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    Subject[RFC][PATCH 00/14] VFS: Introduce superblock configuration context
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    Here are a set of patches to create a superblock configuration context
    prior to setting up a new mount, populating it with the parsed
    options/binary data, creating the superblock 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.

    I've implemented mount context handling for procfs and nfs.

    Significant changes:

    (*) Renamed struct mount_context to struct sb_config and amended various
    variable names.

    (*) Split the sb_config stuff out into its own header.

    (*) Support non-context aware filesystems through a special set of
    sb_config operations.

    (*) Stored the created superblock and root dentry into the sb_config after
    creation rather than directly into a vfsmount. This allows some
    arguments to be removed to various NFS functions.

    (*) Added an explicit superblock-creation step. This allows a created
    superblock to then be mounted multiple times.

    (*) Added a flag to say that the sb_config is degraded and cannot have
    another go at having a superblock creation whilst getting rid of the
    one that says it's already mounted.

    Further developments:

    (*) fsmount() needs to take AT_EMPTY_PATH and similar.

    (*) Implement sb reconfiguration (for now it returns ENOANO).

    (*) 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.

    (*) 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?

    (*) 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 (14):
    Provide a function to create a NUL-terminated string from unterminated data
    Clean up whitespace in fs/namespace.c
    VFS: Make get_mnt_ns() return the namespace
    VFS: Make get_filesystem() return the affected filesystem
    VFS: Provide empty name qstr
    VFS: Introduce a superblock configuration 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: Add superblock config support to procfs
    NFS: Add mount context support.
    Support legacy filesystems
    Add commands to create or update a superblock


    Documentation/filesystems/mounting.txt | 463 ++++++++
    arch/x86/entry/syscalls/syscall_32.tbl | 2
    arch/x86/entry/syscalls/syscall_64.tbl | 2
    fs/Makefile | 3
    fs/dcache.c | 8
    fs/filesystems.c | 3
    fs/fsopen.c | 302 +++++
    fs/gfs2/dir.c | 3
    fs/internal.h | 4
    fs/libfs.c | 17
    fs/mount.h | 3
    fs/namei.c | 3
    fs/namespace.c | 495 +++++++--
    fs/nfs/Makefile | 2
    fs/nfs/client.c | 74 +
    fs/nfs/getroot.c | 75 +
    fs/nfs/internal.h | 142 +--
    fs/nfs/mount.c | 1500 +++++++++++++++++++++++++++
    fs/nfs/namespace.c | 76 +
    fs/nfs/nfs3_fs.h | 2
    fs/nfs/nfs3client.c | 6
    fs/nfs/nfs3proc.c | 2
    fs/nfs/nfs4_fs.h | 4
    fs/nfs/nfs4client.c | 82 +
    fs/nfs/nfs4namespace.c | 208 ++--
    fs/nfs/nfs4proc.c | 3
    fs/nfs/nfs4super.c | 223 ++--
    fs/nfs/proc.c | 2
    fs/nfs/super.c | 1791 ++------------------------------
    fs/nsfs.c | 3
    fs/pipe.c | 3
    fs/proc/inode.c | 50 -
    fs/proc/internal.h | 6
    fs/proc/root.c | 197 +++-
    fs/sb_config.c | 532 ++++++++++
    fs/super.c | 109 +-
    include/linux/dcache.h | 5
    include/linux/fs.h | 13
    include/linux/lsm_hooks.h | 47 +
    include/linux/mount.h | 4
    include/linux/nfs_xdr.h | 7
    include/linux/sb_config.h | 104 ++
    include/linux/security.h | 40 +
    include/linux/string.h | 1
    include/linux/syscalls.h | 2
    include/uapi/linux/magic.h | 1
    kernel/sys_ni.c | 4
    mm/util.c | 24
    samples/fsmount/test-fsmount.c | 78 +
    security/security.c | 45 +
    security/selinux/hooks.c | 202 +++-
    51 files changed, 4592 insertions(+), 2385 deletions(-)
    create mode 100644 Documentation/filesystems/mounting.txt
    create mode 100644 fs/fsopen.c
    create mode 100644 fs/nfs/mount.c
    create mode 100644 fs/sb_config.c
    create mode 100644 include/linux/sb_config.h
    create mode 100644 samples/fsmount/test-fsmount.c

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