[PATCH v5 0/7] Add O_DENY* support for VFS and CIFS/NFS

Pavel Shilovsky
2013-04-09T12:36:30Z

Main changes fom v4:
1) deny_lock_file uses FS_DOES_SHARELOCK flag from fs_flags to determine whether to use VFS locks or not.
2) Make nfs code return -EBUSY for share conflicts (was -EACCESS).

Main changes from v3
1) O_DENYMAND is removed, sharelock mount option is introduced.
2) Patch fcntl.h and VFS patches are united into one.
3) flock/LOCK_MAND is disabled for sharelock mounts.

This patchset adds support of O_DENY* flags for Linux fs layer. These flags can be used by any application that needs share reservations to organize a file access. VFS already has some sort of this capability - now it's done through flock/LOCK_MAND mechanis, but that approach is non-atomic. This patchset build new capabilities on top of the existing one but doesn't bring any changes into the flock call semantic.

These flags can be used by NFS (built-in-kernel) and CIFS (Samba) servers and Wine applications through VFS (for local filesystems) or CIFS/NFS modules. This will help when e.g. Samba and NFS server share the same directory for Windows and Linux users or Wine applications use Samba/NFS share to access the same data from different clients.

According to the previous discussions the most problematic question is how to prevent situations like DoS attacks where e.g /lib/liba.so file can be open with DENYREAD, or smth like this. That's why extra mount option 'sharelock' is added for switching on/off O_DENY* flags processing. It allows us to avoid use of these flags on critical places (like /, /lib) and turn them on if we really want it proccessed that way.

So, we have 3 new flags:
O_DENYREAD - to prevent other opens with read access,
O_DENYWRITE - to prevent other opens with write access,
O_DENYDELETE - to prevent delete operations (this flag is not implemented in VFS and NFS part and only suitable for CIFS module),

The patchset avoid data races problem on newely created files: open with O_CREAT can return the -ETXTBSY error for a successfully created file if this files was locked with a deny lock by another task. Also, it turns flock/LOCK_MAND capability off for mounts with 'sharelock' option. This let us not mix one share reservation approach with another.

The #1 patch adds flags to fcntl and implements VFS part. The patches #3, #4, #5 are related to CIFS-specific changes, #6 and #7 describe NFS and NFSD parts.

The preliminary patch for Samba that replaces the existing use of flock/LOCK_MAND mechanism with O_DENY* flags:
http://git.etersoft.ru/people/piastry/packages/?p=samba.git;a=commitdiff;h=173070262b7f29134ad6b2e49a760017c11aec4a

The future part of open man page patch:

-----
O_DENYREAD, O_DENYWRIYE, O_DENYDELETE - used to inforce a mandatory share reservation scheme of the file access. If these flag is passed, the open fails with -EBUSY in following cases:
1) if O_DENYREAD flag is specified and there is another open with READ access to the file;
2) if O_DENYWRITE flag is specified and there is another open with WRITE access to the file;
3) if READ access is requested and there is another open with O_DENYREAD flags;
4) if WRITE access is requested and there is another open with O_DENYWRITE flags.

If O_DENYDELETE flag is specified and the open succeded, any further unlink operation will fail with -EBUSY untill this open is closed. Now this flag is processed by CIFS filesystems only.

This mechanism is done through flocks. If O_DENY* flags are specified, flock syscall is processed after the open. The share modes are translated into flock according the following rules:
1) !O_DENYREAD -> LOCK_READ | LOCK_MAND
2) !O_DENYWRITE -> LOCK_WRITE | LOCK_MAND
-----

Pavel Shilovsky (7):
fcntl: Introduce new O_DENY* open flags
CIFS: Add share_access parm to open request
CIFS: Add O_DENY* open flags support
CIFS: Use NT_CREATE_ANDX command for forcemand mounts
NFSv4: Add O_DENY* open flags support
NFSD: Pass share reservations flags to VFS
locks: Disable LOCK_MAND support for MS_SHARELOCK mounts

fs/cifs/cifsacl.c | 8 +--
fs/cifs/cifsfs.c | 2 +-
fs/cifs/cifsglob.h | 18 ++++++-
fs/cifs/cifsproto.h | 8 +--
fs/cifs/cifssmb.c | 50 ++++++++++---------
fs/cifs/dir.c | 16 +++---
fs/cifs/file.c | 20 +++++---
fs/cifs/inode.c | 16 +++---
fs/cifs/link.c | 25 +++-------
fs/cifs/readdir.c | 5 +-
fs/cifs/smb1ops.c | 16 +++---
fs/cifs/smb2file.c | 10 ++--
fs/cifs/smb2inode.c | 4 +-
fs/cifs/smb2ops.c | 10 ++--
fs/cifs/smb2pdu.c | 6 +--
fs/cifs/smb2proto.h | 14 +++---
fs/fcntl.c | 5 +-
fs/locks.c | 105 ++++++++++++++++++++++++++++++++++++---
fs/namei.c | 45 +++++++++++++++--
fs/nfs/internal.h | 3 +-
fs/nfs/nfs4proc.c | 4 +-
fs/nfs/nfs4super.c | 9 ++--
fs/nfs/nfs4xdr.c | 21 ++++++--
fs/nfs/super.c | 7 ++-
fs/nfsd/nfs4state.c | 46 ++++++++++++++++-
fs/proc_namespace.c | 1 +
include/linux/fs.h | 8 +++
include/uapi/asm-generic/fcntl.h | 11 ++++
include/uapi/linux/fs.h | 1 +
29 files changed, 364 insertions(+), 130 deletions(-)

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1.8.1.5