_REC ),
+and attach it to the file descriptor.
+This flag is only permitted in conjunction with
+.BR \%OPEN_TREE_CLONE .
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EACCES
+Search permission is denied for one of the directories
+in the path prefix of
+.IR path .
+(See also
+.BR path_resolution (7).)
+.TP
+.B EPERM
+.I flags
+contains
+.B \%OPEN_TREE_CLONE
+but the calling process does not have the required
+.B CAP_SYS_ADMIN
+capability.
+.TP
+.B EBADF
+.I path
+is relative but
+.I dirfd
+is neither
+.B AT_FDCWD
+nor a valid file descriptor.
+.TP
+.B EFAULT
+.I path
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B ELOOP
+Too many symbolic links encountered when resolving
+.IR path .
+.TP
+.B ENAMETOOLONG
+.I path
+is longer than
+.BR PATH_MAX .
+.TP
+.B ENOENT
+A component of
+.I path
+does not exist, or is a dangling symbolic link.
+.TP
+.B ENOENT
+.I path
+is an empty string, but
+.B AT_EMPTY_PATH
+is not specified in
+.IR flags .
+.TP
+.B ENOTDIR
+A component of the path prefix of
+.I path
+is not a directory, or
+.I path
+is relative and
+.I dirfd
+is a file descriptor referring to a file other than a directory.
+.TP
+.B ENOSPC
+The "anonymous" mount namespace
+necessary to contain the
+.B \%OPEN_TREE_CLONE
+detached bind-mount mount object
+could not be allocated,
+as doing so would
+exceed the configured per-user limit
+on the number of mount namespaces
+in the current user namespace.
+(See also
+.BR namespaces (7).)
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit a07b20004793d8926f78d63eb5980559f7813404
+glibc 2.36.
+.SH NOTES
+.SS Anonymous mount namespaces
+The bind-mount mount objects created by
+.BR open_tree ()
+with
+.B \%OPEN_TREE_CLONE
+are not attached to the mount namespace of the calling process.
+Instead, each mount object is attached to
+a newly allocated "anonymous" mount namespace
+associated with the calling process.
+.P
+One of the side-effects of this is that
+(unlike bind-mounts created with
+.BR mount (2)),
+mount propagation
+(as described in
+.BR mount_namespaces (7))
+will not be applied to bind-mounts created by
+.BR open_tree ()
+until the bind-mount is attached with
+.BR move_mount (2),
+at which point the mount
+will be associated with the mount namespace
+where it was mounted
+and mount propagation will resume.
+.SH EXAMPLES
+The following examples show how
+.BR open_tree ()
+can be used in place of more traditional
+.BR mount (2)
+calls with
+.BR MS_BIND .
+.P
+.in +4n
+.EX
+int srcfd;
+\&
+/* mount --bind /var /mnt */
+mount("/var", "/mnt", NULL, MS_BIND, NULL);
+/* ... is equivalent to ... */
+srcfd = open_tree(AT_FDCWD, "/var", OPEN_TREE_CLONE);
+move_mount(srcfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+\&
+/* mount --rbind /var /mnt */
+mount("/var", "/mnt", NULL, MS_BIND|MS_REC, NULL);
+/* ... is equivalent to ... */
+srcfd = open_tree(AT_FDCWD, "/var", OPEN_TREE_CLONE | AT_RECURSIVE);
+move_mount(srcfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+\&
+/* mount --bind /proc/self/fd/100 /proc/self/fd/200/foo */
+mount("/proc/self/fd/100", "/proc/self/fd/200/foo", NULL, MS_BIND, NULL);
+/* ... is equivalent to ... */
+srcfd = open_tree(100, "", AT_EMPTY_PATH | OPEN_TREE_CLONE);
+move_mount(srcfd, "", 200, "foo", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+In addition, you can use the file descriptor returned by
+.BR open_tree ()
+as the
+.I dirfd
+argument to any "*at()" system calls:
+.P
+.in +4n
+.EX
+int dirfd, fd;
+\&
+dirfd = open_tree(AT_FDCWD, "/etc", OPEN_TREE_CLONE);
+fd = openat(dirfd, "passwd", O_RDONLY);
+fchmodat(dirfd, "shadow", 0000, 0);
+close(dirfd);
+close(fd);
+/* The bind-mount is now destroyed. */
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR mount_namespaces (7)
--
2.50.1
^ permalink raw reply related
* [PATCH v2 07/11] move_mount.2: document 'new' mount api
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).
Co-developed-by: David Howells <dhowells@redhat.com>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/move_mount.2 | 609 ++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 609 insertions(+)
diff --git a/man/man2/move_mount.2 b/man/man2/move_mount.2
new file mode 100644
index 000000000000..6a944198f620
--- /dev/null
+++ b/man/man2/move_mount.2
@@ -0,0 +1,609 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH move_mount 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+move_mount \- move or attach mount object to filesystem
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <fcntl.h>" \
+" /* Definition of " AT_* " constants */"
+.BR "#include <sys/mount.h>"
+.P
+.BI "int move_mount(int " from_dirfd ", const char *" from_path ","
+.BI " int " to_dirfd ", const char *" to_path ","
+.BI " unsigned int " flags ");"
+.fi
+.SH DESCRIPTION
+The
+.BR move_mount ()
+system call is part of the suite of file descriptor based mount facilities
+in Linux.
+.P
+.BR move_mount ()
+moves the mount object indicated by
+.I from_dirfd
+and
+.I from_path
+to the path indicated by
+.I to_dirfd
+and
+.IR to_path .
+The mount object being moved
+could be an existing mount point in the current mount namespace,
+or it could be a detached mount object created by
+.BR fsmount (2)
+or
+.BR open_tree (2)
+with
+.BR \%OPEN_TREE_CLONE .
+.P
+To access the source mount object
+or the destination mount point,
+no permissions are required on the object itself,
+but if either pathname is supplied,
+execute (search) permission is required
+on all of the directories specified in
+.I from_path
+or
+.IR to_path .
+.P
+The calling process must have the
+.BR \%CAP_SYS_ADMIN
+capability in order to attach or move a mount object.
+.P
+As with "*at()" system calls,
+.BR move_mount ()
+uses the
+.I from_dirfd
+and
+.I to_dirfd
+arguments
+in conjunction with the
+.I from_path
+and
+.I to_path
+arguments to determine the source and destination objects to operate on
+(respectively), as follows:
+.IP \[bu] 3
+If the pathname given in
+.I *_path
+is absolute, then
+the corresponding
+.I *_dirfd
+is ignored.
+.IP \[bu]
+If the pathname given in
+.I *_path
+is relative and
+the corresponding
+.I *_dirfd
+is the special value
+.BR AT_FDCWD ,
+then
+.I *_path
+is interpreted relative to
+the current working directory
+of the calling process (like
+.BR open (2)).
+.IP \[bu]
+If the pathname given in
+.I *_path
+is relative,
+then it is interpreted relative to
+the directory referred to by
+the corresponding file descriptor
+.I *_dirfd
+(rather than relative to
+the current working directory
+of the calling process,
+as is done by
+.BR open (2)
+for a relative pathname).
+In this case,
+the corresponding
+.I *_dirfd
+must be a directory
+that was opened for reading
+.RB ( O_RDONLY )
+or using the
+.B O_PATH
+flag.
+.IP \[bu]
+If
+.I *_path
+is an empty string,
+and
+.I flags
+contains the appropriate
+.BI \%MOVE_MOUNT_ * _EMPTY_PATH
+flag,
+then the file descriptor referenced by
+the corresponding
+.I *_dirfd
+is operated on directly.
+In this case,
+the corresponding
+.I *_dirfd
+can refer to any type of file,
+not just a directory.
+.IP
+This is the most common mechanism
+used to attach detached mount objects
+to a mount point target.
+.P
+.I flags
+can be used to control aspects of the path lookup
+for both the source and destination objects,
+as well as other properties of the mount operation.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B MOVE_MOUNT_F_EMPTY_PATH
+If
+.I from_path
+is an empty string, operate on the file referred to by
+.I from_dirfd
+(which may have been obtained from
+.BR open (2),
+.BR fsmount (2),
+or
+.BR open_tree (2)).
+In this case,
+.I from_dirfd
+can refer to any type of file,
+not just a directory.
+If
+.I from_dirfd
+is
+.BR AT_FDCWD ,
+the call operates on the current working directory.
+.TP
+.B MOVE_MOUNT_T_EMPTY_PATH
+As with
+.BR \%MOVE_MOUNT_F_EMPTY_PATH ,
+except operating on
+.IR to_dirfd " and " to_path .
+.TP
+.B MOVE_MOUNT_F_SYMLINKS
+If
+.IR from_path
+references a symbolic link,
+then dereference it.
+The default behaviour for
+.BR move_mount ()
+is to
+.I not follow
+symbolic links.
+.TP
+.B MOVE_MOUNT_T_SYMLINKS
+As with
+.BR \%MOVE_MOUNT_F_SYMLINKS ,
+except operating on
+.I to_dirfd
+and
+.IR to_path .
+.TP
+.B MOVE_MOUNT_F_NO_AUTOMOUNT
+Don't automount the terminal ("basename") component of
+.I from_path
+if it is a directory that is an automount point.
+This allows a mount object
+that has an automount point at its root
+to be moved
+and prevents unintended triggering of an automount point.
+This flag has no effect
+if the automount point has already been mounted over.
+.TP
+.B MOVE_MOUNT_T_NO_AUTOMOUNT
+As with
+.BR \%MOVE_MOUNT_F_NO_AUTOMOUNT ,
+except operating on
+.IR to_dirfd " and " to_path .
+This allows an automount point to be manually mounted over.
+.TP
+.BR MOVE_MOUNT_SET_GROUP " (since Linux 5.15)"
+Add the attached (private) mount indicated by
+.I to_dirfd
+and
+.I to_path
+into the mount propagation "peer group"
+of the attached (non-private) mount
+indicated by
+.IR from_dirfd " and " from_path .
+.IP
+Unlike other
+.BR move_mount ()
+operations,
+this operation does not move any actual mount objects.
+Instead, it only updates the metadata
+of existing (attached) mount objects.
+.IP
+This makes it possible to first create a mount tree
+consisting only of private mounts
+and then configure the desired propagation layout afterwards.
+(See the "SHARED SUBTREES" section of
+.BR mount_namespaces (7)
+for more information about mount propagation and peer groups.)
+.TP
+.BR MOVE_MOUNT_BENEATH " (since Linux 6.5)"
+If the path indicated by
+.I to_dirfd
+and
+.I to_path
+is an existing mount object,
+rather than placing the mount object indicated by
+.I from_dirfd
+and
+.I from_path
+on top of the mount stack,
+place it below the current top mount
+on the mount stack.
+.IP
+After using
+.BR \%MOVE_MOUNT_BENEATH ,
+it is possible to
+.BR umount (2)
+the top mount
+in order to reveal the mount
+which was moved beneath it earlier.
+This allows for the seamless (and atomic) replacement
+of intricate mount trees,
+which can further be used
+to "upgrade" a mount tree with a newer version.
+.IP
+This operation has several restrictions:
+.RS
+.IP \[bu] 3
+Mounts cannot be moved beneath the rootfs,
+including the rootfs as configured by
+.BR chroot (2)
+and
+.BR pivot_root (2).
+To mount beneath the rootfs,
+.BR pivot_root (2)
+must be used.
+.IP \[bu]
+The target path indicated by
+.I to_dirfd
+and
+.I to_path
+must be an attached mount object.
+It must not be a detached mount object given by
+.BR open_tree (2)
+with
+.B \%OPEN_TREE_CLONE
+or
+.BR fsmount (2).
+.IP \[bu]
+The current top mount
+of the target path's mount stack
+and its parent mount
+must be in the calling process's mount namespace.
+.IP \[bu]
+The caller must have sufficient privileges
+to unmount the top mount
+of the target path's mount stack,
+to prove they have privileges
+to reveal the underlying mount.
+.IP \[bu]
+Mount propagation events triggered by this
+.BR move_mount ()
+operation
+are calculated based on the parent mount
+of the current top mount
+of the target path's mount stack.
+.IP \[bu]
+The target path's mount
+cannot be a parent mount
+of the source mount object.
+.IP \[bu]
+The source mount object
+must not have any overmounts,
+otherwise it would be possible to create "shadow mounts"
+(i.e., two mounts mounted on the same parent mount at the same mount point).
+.IP \[bu]
+It is not possible to move a mount
+beneath a top mount
+if the parent mount
+of the current top mount
+propagates to the top mount itself.
+Otherwise,
+.B \%MOVE_MOUNT_BENEATH
+would cause the mount object
+to be propagated
+to the top mount
+from the parent mount,
+defeating the purpose of using
+.BR \%MOVE_MOUNT_BENEATH .
+.IP \[bu]
+It is not possible to move a mount
+beneath a top mount
+if the parent mount
+of the current top mount
+propagates to the mount object
+being mounted beneath.
+Otherwise, this would cause a similar propagation issue
+to the previous point,
+also defeating the purpose of using
+.BR \%MOVE_MOUNT_BENEATH .
+.RE
+.RE
+.P
+If
+.BR move_mount ()
+is called repeatedly
+with a file descriptor
+that refers to a mount object,
+then the object will be attached (or moved)
+the first time
+and then moved again and again,
+detaching it from the previous mount point each time.
+.SH RETURN VALUE
+On success,
+.BR move_mount ()
+returns 0.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EACCES
+Search permission is denied
+for one of the directories
+in the path prefix of one of
+.IR from_path " or " to_path .
+(See also
+.BR path_resolution (7).)
+.TP
+.B EBADF
+One of
+.I from_dirfd
+or
+.I to_dirfd
+is not a valid file descriptor.
+.TP
+.B EFAULT
+One of
+.I from_path
+or
+.I to_path
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B EINVAL
+The path indicated by
+.IR from_dirfd " and " from_path
+is not a mount object.
+.TP
+.B EINVAL
+The mount object type
+of the source mount object and target inode
+are not compatible
+(i.e., the source is a file but the target is a directory, or vice-versa).
+.TP
+.B EINVAL
+The source mount object or target path
+are not in the calling process's mount namespace
+(or an anonymous mount namespace of the calling process).
+.TP
+.B EINVAL
+The source mount object's parent mount
+has shared mount propagation,
+and thus cannot be moved
+(as described in
+.BR mount_namespaces (7)).
+.TP
+.B EINVAL
+The source mount has
+.B MS_UNBINDABLE
+child mounts
+but the target path
+resides on a mount tree with shared mount propagation,
+which would otherwise cause the unbindable mounts to be propagated
+(as described in
+.BR mount_namespaces (7)).
+.TP
+.B EINVAL
+.B \%MOVE_MOUNT_BENEATH
+was attempted,
+but one of the listed restrictions was violated.
+.TP
+.B ELOOP
+Too many symbolic links encountered
+when resolving one of
+.I from_path
+or
+.IR to_path .
+.TP
+.B ENAMETOOLONG
+One of
+.I from_path
+or
+.I to_path
+is longer than
+.BR PATH_MAX .
+.TP
+.B ENOENT
+A component of one of
+.I from_path
+or
+.I to_path
+does not exist.
+.TP
+.B ENOENT
+One of
+.I from_path
+or
+.I to_path
+is an empty string,
+but the corresponding
+.BI MOVE_MOUNT_ * _EMPTY_PATH
+flag is not specified in
+.IR flags .
+.TP
+.B ENOTDIR
+A component of the path prefix of one of
+.I from_path
+or
+.I to_path
+is not a directory,
+or one of
+.I from_path
+or
+.I to_path
+is relative
+and the corresponding
+.I from_dirfd
+or
+.I to_dirfd
+is a file descriptor referring to a file other than a directory.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit 2db154b3ea8e14b04fee23e3fdfd5e9d17fbc6ae
+glibc 2.36.
+.SH EXAMPLES
+.BR move_mount ()
+can be used to move attached mounts like the following:
+.P
+.in +4n
+.EX
+move_mount(AT_FDCWD, "/a", AT_FDCWD, "/b", 0);
+.EE
+.in
+.P
+This would move the mount object mounted on
+.I /a
+to
+.IR /b .
+The above procedure is functionally equivalent to
+the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/a", "/b", NULL, MS_MOVE, NULL);
+.EE
+.in
+.P
+.BR move_mount ()
+can also be used in conjunction with file descriptors returned from
+.BR open_tree (2)
+or
+.BR open (2):
+.P
+.in +4n
+.EX
+int fd = open_tree(AT_FDCWD, "/mnt", 0); /* or open("/mnt", O_PATH); */
+move_mount(fd, "", AT_FDCWD, "/mnt2", MOVE_MOUNT_F_EMPTY_PATH);
+move_mount(fd, "", AT_FDCWD, "/mnt3", MOVE_MOUNT_F_EMPTY_PATH);
+move_mount(fd, "", AT_FDCWD, "/mnt4", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+This would move the mount object mounted at
+.I /mnt
+to
+.IR /mnt2 ,
+then
+.IR /mnt3 ,
+then
+.IR /mnt4 .
+.P
+If the source mount object
+indicated by
+.I from_dirfd
+and
+.I from_path
+is a detached mount object,
+.BR move_mount ()
+can be used to attach it to a mount point:
+.P
+.in +4n
+.EX
+int fsfd, mntfd;
+\&
+fsfd = fsopen("ext4", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/sda1", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NODEV);
+move_mount(mntfd, "", AT_FDCWD, "/home", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+This creates a new filesystem configuration context for ext4,
+configures it,
+creates a mount object,
+and then attaches it to
+.IR /home .
+The above procedure is functionally equivalent to
+the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/dev/sda1", "/home", "ext4", MS_NODEV, "user_xattr");
+.EE
+.in
+.P
+This also works with detached bind-mounts created with
+.BR open_tree (2)
+with
+.BR OPEN_TREE_CLONE :
+.P
+.in +4n
+.EX
+int mntfd = open_tree(AT_FDCWD, "/home/cyphar", OPEN_TREE_CLONE);
+move_mount(mntfd, "", AT_FDCWD, "/root", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+This creates a new detached bind-mount mount object of
+.IR /home/cyphar ,
+and then attaches it to
+.IR /root .
+The above procedure is functionally equivalent to
+the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/home/cyphar", "/root", NULL, MS_BIND, NULL);
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)
+
--
2.50.1
^ permalink raw reply related
* [PATCH v2 06/11] fsmount.2: document 'new' mount api
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).
Co-developed-by: David Howells <dhowells@redhat.com>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/fsmount.2 | 209 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 209 insertions(+)
diff --git a/man/man2/fsmount.2 b/man/man2/fsmount.2
new file mode 100644
index 000000000000..8c264c3d5aba
--- /dev/null
+++ b/man/man2/fsmount.2
@@ -0,0 +1,209 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsmount 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fsmount \- instantiate mount object from filesystem context
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <sys/mount.h>"
+.P
+.BI "int fsmount(int " fsfd ", unsigned int " flags ", \
+unsigned int " attr_flags ");"
+.fi
+.SH DESCRIPTION
+The
+.BR fsmount ()
+system call is part of the suite of file descriptor based mount facilities
+in Linux.
+.P
+.BR fsmount ()
+takes the created filesystem instance
+referenced by the filesystem context
+associated with the file descriptor
+.I fsfd
+and creates a new mount object
+for the root of the filesystem instance,
+which is then attached to a new file descriptor
+and returned.
+In order to create a mount object with
+.BR fsmount (),
+the calling process must have the
+.BR \%CAP_SYS_ADMIN
+capability.
+.P
+The filesystem context must have been created with a call to
+.BR fsopen (2)
+and then had a filesystem instance instantiated with a call to
+.BR fsconfig (2)
+with
+.B \%FSCONFIG_CMD_CREATE
+or
+.B \%FSCONFIG_CMD_CREATE_EXCL
+in order to be in the correct state
+for this operation.
+.P
+As with file descriptors returned from
+.BR open_tree (2)
+called with
+.BR OPEN_TREE_CLONE ,
+the returned file descriptor
+can then be used with
+.BR move_mount (2),
+.BR mount_setattr (2),
+or other such system calls
+to do further mount operations.
+This mount object will be unmounted and destroyed
+when the file descriptor is closed
+if it was not otherwise mounted somewhere else
+by calling
+.BR move_mount (2).
+The returned file descriptor
+also acts the same as one produced by
+.BR open (2)
+with
+.BR O_PATH ,
+meaning it can also be used as a
+.I dirfd
+argument
+to "*at()" system calls.
+.P
+.I flags
+controls the creation of the returned file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSMOUNT_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.RE
+.P
+.I attr_flags
+specifies the mount attributes
+for the created mount object
+and accepts the same set of
+.BI \%MOUNT_ATTR_ *
+flags as
+.BR mount_setattr (2),
+except for flags such as
+.B \%MOUNT_ATTR_IDMAP
+which require specifying additional fields in
+.BR mount_attr (2type).
+.P
+If the
+.BR fsmount ()
+operation is successful,
+the filesystem context
+associated with the file descriptor
+.I fsfd
+is reset
+and placed into a reconfiguration state,
+similar to the one produced by
+.BR fspick (2).
+You may coninue to use
+.BR fsconfig (2)
+with the
+.B \%FSCONFIG_CMD_RECONFIGURE
+command
+to reconfigure the filesystem instance.
+.P
+Unlike
+.BR open_tree (2),
+.BR fsmount ()
+can only be called once
+to produce a mount object
+for a given filesystem configuration context.
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EBUSY
+The filesystem context attached to
+.I fsfd
+is not in the right state
+to be used by
+.BR fsmount ().
+.TP
+.B EINVAL
+.I flags
+had an invalid flag set.
+.TP
+.B EINVAL
+.I attr_flags
+had an invalid
+.BI MOUNT_ATTR_ *
+flag set.
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B ENOSPC
+The "anonymous" mount namespace
+necessary to contain the new mount object
+could not be allocated,
+as doing so would
+exceed the configured per-user limit
+on the number of mount namespaces
+in the current user namespace.
+(See also
+.BR namespaces (7).)
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit 93766fbd2696c2c4453dd8e1070977e9cd4e6b6d
+glibc 2.36.
+.SH EXAMPLES
+.in +4n
+.EX
+int fsfd, mntfd, tmpfd;
+\&
+fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NODEV | MOUNT_ATTR_NOEXEC);
+\&
+/* Create a new file without attaching the mount object. */
+int tmpfd = openat(mntfd, "tmpfile", O_CREAT | O_EXCL | O_RDWR, 0600);
+unlinkat(mntfd, "tmpfile", 0);
+\&
+/* Attach the mount object to "/tmp". */
+move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)
+
--
2.50.1
^ permalink raw reply related
* [PATCH v2 05/11] fsconfig.2: document 'new' mount api
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).
Co-developed-by: David Howells <dhowells@redhat.com>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/fsconfig.2 | 559 ++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 559 insertions(+)
diff --git a/man/man2/fsconfig.2 b/man/man2/fsconfig.2
new file mode 100644
index 000000000000..e2121b7a6b68
--- /dev/null
+++ b/man/man2/fsconfig.2
@@ -0,0 +1,559 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsconfig 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fsconfig \- configure new or existing filesystem context
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <sys/mount.h>"
+.P
+.BI "int fsconfig(int " fd ", unsigned int " cmd ","
+.BI " const char *" key ", const void *" value ", int " aux ");"
+.fi
+.SH DESCRIPTION
+The
+.BR fsconfig ()
+system call is part of the suite of file descriptor based mount facilities
+in Linux.
+.P
+.BR fsconfig ()
+is used to supply parameters to
+and issue commands against
+the filesystem configuration context
+associated with the file descriptor
+.IR fd .
+Filesystem configuration contexts can be created with
+.BR fsopen (2)
+or instantiated from an extant filesystem instance with
+.BR fspick (2).
+.P
+The
+.I cmd
+argument indicates the command to be issued.
+Some commands supply parameters to the context
+(equivalent to mount options specified with
+.BR mount (8)),
+while others are meta-operations on the filesystem context.
+The list of valid
+.I cmd
+values are:
+.RS
+.TP
+.B FSCONFIG_SET_FLAG
+Set the flag parameter named by
+.IR key .
+.I value
+must be NULL,
+and
+.I aux
+must be 0.
+.TP
+.B FSCONFIG_SET_STRING
+Set the string parameter named by
+.I key
+to the value specified by
+.IR value .
+.I value
+points to a null-terminated string,
+and
+.I aux
+must be 0.
+.TP
+.B FSCONFIG_SET_BINARY
+Set the blob parameter named by
+.I key
+to the contents of the binary blob
+specified by
+.IR value .
+.I value
+points to
+the start of a buffer
+that is
+.I aux
+bytes in length.
+.TP
+.B FSCONFIG_SET_FD
+Set the file parameter named by
+.I key
+to the file referenced by the file descriptor
+.IR aux .
+.I value
+must be NULL.
+.IP
+You may also use
+.B \%FSCONFIG_SET_STRING
+for file parameters,
+with
+.I value
+set to a null-terminated string
+containing a base-10 representation
+of the file descriptor number.
+This mechanism is primarily intended for compatibility with older
+.BR mount (2)-based
+programs.
+.TP
+.B FSCONFIG_SET_PATH
+Set the path parameter named by
+.I key
+to the object at a provided path,
+resolved in a similar manner to
+.BR openat (2).
+.I value
+points to a null-terminated pathname string,
+and
+.I aux
+is equivalent to the
+.I dirfd
+argument to
+.BR openat (2).
+.IP
+You may also use
+.B \%FSCONFIG_SET_STRING
+for path parameters,
+the behaviour of which is equivalent to
+.B \%FSCONFIG_SET_PATH
+with
+.I aux
+set to
+.BR AT_FDCWD .
+.TP
+.B FSCONFIG_SET_PATH_EMPTY
+As with
+.BR \%FSCONFIG_SET_PATH ,
+except that if
+.I value
+is an empty string,
+the file descriptor specified by
+.I aux
+may be any type of file
+(not just a directory)
+and will be used as the path parameter value,
+equivalent to the behaviour of
+.B \%AT_EMPTY_PATH
+with most "*at()" system calls.
+If
+.I aux
+is
+.BR AT_FDCWD ,
+the call operates on the current working directory
+of the calling process.
+.IP
+Note that this behaviour with empty paths is distinct in some subtle ways to
+.BR \%FSCONFIG_SET_FD .
+.B \%FSCONFIG_SET_FD
+indicates that the underlying file
+for the file descriptor
+.I aux
+should be used as the parameter value directly;
+.B \%FSCONFIG_SET_PATH_EMPTY
+indicates that the underlying file
+for the file descriptor
+.I aux
+should be re-opened by the filesystem driver,
+and the newly created file description
+should be used as the parameter value.
+This can result in slightly different behaviour
+when dealing with special files
+or files sourced from pseudofilesystems.
+Filesystems may also choose to only support one kind of parameter,
+and so a parameter that accepts
+.B FSCONFIG_SET_FD
+may not work with
+.BR FSCONFIG_SET_PATH ( _EMPTY )
+(or vice-versa).
+.TP
+.B FSCONFIG_CMD_CREATE
+This command instructs the filesystem driver
+to instantiate an instance of the filesystem in the kernel
+with the parameters set in the filesystem configuration context
+referenced by the file descriptor
+.IR fd .
+.IR key " and " value
+must be NULL,
+and
+.I aux
+must be 0.
+.IP
+If this operation succeeds,
+the filesystem context
+associated with file descriptor
+.I fd
+now references the created filesystem instance,
+and is placed into a special "needs-mount" mode
+that allows you to use
+.BR fsmount (2)
+to create a mount object from the filesystem instance.
+.IP
+This is intended for use with filesystem configuration contexts created with
+.BR fsopen (2).
+In order to create a filesystem instance,
+the calling process must have the
+.B \%CAP_SYS_ADMIN
+capability.
+.IP
+Note that the Linux kernel reuses filesystem instances
+for many filesystems,
+so (depending on the filesystem being configured and parameters used)
+it is possible for the filesystem instance "created" by
+.B \%FSCONFIG_CMD_CREATE
+to, in fact, be a reference
+to an existing filesystem instance in the kernel.
+The kernel will attempt to merge the specified parameters
+of this filesystem configuration context
+with those of the filesystem instance being reused,
+but some parameters may be
+.IR "silently ignored" .
+.IP
+Programs that need to ensure
+that they create a new filesystem instance
+with specific parameters
+(notably, security-related parameters such as "acl" to enable
+POSIX ACLs as described in
+.BR acl (5))
+should use
+.B \%FSCONFIG_CMD_CREATE_EXCL
+instead.
+.TP
+.BR FSCONFIG_CMD_CREATE_EXCL " (since Linux 6.6)"
+.\" commit 22ed7ecdaefe0cac0c6e6295e83048af60435b13
+As with
+.BR \%FSCONFIG_CMD_CREATE ,
+except that the kernel is instructed
+to create a new filesystem instance
+("superblock" in kernel developer parlance)
+rather than reusing an existing one.
+.IP
+If this is not possible
+(such as with disk-backed filesystems
+where multiple filesystem instances
+using the same filesystem driver
+and writing to the same underlying device
+could result in data corruption),
+this operation will incur
+an
+.B EBUSY
+error.
+.IP
+As a result (unlike
+.BR \%FSCONFIG_CMD_CREATE ),
+if this command succeeds
+then the calling process can be sure that
+all of the parameters successfully configured with
+.BR fsconfig ()
+will actually be applied
+to the created filesystem instance.
+.TP
+.B FSCONFIG_CMD_RECONFIGURE
+This command instructs the filesystem driver
+to apply the parameters set in this filesystem configuration context
+to an already existing filesystem instance.
+.IP
+This is primarily intended for use with
+.BR fspick (2),
+but may also be used to modify the parameters of filesystem instance after
+.BR \%FSCONFIG_CMD_CREATE
+was used to create it
+and a mount object was created using
+.BR fsmount (2).
+In order to reconfigure an extant filesystem instance,
+the calling process must have the
+.B CAP_SYS_ADMIN
+capability.
+.IP
+Once this operation succeeds, the filesystem context is reset
+but remains in reconfiguration mode
+and thus can be used for subsequent
+.B \%FSCONFIG_CMD_RECONFIGURE
+commands.
+.SH RETURN VALUE
+On success,
+.BR fsconfig ()
+returns 0.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+If an error occurs, the filesystem driver may provide
+additional information about the error
+through the message retrieval interface for filesystem configuration contexts.
+This additional information can be retrieved at any time by calling
+.BR read (2)
+on the filesystem instance or filesystem configuration context referenced by
+the file descriptor
+.IR fd .
+(See the "Message retrieval interface" subsection in
+.BR fsopen (2)
+for more details on the message format.)
+.P
+Even after an error occurs,
+the filesystem configuration context is
+.I not
+invalidated,
+and thus can still be used with other
+.BR fsconfig ()
+commands.
+This means that users can probe support for mount parameters
+on a per-parameter basis,
+and adjust which parameters they wish to set.
+.P
+The error values given below result from filesystem type independent errors.
+Each filesystem type may have its own special errors
+and its own special behavior.
+See the Linux kernel source code for details.
+.TP
+.B EACCES
+A component of a path
+provided as a path parameter
+was not searchable.
+(See also
+.BR path_resolution (7).)
+.TP
+.B EACCES
+.B \%FSCONFIG_CMD_CREATE
+was attempted
+for a read-only filesystem
+without specifying the
+.RB ' ro '
+flag parameter.
+.TP
+.B EACCES
+A specified block device parameter
+is located on a filesystem
+mounted with the
+.B \%MS_NODEV
+option.
+.TP
+.B EBADF
+The file descriptor given by
+.I fd
+(or possibly by
+.IR aux ,
+depending on the command)
+is invalid.
+.TP
+.B EBUSY
+The filesystem context attached to
+.I fd
+is in the wrong state
+for the given command.
+.TP
+.B EBUSY
+The filesystem instance cannot be reconfigured as read-only
+with
+.B \%FSCONFIG_CMD_RECONFIGURE
+because some programs
+still hold files open for writing.
+.TP
+.B EBUSY
+A new filesystem instance was requested with
+.B \%FSCONFIG_CMD_CREATE_EXCL
+but a matching superblock already existed.
+.TP
+.B EFAULT
+One of the pointer arguments
+points to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+.I fd
+does not refer to
+a filesystem configuration context
+or filesystem instance.
+.TP
+.B EINVAL
+One of the values of
+.IR name ,
+.IR value ,
+and/or
+.I aux
+were set to a non-zero value when
+.I cmd
+required that they be zero
+(or NULL).
+.TP
+.B EINVAL
+The parameter named by
+.I name
+cannot be set
+using the type specified with
+.IR cmd .
+.TP
+.B EINVAL
+One of the source parameters
+referred to
+an invalid superblock.
+.TP
+.B ELOOP
+Too many links encountered
+during pathname resolution
+of a path argument.
+.TP
+.B ENAMETOOLONG
+A path argument was longer than
+.BR PATH_MAX .
+.TP
+.B ENOENT
+A path argument had a non-existent component.
+.TP
+.B ENOENT
+A path argument is an empty string,
+but
+.I cmd
+is not
+.BR \%FSCONFIG_SET_PATH_EMPTY .
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B ENOTBLK
+The parameter named by
+.I name
+must be a block device,
+but the provided parameter value was not a block device.
+.TP
+.B ENOTDIR
+A component of the path prefix
+of a path argument
+was not a directory.
+.TP
+.B EOPNOTSUPP
+The command given by
+.I cmd
+is not valid.
+.TP
+.B ENXIO
+The major number
+of a block device parameter
+is out of range.
+.TP
+.B EPERM
+The command given by
+.I cmd
+was
+.BR \%FSCONFIG_CMD_CREATE ,
+.BR \%FSCONFIG_CMD_CREATE_EXCL ,
+or
+.BR \% FSCONFIG_CMD_RECONFIGURE ,
+but the calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit ecdab150fddb42fe6a739335257949220033b782
+glibc 2.36.
+.SH EXAMPLES
+To illustrate the different kinds of flags that can be configured with
+.BR fsconfig (),
+here are a few examples of some different filesystems being created:
+.P
+.in +4n
+.EX
+int fsfd, mntfd;
+\&
+fsfd = fsopen("tmpfs", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "inode64", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "uid", "1234", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "huge", "never", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "casefold", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOEXEC);
+move_mount(mntfd, "", AT_FDCWD, "/tmp", MOVE_MOUNT_F_EMPTY_PATH);
+\&
+fsfd = fsopen("erofs", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "/dev/loop0", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE_EXCL, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_NOSUID);
+move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+Some filesystems have different behaviour
+when using
+.BR fsconfig ()
+to set the same parameter
+named by
+.I key
+multiple times:
+.P
+.in +4n
+.EX
+\&
+int fsfd, mntfd, lowerdirfd;
+\&
+lowerdirfd = open("/o/ctr/lower1", O_DIRECTORY | O_CLOEXEC);
+fsfd = fsopen("overlay", FSOPEN_CLOEXEC);
+/* "lowerdir+" appends to the lower dir stack */
+fsconfig(fsfd, FSCONFIG_SET_FD, "lowerdir+", NULL, lowerdirfd);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "lowerdir+", "/o/ctr/lower2", 0);
+.\" fsconfig(fsfd, FSCONFIG_SET_PATH, "lowerdir+", "/o/ctr/lower3", AT_FDCWD);
+.\" fsconfig(fsfd, FSCONFIG_SET_PATH_EMPTY, "lowerdir+", "", lowerdirfd);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "xino", "auto", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "nfs_export", "off", 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+Other filesystems allow you to use different
+.BI FSCONFIG_SET_ *
+commands for the same parameter
+named by
+.IR key :
+.P
+.in +4n
+.EX
+\&
+int fsfd, mntfd, nsfd, nsdirfd;
+\&
+nsfd = open("/proc/self/ns/pid", O_PATH);
+nsdirfd = open("/proc/1/ns", O_DIRECTORY);
+\&
+fsfd = fsopen("proc", FSOPEN_CLOEXEC);
+/* "pidns" changes the value each time. */
+fsconfig(fsfd, FSCONFIG_SET_PATH, "pidns", "/proc/self/ns/pid", AT_FDCWD);
+fsconfig(fsfd, FSCONFIG_SET_PATH, "pidns", "pid", NULL, nsdirfd);
+fsconfig(fsfd, FSCONFIG_SET_PATH_EMPTY, "pidns", "", nsfd);
+fsconfig(fsfd, FSCONFIG_SET_FD, "pidns", NULL, nsfd);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+move_mount(mntfd, "", AT_FDCWD, "/proc", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+And here is an example of how
+.BR fspick (2)
+can be used with
+.BR fsconfig ()
+to reconfigure the parameters
+of an extant filesystem instance
+attached to
+.IR /proc :
+.P
+.in +4n
+.EX
+int fsfd = fspick(AT_FDCWD, "/proc", FSPICK_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "ptraceable", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "subset", "pid", 0);
+fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
+.EE
+.in
+.SH SEE ALSO
+.BR fsmount (2),
+.BR fsopen (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)
+
--
2.50.1
^ permalink raw reply related
* [PATCH v2 04/11] fspick.2: document 'new' mount api
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).
Co-developed-by: David Howells <dhowells@redhat.com>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/fspick.2 | 305 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 305 insertions(+)
diff --git a/man/man2/fspick.2 b/man/man2/fspick.2
new file mode 100644
index 000000000000..5215d706428b
--- /dev/null
+++ b/man/man2/fspick.2
@@ -0,0 +1,305 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fspick 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fspick \- select filesystem for reconfiguration
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <fcntl.h>" \
+" /* Definition of " AT_* " constants */"
+.BR "#include <sys/mount.h>"
+.P
+.BI "int fspick(int " dirfd ", const char *" path ", unsigned int " flags ");"
+.fi
+.SH DESCRIPTION
+The
+.BR fspick ()
+system call is part of
+the suite of file descriptor based mount facilities
+in Linux.
+.P
+.BR fspick()
+creates a new filesystem configuration context
+for the filesystem instance
+associated with the path described by
+.IR dirfd
+and
+.IR path ,
+places it into reconfiguration mode
+(similar to
+.BR mount (8)
+with the
+.I -o remount
+option),
+and attaches it to a new file descriptor,
+which is then returned.
+The calling process must have the
+.BR CAP_SYS_ADMIN
+capability in order to create a new filesystem configuration context.
+.P
+The resultant file descriptor can be used with
+.BR fsconfig (2)
+to specify the desired set of changes
+to mount parameters
+of the filesystem instance.
+Once the desired set of changes have been configured,
+the changes can be effectuated by calling
+.BR fsconfig (2)
+with the
+.B \%FSCONFIG_CMD_RECONFIGURE
+command.
+.P
+As with "*at()" system calls,
+.BR fspick ()
+uses the
+.I dirfd
+argument in conjunction with the
+.I path
+argument to determine the path to operate on, as follows:
+.IP \[bu] 3
+If the pathname given in
+.I path
+is absolute, then
+.I dirfd
+is ignored.
+.IP \[bu]
+If the pathname given in
+.I path
+is relative and
+.I dirfd
+is the special value
+.BR AT_FDCWD ,
+then
+.I path
+is interpreted relative to
+the current working directory
+of the calling process (like
+.BR open (2)).
+.IP \[bu]
+If the pathname given in
+.I path
+is relative,
+then it is interpreted relative to
+the directory referred to by the file descriptor
+.I dirfd
+(rather than relative to
+the current working directory
+of the calling process,
+as is done by
+.BR open (2)
+for a relative pathname).
+In this case,
+.I dirfd
+must be a directory
+that was opened for reading
+.RB ( O_RDONLY )
+or using the
+.B O_PATH
+flag.
+.IP \[bu]
+If
+.I path
+is an empty string,
+and
+.I flags
+contains
+.BR \%FSPICK_EMPTY_PATH ,
+then the file descriptor referenced by
+.I dirfd
+is operated on directly.
+In this case,
+.I dirfd
+can refer to any type of file,
+not just a directory.
+.P
+.I flags
+can be used to control aspects of how
+.I path
+is resolved
+and properties of the returned file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSPICK_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.TP
+.B FSPICK_EMPTY_PATH
+If
+.I path
+is an empty string,
+operate on the file referred to by
+.I dirfd
+(which may have been obtained from
+.BR open (2),
+.BR fsmount (2),
+or
+.BR open_tree (2)).
+In this case,
+.I dirfd
+can refer to any type of file, not just a directory.
+If
+.I dirfd
+is
+.BR \%AT_FDCWD ,
+the call operates on the current working directory
+of the calling process.
+.TP
+.B FSPICK_SYMLINK_NOFOLLOW
+Do not follow symbolic links
+in the terminal component of
+.IR path .
+If
+.I path
+references a symbolic link,
+the returned filesystem context will reference
+the filesystem that the symbolic link itself resides on.
+.TP
+.B FSPICK_NO_AUTOMOUNT
+Do not follow automounts in the terminal component of
+.IR path .
+This allows you to configure the underlying automount point
+without triggering the automount.
+This flag has no effect if the automount point has already been mounted over.
+.RE
+.P
+As with filesystem contexts created with
+.BR fsopen (2),
+the file descriptor returned by
+.BR fspick ()
+may be queried for message strings at any time by calling
+.BR read (2)
+on the file descriptor.
+(See the "Message retrieval interface" subsection in
+.BR fsopen (2)
+for more details on the message format.)
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EACCES
+Search permission is denied
+for one of the directories
+in the path prefix of
+.IR path .
+(See also
+.BR path_resolution (7).)
+.TP
+.B EBADF
+.I path
+is relative but
+.I dirfd
+is neither
+.B AT_FDCWD
+nor a valid file descriptor.
+.TP
+.B EFAULT
+.I path
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B ELOOP
+Too many symbolic links encountered when resolving
+.IR path .
+.TP
+.B ENAMETOOLONG
+.I path
+is longer than
+.BR PATH_MAX .
+.TP
+.B ENOENT
+A component of
+.I path
+does not exist,
+or is a dangling symbolic link.
+.TP
+.B ENOENT
+.I path
+is an empty string, but
+.B \%FSPICK_EMPTY_PATH
+is not specified in
+.IR flags .
+.TP
+.B ENOTDIR
+A component of the path prefix of
+.I path
+is not a directory, or
+.I path
+is relative and
+.I dirfd
+is a file descriptor referring to a file other than a directory.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit cf3cba4a429be43e5527a3f78859b1bfd9ebc5fb
+glibc 2.36.
+.SH EXAMPLES
+The following example sets the read-only flag
+on the filesystem instance referenced by
+the mount object attached at
+.IR /tmp .
+.P
+.in +4n
+.EX
+int fsfd = fspick(AT_FDCWD, "/tmp", FSPICK_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0);
+.EE
+.in
+.P
+The above procedure is functionally equivalent to the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount(NULL, "/tmp", NULL, MS_REMOUNT | MS_RDONLY, NULL);
+.EE
+.in
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fsopen (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)
+
--
2.50.1
^ permalink raw reply related
* [PATCH v2 03/11] fsopen.2: document 'new' mount api
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
This is loosely based on the original documentation written by David
Howells and later maintained by Christian Brauner, but has been
rewritten to be more from a user perspective (as well as fixing a few
critical mistakes).
Co-developed-by: David Howells <dhowells@redhat.com>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/fsopen.2 | 319 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 319 insertions(+)
diff --git a/man/man2/fsopen.2 b/man/man2/fsopen.2
new file mode 100644
index 000000000000..ad38ef0782be
--- /dev/null
+++ b/man/man2/fsopen.2
@@ -0,0 +1,319 @@
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH fsopen 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+fsopen \- create a new filesystem context
+.SH LIBRARY
+Standard C library
+.RI ( libc ,\~ \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <sys/mount.h>"
+.P
+.BI "int fsopen(const char *" fsname ", unsigned int " flags ");"
+.fi
+.SH DESCRIPTION
+The
+.BR fsopen ()
+system call is part of the suite of file descriptor based mount facilities in
+Linux.
+.P
+.BR fsopen ()
+creates a blank filesystem configuration context within the kernel
+for the filesystem named by
+.IR fsname ,
+puts the context into creation mode and attaches it to a file descriptor,
+which is then returned.
+The calling process must have the
+.B \%CAP_SYS_ADMIN
+capability in order to create a new filesystem configuration context.
+.P
+A filesystem configuration context is an in-kernel representation of a pending
+transaction,
+containing a set of configuration parameters that are to be applied
+when creating a new instance of a filesystem
+(or modifying the configuration of an existing filesystem instance,
+such as when using
+.BR fspick (2)).
+.P
+After obtaining a filesystem configuration context with
+.BR fsopen (),
+the general workflow for operating on the context looks like the following:
+.IP (1) 5
+Pass the filesystem context file descriptor to
+.BR fsconfig (2)
+to specify any desired filesystem parameters.
+This may be done as many times as necessary.
+.IP (2)
+Pass the same filesystem context file descriptor to
+.BR fsconfig (2)
+with
+.B \%FSCONFIG_CMD_CREATE
+to create an instance of the configured filesystem.
+.IP (3)
+Pass the same filesystem context file descriptor to
+.BR fsmount (2)
+to create a new mount object for the root of the filesystem,
+which is then attached to a new file descriptor.
+This also places the filesystem context file descriptor into reconfiguration
+mode,
+similar to the mode produced by
+.BR fspick (2).
+.IP (4)
+Use the mount object file descriptor as a
+.I dirfd
+argument to "*at()" system calls;
+or attach the mount object to a mount point
+by passing the mount object file descriptor to
+.BR move_mount (2).
+.P
+A filesystem context will move between different modes throughout its
+lifecycle
+(such as the creation phase when created with
+.BR fsopen (),
+the reconfiguration phase when an existing filesystem instance is selected by
+.BR fspick (2),
+and the intermediate "needs-mount" phase between
+.\" FS_CONTEXT_NEEDS_MOUNT is the term the kernel uses for this.
+.BR \%FSCONFIG_CMD_CREATE
+and
+.BR fsmount (2)),
+which has an impact on what operations are permitted on the filesystem context.
+.P
+The file descriptor returned by
+.BR fsopen ()
+also acts as a channel for filesystem drivers to provide more comprehensive
+error, warning, and information messages
+than are normally provided through the standard
+.BR errno (3)
+interface for system calls.
+If an error occurs at any time during the workflow mentioned above,
+calling
+.BR read (2)
+on the filesystem context file descriptor will retrieve any ancillary
+information about the encountered errors.
+(See the "Message retrieval interface" section for more details on the message
+format.)
+.P
+.I flags
+can be used to control aspects of the creation of the filesystem configuration
+context file descriptor.
+A value for
+.I flags
+is constructed by bitwise ORing
+zero or more of the following constants:
+.RS
+.TP
+.B FSOPEN_CLOEXEC
+Set the close-on-exec
+.RB ( FD_CLOEXEC )
+flag on the new file descriptor.
+See the description of the
+.B O_CLOEXEC
+flag in
+.BR open (2)
+for reasons why this may be useful.
+.RE
+.P
+A list of filesystems supported by the running kernel
+(and thus a list of valid values for
+.IR fsname )
+can be obtained from
+.IR /proc/filesystems .
+(See also
+.BR proc_filesystems (5).)
+.SS Message retrieval interface
+When doing operations on a filesystem configuration context,
+the filesystem driver may choose to provide ancillary information to userspace
+in the form of message strings.
+.P
+The filesystem context file descriptors returned by
+.BR fsopen ()
+and
+.BR fspick (2)
+may be queried for message strings at any time by calling
+.BR read (2)
+on the file descriptor.
+Each call to
+.BR read (2)
+will return a single message,
+prefixed to indicate its class:
+.RS
+.TP
+.B "e <message>"
+An error message was logged.
+This is usually associated with an error being returned from the corresponding
+system call which triggered this message.
+.TP
+.B "w <message>"
+A warning message was logged.
+.TP
+.B "i <message>"
+An informational message was logged.
+.RE
+.P
+Messages are removed from the queue as they are read.
+Note that the message queue has limited depth,
+so it is possible for messages to get lost.
+If there are no messages in the message queue,
+.B read(2)
+will return no data and
+.I errno
+will be set to
+.BR \%ENODATA .
+If the
+.I buf
+argument to
+.BR read (2)
+is not large enough to contain the message,
+.BR read (2)
+will return no data and
+.I errno
+will be set to
+.BR \%EMSGSIZE .
+.P
+If there are multiple filesystem context file descriptors referencing the same
+filesystem instance
+(such as if you call
+.BR fspick (2)
+multiple times for the same mount),
+each one gets its own independent message queue.
+This does not apply to file descriptors that were duplicated with
+.BR dup (2).
+.P
+Messages strings will usually be prefixed by the filesystem driver that logged
+the message, though this may not always be the case.
+See the Linux kernel source code for details.
+.SH RETURN VALUE
+On success, a new file descriptor is returned.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EFAULT
+.I fsname
+is NULL
+or a pointer to a location
+outside the calling process's accessible address space.
+.TP
+.B EINVAL
+.I flags
+had an invalid flag set.
+.TP
+.B EMFILE
+The calling process has too many open files to create more.
+.TP
+.B ENFILE
+The system has too many open files to create more.
+.TP
+.B ENODEV
+The filesystem named by
+.I fsname
+is not supported by the kernel.
+.TP
+.B ENOMEM
+The kernel could not allocate sufficient memory to complete the operation.
+.TP
+.B EPERM
+The calling process does not have the required
+.B \%CAP_SYS_ADMIN
+capability.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 5.2.
+.\" commit 24dcb3d90a1f67fe08c68a004af37df059d74005
+glibc 2.36.
+.SH EXAMPLES
+To illustrate the workflow for creating a new mount,
+the following is an example of how to mount an
+.BR ext4 (5)
+filesystem stored on
+.I /dev/sdb1
+onto
+.IR /mnt .
+.P
+.in +4n
+.EX
+int fsfd, mntfd;
+\&
+fsfd = fsopen("ext4", FSOPEN_CLOEXEC);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "ro", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_PATH, "source", "/dev/sdb1", AT_FDCWD);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "noatime", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "acl", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "user_xattr", NULL, 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "iversion", NULL, 0)
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, FSMOUNT_CLOEXEC, MOUNT_ATTR_RELATIME);
+move_mount(mntfd, "", AT_FDCWD, "/mnt", MOVE_MOUNT_F_EMPTY_PATH);
+.EE
+.in
+.P
+First, an ext4 configuration context is created and attached to the file
+descriptor
+.IR fsfd .
+Then, a series of parameters
+(such as the source of the filesystem)
+are provided using
+.BR fsconfig (2),
+followed by the filesystem instance being created with
+.BR \%FSCONFIG_CMD_CREATE .
+.BR fsmount (2)
+is then used to create a new mount object attached to the file descriptor
+.IR mntfd ,
+which is then attached to the intended mount point using
+.BR move_mount (2).
+.P
+The above procedure is functionally equivalent to the following mount operation
+using
+.BR mount (2):
+.P
+.in +4n
+.EX
+mount("/dev/sdb1", "/mnt", "ext4", MS_RELATIME,
+ "ro,noatime,acl,user_xattr,iversion");
+.EE
+.in
+.P
+And here's an example of creating a mount object
+of an NFS server share
+and setting a Smack security module label.
+However, instead of attaching it to a mount point,
+the program uses the mount object directly
+to open a file from the NFS share.
+.P
+.in +4n
+.EX
+int fsfd, mntfd, fd;
+\&
+fsfd = fsopen("nfs", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "source", "example.com/pub/linux", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "nfsvers", "3", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "rsize", "65536", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "wsize", "65536", 0);
+fsconfig(fsfd, FSCONFIG_SET_STRING, "smackfsdef", "foolabel", 0);
+fsconfig(fsfd, FSCONFIG_SET_FLAG, "rdma", NULL, 0);
+fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+mntfd = fsmount(fsfd, 0, MOUNT_ATTR_NODEV);
+fd = openat(mntfd, "src/linux-5.2.tar.xz", O_RDONLY);
+.EE
+.in
+.P
+Unlike the previous example,
+this operation has no trivial equivalent with
+.BR mount (2),
+as it was not previously possible to create a mount object
+that is not attached to any mount point.
+.SH SEE ALSO
+.BR fsconfig (2),
+.BR fsmount (2),
+.BR fspick (2),
+.BR mount (2),
+.BR mount_setattr (2),
+.BR move_mount (2),
+.BR open_tree (2),
+.BR mount_namespaces (7)
--
2.50.1
^ permalink raw reply related
* [PATCH v2 02/11] mount_setattr.2: move mount_attr struct to mount_attr.2type
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
As with open_how(2type), it makes sense to move this to a separate man
page. In addition, future man pages added in this patchset will want to
reference mount_attr(2type).
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/mount_setattr.2 | 17 ++++---------
man/man2type/mount_attr.2type | 58 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 63 insertions(+), 12 deletions(-)
diff --git a/man/man2/mount_setattr.2 b/man/man2/mount_setattr.2
index c96f0657f046..d44fafc93a20 100644
--- a/man/man2/mount_setattr.2
+++ b/man/man2/mount_setattr.2
@@ -114,18 +114,11 @@ .SH DESCRIPTION
.I attr
argument of
.BR mount_setattr ()
-is a structure of the following form:
-.P
-.in +4n
-.EX
-struct mount_attr {
- __u64 attr_set; /* Mount properties to set */
- __u64 attr_clr; /* Mount properties to clear */
- __u64 propagation; /* Mount propagation type */
- __u64 userns_fd; /* User namespace file descriptor */
-};
-.EE
-.in
+is a pointer to a
+.I mount_attr
+structure,
+described in
+.BR mount_attr (2type).
.P
The
.I attr_set
diff --git a/man/man2type/mount_attr.2type b/man/man2type/mount_attr.2type
new file mode 100644
index 000000000000..b7a3ace6b3b9
--- /dev/null
+++ b/man/man2type/mount_attr.2type
@@ -0,0 +1,58 @@
+
+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH mount_attr 2type (date) "Linux man-pages (unreleased)"
+.SH NAME
+mount_attr \- what mount properties to set and clear
+.SH LIBRARY
+Linux kernel headers
+.SH SYNOPSIS
+.EX
+.B #include <sys/mount.h>
+.P
+.B struct mount_attr {
+.BR " __u64 attr_set;" " /* Mount properties to set */"
+.BR " __u64 attr_clr;" " /* Mount properties to clear */"
+.BR " __u64 propagation;" " /* Mount propagation type */"
+.BR " __u64 userns_fd;" " /* User namespace file descriptor */"
+ /* ... */
+.B };
+.EE
+.SH DESCRIPTION
+Specifies which mount properties should be changed with
+.BR mount_setattr (2).
+.P
+The fields are as follows:
+.TP
+.I .attr_set
+This field specifies which
+.BI MOUNT_ATTR_ *
+attribute flags to set.
+.TP
+.I .attr_clr
+This fields specifies which
+.BI MOUNT_ATTR_ *
+attribute flags to clear.
+.TP
+.I .propagation
+This field specifies what mount propagation will be applied.
+The valid values of this field are the same propagation types described in
+.BR mount_namespaces (7).
+.TP
+.I .userns_fd
+This fields specifies a file descriptor that indicates which user namespace to
+use as a reference for ID-mapped mounts with
+.BR MOUNT_ATTR_IDMAP .
+.SH VERSIONS
+Extra fields may be appended to the structure,
+with a zero value in a new field resulting in
+the kernel behaving as though that extension field was not present.
+Therefore, a user
+.I must
+zero-fill this structure on initialization.
+.SH STANDARDS
+Linux.
+.SH SEE ALSO
+.BR mount_setattr (2)
--
2.50.1
^ permalink raw reply related
* [PATCH v2 01/11] mount_setattr.2: document glibc >= 2.36 syscall wrappers
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
In-Reply-To: <20250807-new-mount-api-v2-0-558a27b8068c@cyphar.com>
Glibc 2.36 added syscall wrappers for the entire family of fd-based
mount syscalls, including mount_setattr(2). Thus it's no longer
necessary to instruct users to do raw syscall(2) operations.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
man/man2/mount_setattr.2 | 45 +++++++--------------------------------------
1 file changed, 7 insertions(+), 38 deletions(-)
diff --git a/man/man2/mount_setattr.2 b/man/man2/mount_setattr.2
index 60d9cf9de8aa..c96f0657f046 100644
--- a/man/man2/mount_setattr.2
+++ b/man/man2/mount_setattr.2
@@ -10,21 +10,12 @@ .SH LIBRARY
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
-.BR "#include <linux/fcntl.h>" " /* Definition of " AT_* " constants */"
-.BR "#include <linux/mount.h>" " /* Definition of " MOUNT_ATTR_* " constants */"
-.BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */"
-.B #include <unistd.h>
+.BR "#include <fcntl.h>" " /* Definition of " AT_* " constants */"
+.B #include <sys/mount.h>
.P
-.BI "int syscall(SYS_mount_setattr, int " dirfd ", const char *" path ,
-.BI " unsigned int " flags ", struct mount_attr *" attr \
-", size_t " size );
+.BI "int mount_setattr(int " dirfd ", const char *" path ", unsigned int " flags ","
+.BI " struct mount_attr *" attr ", size_t " size );"
.fi
-.P
-.IR Note :
-glibc provides no wrapper for
-.BR mount_setattr (),
-necessitating the use of
-.BR syscall (2).
.SH DESCRIPTION
The
.BR mount_setattr ()
@@ -586,6 +577,7 @@ .SH HISTORY
.\" commit 7d6beb71da3cc033649d641e1e608713b8220290
.\" commit 2a1867219c7b27f928e2545782b86daaf9ad50bd
.\" commit 9caccd41541a6f7d6279928d9f971f6642c361af
+glibc 2.36.
.SH NOTES
.SS ID-mapped mounts
Creating an ID-mapped mount makes it possible to
@@ -914,37 +906,14 @@ .SH EXAMPLES
#include <err.h>
#include <fcntl.h>
#include <getopt.h>
-#include <linux/mount.h>
-#include <linux/types.h>
+#include <sys/mount.h>
+#include <sys/types.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
-#include <sys/syscall.h>
#include <unistd.h>
\&
-static inline int
-mount_setattr(int dirfd, const char *path, unsigned int flags,
- struct mount_attr *attr, size_t size)
-{
- return syscall(SYS_mount_setattr, dirfd, path, flags,
- attr, size);
-}
-\&
-static inline int
-open_tree(int dirfd, const char *filename, unsigned int flags)
-{
- return syscall(SYS_open_tree, dirfd, filename, flags);
-}
-\&
-static inline int
-move_mount(int from_dirfd, const char *from_path,
- int to_dirfd, const char *to_path, unsigned int flags)
-{
- return syscall(SYS_move_mount, from_dirfd, from_path,
- to_dirfd, to_path, flags);
-}
-\&
static const struct option longopts[] = {
{"map\-mount", required_argument, NULL, \[aq]a\[aq]},
{"recursive", no_argument, NULL, \[aq]b\[aq]},
--
2.50.1
^ permalink raw reply related
* [PATCH v2 00/11] man2: add man pages for 'new' mount API
From: Aleksa Sarai @ 2025-08-06 17:44 UTC (permalink / raw)
To: Alejandro Colomar
Cc: Michael T. Kerrisk, Alexander Viro, Jan Kara, Askar Safin,
G. Branden Robinson, linux-man, linux-api, linux-fsdevel,
linux-kernel, David Howells, Christian Brauner, Aleksa Sarai
Back in 2019, the new mount API was merged into mainline[1]. David Howells
then set about writing man pages for these new APIs, and sent some
patches back in 2020[2]. Unfortunately, these patches were never merged,
which meant that these APIs were practically undocumented for many
years -- arguably this may have been a contributing factor to the
relatively slow adoption of these new (far better) APIs. I have often
discovered that many folks are unaware of the read(2)-based message
retrieval interface provided by filesystem context file descriptors.
In 2024, Christian Brauner set aside some time to provide some
documentation of these new APIs and so adapted David Howell's original
man pages into the easier-to-edit Markdown format and published them on
GitHub[3]. These have been maintained since, including updated
information on new features added since David Howells's 2020 draft pages
(such as MOVE_MOUNT_BENEATH).
While this was a welcome improvement to the previous status quo (that
had lasted over 6 years), speaking personally my experience is that not
having access to these man pages from the terminal has been a fairly
common painpoint.
So, this is a modern version of the man pages for these APIs, in the hopes
that we can finally (7 years later) get proper documentation for these
APIs in the man-pages project.
One important thing to note is that most of these were re-written by me,
with very minimal copying from the versions available from Christian[2].
The reasons for this are two-fold:
* Both Howells's original version and Christian's maintained versions
contain crucial mistakes that I have been bitten by in the past (the
most obvious being that all of these APIs were merged in Linux 5.2,
but the man pages all claim they were merged in different versions.)
* As the man pages appear to have been written from Howells's
perspective while implementing them, some of the wording is a little
too tied to the implementation (or appears to describe features that
don't really exist in the merged versions of these APIs).
I decided that the best way to resolve these issues is to rewrite them
from the perspective of an actual user of these APIs (me), and check
that we do not repeat the mistakes I found in the originals.
I have also done my best to resolve the issues raised by Michael Kerrisk
on the original patchset sent by Howells[1].
In addition, I have also included a man page for open_tree_attr(2) (as a
subsection of the new open_tree(2) man page), which was merged in Linux
6.15.
[1]: https://lore.kernel.org/all/20190507204921.GL23075@ZenIV.linux.org.uk/
[2]: https://lore.kernel.org/linux-man/159680892602.29015.6551860260436544999.stgit@warthog.procyon.org.uk/
[3]: https://github.com/brauner/man-pages-md
Co-developed-by: David Howells <dhowells@redhat.com>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
Changes in v2:
- `make -R lint-man`. [Alejandro Colomar]
- `sed -i s|Glibc|glibc|g`. [Alejandro Colomar]
- `sed -i s|pathname|path|g` [Alejandro Colomar]
- Clean up macro usage, example code, and synopsis. [Alejandro Colomar]
- Try to use semantic newlines. [Alejandro Colomar]
- Make sure the usage of "filesystem context", "filesystem instance",
and "mount object" are consistent. [Askar Safin]
- Avoid referring to these syscalls without an "at" suffix as "*at()
syscalls". [Askar Safin]
- Use \% to avoid hyphenation of constants. [Askar Safin, G. Branden Robinson]
- Add a new subsection to mount_setattr(2) to describe the distinction
between mount attributes and filesystem parameters.
- (Under protest) double-space-after-period formatted commit messages.
- v1: <https://lore.kernel.org/r/20250806-new-mount-api-v1-0-8678f56c6ee0@cyphar.com>
---
Aleksa Sarai (11):
mount_setattr.2: document glibc >= 2.36 syscall wrappers
mount_setattr.2: move mount_attr struct to mount_attr.2type
fsopen.2: document 'new' mount api
fspick.2: document 'new' mount api
fsconfig.2: document 'new' mount api
fsmount.2: document 'new' mount api
move_mount.2: document 'new' mount api
open_tree.2: document 'new' mount api
mount_setattr.2: mirror opening sentence from fsopen(2)
open_tree_attr.2, open_tree.2: document new open_tree_attr() api
fsconfig.2, mount_setattr.2: add note about attribute-parameter distinction
man/man2/fsconfig.2 | 566 +++++++++++++++++++++++++++++++++++++++
man/man2/fsmount.2 | 209 +++++++++++++++
man/man2/fsopen.2 | 319 ++++++++++++++++++++++
man/man2/fspick.2 | 305 +++++++++++++++++++++
man/man2/mount_setattr.2 | 105 ++++----
man/man2/move_mount.2 | 609 ++++++++++++++++++++++++++++++++++++++++++
man/man2/open_tree.2 | 479 +++++++++++++++++++++++++++++++++
man/man2/open_tree_attr.2 | 1 +
man/man2type/mount_attr.2type | 58 ++++
9 files changed, 2600 insertions(+), 51 deletions(-)
---
base-commit: f23e8249a6dcf695d38055483802779c36aedbba
change-id: 20250802-new-mount-api-436db984f432
Best regards,
--
Aleksa Sarai <cyphar@cyphar.com>
^ permalink raw reply
* Re: [PATCH v4 2/4] procfs: add "pidns" mount option
From: Aleksa Sarai @ 2025-08-06 14:12 UTC (permalink / raw)
To: Askar Safin
Cc: amir73il, brauner, corbet, jack, linux-api, linux-doc,
linux-fsdevel, linux-kernel, linux-kselftest, luto, shuah, viro
In-Reply-To: <20250806102501.75104-1-safinaskar@zohomail.com>
[-- Attachment #1: Type: text/plain, Size: 1539 bytes --]
On 2025-08-06, Askar Safin <safinaskar@zohomail.com> wrote:
> > I just realised that we probably also want to support FSCONFIG_SET_PATH
>
> I just checked kernel code. Indeed nobody uses FSCONFIG_SET_PATH.
> Moreover, fsparam_path macro is present since 5.1. And for all this
> time nobody used it. So, let's just remove FSCONFIG_SET_PATH. Nobody
> used it, so this will not break anything.
>
> If you okay with that, I can submit patch, removing it.
I would prefer you didn't -- "*at()" semantics are very useful to a lot
of programs (*especially* AT_EMPTY_PATH). I would like the pidns= stuff
to support it, and probably also overlayfs...
I suspect the primary issue is that when migrating to the new mount API,
filesystem devs just went with the easiest thing to use
(FSCONFIG_SET_STRING) even though FSCONFIG_SET_PATH would be better. I
suspect the lack of documentation around fsconfig(2) played a part too.
My impression is that interest in the minutia about fsconfig(2) is quite
low on the list of priorities for most filesystem devs, and so the neat
aspects of fsconfig(2) haven't been fully utilised. (In LPC last year,
we struggled to come to an agreement on how filesystems should use the
read(2)-based error interface.)
We can very easily move fsparam_string() or fsparam_file_or_string()
parameters to fsparam_path() and a future fsparam_file_or_path(). I
would much prefer that as a user.
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]
^ permalink raw reply
* Re: [PATCH v2 31/32] libluo: introduce luoctl
From: Pratyush Yadav @ 2025-08-06 12:02 UTC (permalink / raw)
To: Pasha Tatashin
Cc: Pratyush Yadav, Steven Rostedt, Jason Gunthorpe, Thomas Gleixner,
jasonmiu, graf, changyuanl, rppt, dmatlack, rientjes, corbet,
rdunlap, ilpo.jarvinen, kanie, ojeda, aliceryhl, masahiroy, akpm,
tj, yoann.congal, mmaurer, roman.gushchin, chenridong, axboe,
mark.rutland, jannh, vincent.guittot, hannes, dan.j.williams,
david, joel.granados, anna.schumaker, song, zhangguopeng, linux,
linux-kernel, linux-doc, linux-mm, gregkh, mingo, bp, dave.hansen,
x86, hpa, rafael, dakr, bartosz.golaszewski, cw00.choi,
myungjoo.ham, yesanishhere, Jonathan.Cameron, quic_zijuhu,
aleksander.lobakin, ira.weiny, andriy.shevchenko, leon, lukas,
bhelgaas, wagi, djeffery, stuart.w.hayes, lennart, brauner,
linux-api, linux-fsdevel, saeedm, ajayachandra, parav, leonro,
witu
In-Reply-To: <CA+CK2bA=pmEtNWc5nN2hWcepq_+8HtbH2mTP2UUgabZ8ERaROw@mail.gmail.com>
Hi Pasha,
On Tue, Aug 05 2025, Pasha Tatashin wrote:
>> To add some context: one of the reasons to include it in the series as
>> an RFC at the end was to showcase the userspace side of the API and have
>> a way for people to see how it can be used. Seeing an API in action
>> provides useful context for reviewing patches.
>>
>> I think Pasha forgot to add the RFC tags when he created v2, since it is
>> only meant to be RFC right now and not proper patches.
>
> Correct, I accidently removed RFC from memfd patches in the version. I
> will include memfd preservation as RFCv1 in v3 submission.
I didn't mean this for the memfd patches, only for libluo.
I think the memfd patches are in decent shape. They aren't pristine, but
I do think they are good enough to land and be improved iteratively.
If you think otherwise, then what do you reckon needs to be done to make
them _not_ RFC?
--
Regards,
Pratyush Yadav
^ permalink raw reply
* Re: [PATCH v4 2/4] procfs: add "pidns" mount option
From: Askar Safin @ 2025-08-06 10:25 UTC (permalink / raw)
To: cyphar
Cc: amir73il, brauner, corbet, jack, linux-api, linux-doc,
linux-fsdevel, linux-kernel, linux-kselftest, luto, shuah, viro
In-Reply-To: <2025-08-05.1754378656-steep-harps-muscled-mailroom-lively-gosling-VVGNTP@cyphar.com>
> I just realised that we probably also want to support FSCONFIG_SET_PATH
I just checked kernel code. Indeed nobody uses FSCONFIG_SET_PATH. Moreover, fsparam_path macro is present since 5.1. And for all this time nobody used it. So, let's just remove FSCONFIG_SET_PATH. Nobody used it, so this will not break anything.
If you okay with that, I can submit patch, removing it.
--
Askar Safin
^ permalink raw reply
* [PATCH v2 2/2] vfs: output mount_too_revealing() errors to fscontext
From: Aleksa Sarai @ 2025-08-06 6:07 UTC (permalink / raw)
To: Alexander Viro, Christian Brauner, Jan Kara
Cc: David Howells, linux-api, linux-kernel, linux-fsdevel,
Aleksa Sarai
In-Reply-To: <20250806-errorfc-mount-too-revealing-v2-0-534b9b4d45bb@cyphar.com>
It makes little sense for fsmount() to output the warning message when
mount_too_revealing() is violated to kmsg. Instead, the warning should
be output (with a "VFS" prefix) to the fscontext log. In addition,
include the same log message for mount_too_revealing() when doing a
regular mount for consistency.
With the newest fsopen()-based mount(8) from util-linux, the error
messages now look like
# mount -t proc proc /tmp
mount: /tmp: fsmount() failed: VFS: Mount too revealing.
dmesg(1) may have more information after failed mount system call.
which could finally result in mount_too_revealing() errors being easier
for users to detect and understand.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
fs/namespace.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index 55f28cebbe7d..1e1c2c257e2e 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3820,8 +3820,10 @@ static int do_new_mount_fc(struct fs_context *fc, struct path *mountpoint,
int error;
error = security_sb_kern_mount(sb);
- if (!error && mount_too_revealing(sb, &mnt_flags))
+ if (!error && mount_too_revealing(sb, &mnt_flags)) {
+ errorfcp(fc, "VFS", "Mount too revealing");
error = -EPERM;
+ }
if (unlikely(error)) {
fc_drop_locked(fc);
@@ -4547,7 +4549,7 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
ret = -EPERM;
if (mount_too_revealing(fc->root->d_sb, &mnt_flags)) {
- pr_warn("VFS: Mount too revealing\n");
+ errorfcp(fc, "VFS", "Mount too revealing");
goto err_unlock;
}
--
2.50.1
^ permalink raw reply related
* [PATCH v2 1/2] fscontext: add custom-prefix log helpers
From: Aleksa Sarai @ 2025-08-06 6:07 UTC (permalink / raw)
To: Alexander Viro, Christian Brauner, Jan Kara
Cc: David Howells, linux-api, linux-kernel, linux-fsdevel,
Aleksa Sarai
In-Reply-To: <20250806-errorfc-mount-too-revealing-v2-0-534b9b4d45bb@cyphar.com>
Sometimes, errors associated with an fscontext come from the VFS or
otherwise outside of the filesystem driver itself. However, the default
logging of errorfc will always prefix the message with the filesystem
name.
So, add some *fcp() wrappers that allow for custom prefixes to be used
when emitting information to the fscontext log.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
include/linux/fs_context.h | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index 7773eb870039..671f031be173 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -186,10 +186,12 @@ struct fc_log {
extern __attribute__((format(printf, 4, 5)))
void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt, ...);
-#define __logfc(fc, l, fmt, ...) logfc((fc)->log.log, NULL, \
- l, fmt, ## __VA_ARGS__)
-#define __plog(p, l, fmt, ...) logfc((p)->log, (p)->prefix, \
- l, fmt, ## __VA_ARGS__)
+#define __logfc(fc, l, fmt, ...) \
+ logfc((fc)->log.log, NULL, (l), (fmt), ## __VA_ARGS__)
+#define __plogp(p, prefix, l, fmt, ...) \
+ logfc((p)->log, (prefix), (l), (fmt), ## __VA_ARGS__)
+#define __plog(p, l, fmt, ...) __plogp(p, (p)->prefix, l, fmt, ## __VA_ARGS__)
+
/**
* infof - Store supplementary informational message
* @fc: The context in which to log the informational message
@@ -201,6 +203,8 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
#define infof(fc, fmt, ...) __logfc(fc, 'i', fmt, ## __VA_ARGS__)
#define info_plog(p, fmt, ...) __plog(p, 'i', fmt, ## __VA_ARGS__)
#define infofc(fc, fmt, ...) __plog((&(fc)->log), 'i', fmt, ## __VA_ARGS__)
+#define infofcp(fc, prefix, fmt, ...) \
+ __plogp((&(fc)->log), prefix, 'i', fmt, ## __VA_ARGS__)
/**
* warnf - Store supplementary warning message
@@ -213,6 +217,8 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
#define warnf(fc, fmt, ...) __logfc(fc, 'w', fmt, ## __VA_ARGS__)
#define warn_plog(p, fmt, ...) __plog(p, 'w', fmt, ## __VA_ARGS__)
#define warnfc(fc, fmt, ...) __plog((&(fc)->log), 'w', fmt, ## __VA_ARGS__)
+#define warnfcp(fc, prefix, fmt, ...) \
+ __plogp((&(fc)->log), prefix, 'w', fmt, ## __VA_ARGS__)
/**
* errorf - Store supplementary error message
@@ -225,6 +231,8 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
#define errorf(fc, fmt, ...) __logfc(fc, 'e', fmt, ## __VA_ARGS__)
#define error_plog(p, fmt, ...) __plog(p, 'e', fmt, ## __VA_ARGS__)
#define errorfc(fc, fmt, ...) __plog((&(fc)->log), 'e', fmt, ## __VA_ARGS__)
+#define errorfcp(fc, prefix, fmt, ...) \
+ __plogp((&(fc)->log), prefix, 'e', fmt, ## __VA_ARGS__)
/**
* invalf - Store supplementary invalid argument error message
@@ -237,5 +245,7 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
#define invalf(fc, fmt, ...) (errorf(fc, fmt, ## __VA_ARGS__), -EINVAL)
#define inval_plog(p, fmt, ...) (error_plog(p, fmt, ## __VA_ARGS__), -EINVAL)
#define invalfc(fc, fmt, ...) (errorfc(fc, fmt, ## __VA_ARGS__), -EINVAL)
+#define invalfcp(fc, prefix, fmt, ...) \
+ (errorfcp(fc, prefix, fmt, ## __VA_ARGS__), -EINVAL)
#endif /* _LINUX_FS_CONTEXT_H */
--
2.50.1
^ permalink raw reply related
* [PATCH v2 0/2] vfs: output mount_too_revealing() errors to fscontext
From: Aleksa Sarai @ 2025-08-06 6:07 UTC (permalink / raw)
To: Alexander Viro, Christian Brauner, Jan Kara
Cc: David Howells, linux-api, linux-kernel, linux-fsdevel,
Aleksa Sarai
It makes little sense for fsmount() to output the warning message when
mount_too_revealing() is violated to kmsg. Instead, the warning should
be output (with a "VFS" prefix) to the fscontext log. In addition,
include the same log message for mount_too_revealing() when doing a
regular mount for consistency.
With the newest fsopen()-based mount(8) from util-linux, the error
messages now look like
# mount -t proc proc /tmp
mount: /tmp: fsmount() failed: VFS: Mount too revealing.
dmesg(1) may have more information after failed mount system call.
which could finally result in mount_too_revealing() errors being easier
for users to detect and understand.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
Changes in v2:
- Log before setting retval. [Al Viro]
- v1: <https://lore.kernel.org/r/20250806-errorfc-mount-too-revealing-v1-0-536540f51560@cyphar.com>
---
Aleksa Sarai (2):
fscontext: add custom-prefix log helpers
vfs: output mount_too_revealing() errors to fscontext
fs/namespace.c | 6 ++++--
include/linux/fs_context.h | 18 ++++++++++++++----
2 files changed, 18 insertions(+), 6 deletions(-)
---
base-commit: 66639db858112bf6b0f76677f7517643d586e575
change-id: 20250805-errorfc-mount-too-revealing-5d9f670ba770
Best regards,
--
Aleksa Sarai <cyphar@cyphar.com>
^ permalink raw reply
* Re: [PATCH 2/2] vfs: output mount_too_revealing() errors to fscontext
From: Aleksa Sarai @ 2025-08-06 6:06 UTC (permalink / raw)
To: Al Viro
Cc: Christian Brauner, Jan Kara, David Howells, linux-api,
linux-kernel, linux-fsdevel
In-Reply-To: <20250806054116.GE222315@ZenIV>
[-- Attachment #1: Type: text/plain, Size: 597 bytes --]
On 2025-08-06, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Wed, Aug 06, 2025 at 02:48:30PM +1000, Aleksa Sarai wrote:
>
> > error = security_sb_kern_mount(sb);
> > - if (!error && mount_too_revealing(sb, &mnt_flags))
> > + if (!error && mount_too_revealing(sb, &mnt_flags)) {
> > error = -EPERM;
> > + errorfcp(fc, "VFS", "Mount too revealing");
> > + }
>
> Hmm... For aesthetics sake, I'd probably do logging first; otherwise
> fine by me.
Good point, I'll send a v2.
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]
^ permalink raw reply
page: next (older) | prev (newer) | latest
- recent:[subjects (threaded)|topics (new)|topics (active)]
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox