Re: [RFC][PATCH 0/9] VFS: Introduce mount context

[Jeff Layton <jlayton@poochiereds.net>]

On Wed, 2017-05-03 at 17:04 +0100, David Howells wrote:
> Here are a set of patches to create a mount context prior to setting up a
> new mount, populating it with the parsed options/binary data and then
> effecting the mount.
> 
> This allows namespaces and other information to be conveyed through the
> mount procedure.  It also allows extra error information to be returned
> (so many things can go wrong during a mount that a small integer isn't
> really sufficient to convey the issue).
> 
> This also allows Miklós Szeredi's idea of doing:
> 
> 	fd = fsopen("nfs");
> 	write(fd, "option=val", ...);
> 	fsmount(fd, "/mnt");
> 
> that he presented at LSF-2017 to be implemented (see the relevant patches
> in the series), to which I can add:
> 
> 	read(fd, error_buffer, ...);
> 
> to read back any error message.  I didn't use netlink as that would make it
> depend on CONFIG_NET and would introduce network namespacing issues.
> 

Nice work!

> I've implemented mount context handling for procfs and nfs.
> 
> Further developments:
> 
>  (*) Implement mount context support in more filesystems, ext4 being next
>      on my list.
> 
>  (*) Move the walk-from-root stuff that nfs has to generic code so that you
>      can do something akin to:
> 
> 	mount /dev/sda1:/foo/bar /mnt
> 
>      See nfs_follow_remote_path() and mount_subtree().  This is slightly
>      tricky in NFS as we have to prevent referral loops.
> 

':' is a legitimate character in a path component. How will you
distinguish that case?

>  (*) Move the pid_ns pointer from struct mount_context to struct
>      proc_mount_context as I'm not sure it's necessary for anything other
>      than procfs.
> 


>  (*) Work out how to get at the error message incurred by submounts
>      encountered during nfs_follow_remote_path().
> 
>      Should the error message be moved to task_struct and made more
>      general, perhaps retrieved with a prctl() function?
> 

Now that's an interesting idea.

>  (*) Clean up/consolidate the security functions.  Possibly add a
>      validation hook to be called at the same time as the mount context
>      validate op.
> 
> The patches can be found here also:
> 
> 	http://git.kernel.org/cgit/linux/kernel/git/dhowells/linux-fs.git/log/?h=mount-context
> 
> David
> ---
> David Howells (9):
>       Provide a function to create a NUL-terminated string from unterminated data
>       Clean up whitespace in fs/namespace.c
>       VFS: Introduce a mount context
>       Implement fsopen() to prepare for a mount
>       Implement fsmount() to effect a pre-configured mount
>       Sample program for driving fsopen/fsmount
>       procfs: Move proc_fill_super() to fs/proc/root.c
>       proc: Support the mount context in procfs
>       NFS: Support the mount context and fsopen()
> 
> 
>  Documentation/filesystems/mounting.txt |  445 ++++++++
>  arch/x86/entry/syscalls/syscall_32.tbl |    2 
>  arch/x86/entry/syscalls/syscall_64.tbl |    2 
>  fs/Makefile                            |    3 
>  fs/fsopen.c                            |  295 +++++
>  fs/internal.h                          |    2 
>  fs/mount.h                             |    3 
>  fs/mount_context.c                     |  343 ++++++
>  fs/namespace.c                         |  367 ++++++-
>  fs/nfs/Makefile                        |    2 
>  fs/nfs/client.c                        |   18 
>  fs/nfs/internal.h                      |  127 +-
>  fs/nfs/mount.c                         | 1539 ++++++++++++++++++++++++++++
>  fs/nfs/namespace.c                     |   75 +
>  fs/nfs/nfs3_fs.h                       |    2 
>  fs/nfs/nfs3client.c                    |    6 
>  fs/nfs/nfs3proc.c                      |    1 
>  fs/nfs/nfs4_fs.h                       |    4 
>  fs/nfs/nfs4client.c                    |   80 +
>  fs/nfs/nfs4namespace.c                 |  207 ++--
>  fs/nfs/nfs4proc.c                      |    1 
>  fs/nfs/nfs4super.c                     |  184 ++-
>  fs/nfs/proc.c                          |    1 
>  fs/nfs/super.c                         | 1729 ++------------------------------
>  fs/proc/inode.c                        |   50 -
>  fs/proc/internal.h                     |    6 
>  fs/proc/root.c                         |  194 +++-
>  fs/super.c                             |   50 +
>  include/linux/fs.h                     |   11 
>  include/linux/lsm_hooks.h              |   43 +
>  include/linux/mount.h                  |   67 +
>  include/linux/nfs_xdr.h                |    7 
>  include/linux/security.h               |   35 +
>  include/linux/string.h                 |    1 
>  include/linux/syscalls.h               |    2 
>  include/uapi/linux/magic.h             |    1 
>  kernel/sys_ni.c                        |    4 
>  mm/util.c                              |   22 
>  samples/fsmount/test-fsmount.c         |   79 +
>  security/security.c                    |   39 +
>  security/selinux/hooks.c               |  192 ++++
>  41 files changed, 4148 insertions(+), 2093 deletions(-)
>  create mode 100644 Documentation/filesystems/mounting.txt
>  create mode 100644 fs/fsopen.c
>  create mode 100644 fs/mount_context.c
>  create mode 100644 fs/nfs/mount.c
>  create mode 100644 samples/fsmount/test-fsmount.c
> 

-- 
Jeff Layton <jlayton@poochiereds.net>