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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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