Re: [RFC PATCH 1/2] mm: restrictedmem: Allow userspace to specify mount_path for memfd_restricted

[Ackerley Tng <ackerleytng@google.com>]

"Kirill A. Shutemov" <kirill@shutemov.name> writes:

> On Thu, Feb 16, 2023 at 12:41:16AM +0000, Ackerley Tng wrote:
>> By default, the backing shmem file for a restrictedmem fd is created
>> on shmem's kernel space mount.

>> With this patch, an optional tmpfs mount can be specified, which will
>> be used as the mountpoint for backing the shmem file associated with a
>> restrictedmem fd.

>> This change is modeled after how sys_open() can create an unnamed
>> temporary file in a given directory with O_TMPFILE.

>> This will help restrictedmem fds inherit the properties of the
>> provided tmpfs mounts, for example, hugepage allocation hints, NUMA
>> binding hints, etc.

>> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
>> ---
>>   include/linux/syscalls.h           |  2 +-
>>   include/uapi/linux/restrictedmem.h |  8 ++++
>>   mm/restrictedmem.c                 | 63 +++++++++++++++++++++++++++---
>>   3 files changed, 66 insertions(+), 7 deletions(-)
>>   create mode 100644 include/uapi/linux/restrictedmem.h

>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>> index f9e9e0c820c5..4b8efe9a8680 100644
>> --- a/include/linux/syscalls.h
>> +++ b/include/linux/syscalls.h
>> @@ -1056,7 +1056,7 @@ asmlinkage long sys_memfd_secret(unsigned int  
>> flags);
>>   asmlinkage long sys_set_mempolicy_home_node(unsigned long start,  
>> unsigned long len,
>>   					    unsigned long home_node,
>>   					    unsigned long flags);
>> -asmlinkage long sys_memfd_restricted(unsigned int flags);
>> +asmlinkage long sys_memfd_restricted(unsigned int flags, const char  
>> __user *mount_path);

>>   /*
>>    * Architecture-specific system calls

> I'm not sure what the right practice now: do we provide string that
> contains mount path or fd that represents the filesystem (returned from
> fsmount(2) or open_tree(2)).

> fd seems more flexible: it allows to specify unbind mounts.

I tried out the suggestion of passing fds to memfd_restricted() instead
of strings.

One benefit I see of using fds is interface uniformity: it feels more
aligned with other syscalls like fsopen(), fsconfig(), and fsmount() in
terms of using and passing around fds.

Other than being able to use a mount without a path attached to the
mount, are there any other benefits of using fds over using the path string?

Should I post the patches that allows specifying a mount using fds?
Should I post them as a separate RFC, or as a new revision to this RFC?