[Pkg-samba-maint] Bug#483507: mount.cifs: brl option does not produce documented semantics for fcntl(F_SETLK)

[Daniel Kahn Gillmor]

Package: smbfs
Version: 1:3.0.28a-3
Severity: normal

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In http://bugs.debian.org/483502, i wrote:

> locking a certain range of a file with a read lock and then locking
> the same range with a write lock gives an error when a CIFS share is
> mounted with "brl".  This doesn't make much sense to me, but i'm
> willing to accept it as intended for now.

After doing more reading, it still doesn't make sense: the filesystem
is behaving with non-POSIX semantics, which causes breakage in other
packages (for example, see http://bugs.debian.org/483216).

In particular, the semantics specified by fcntl(2) for setting a lock
state:

      A single process can hold only one type of lock on a file
      region; if a new lock is applied to an already-locked region,
      then the existing lock is converted to the new lock type.

(this appears to be true for both advisory *and* mandatory locks)

However, when accessing a CIFS share mounted with with "brl" (standard
CIFS-style server-side byte-range locking), converting a read lock to
a write lock gives a "permission denied" error.

The same error does *not* occur when using the CIFS share mounted with
"nobrl", or when attempting the same series of operations against a
tmpfs (whether or not the tmpfs is mounted with -o mand).  Shouldn't
mount.cifs do its best to mimic the expected semantics of
fcntl(F_SETLK) using the underlying CIFS semantics?  If CIFS has no
explicit lock upgrade/downgrade operations, as long as the protocol is
capable of dropping a lock and acquiring a new one, these two
primitives used in sequence could be used to emulate a lock
upgrade/downgrade.

Attached is the test program that displays this behavior.  It should
fail with "write lock failed: Permission denied" when used on a file
on a CIFS mount, and it has succeeded quietly on every other
filesystem i've tried.  the latest version of the source can be found
at
http://cmrg.fifthhorseman.net/browser/trunk/test/testlocking/testlocking.c

Thanks for maintaining samba in debian!

Regards,

	--dkg

- -- System Information:
Debian Release: lenny/sid
  APT prefers testing
  APT policy: (500, 'testing'), (200, 'unstable'), (101, 'experimental')
Architecture: i386 (i686)

Kernel: Linux 2.6.24-1-686 (SMP w/1 CPU core)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages smbfs depends on:
ii  libc6                        2.7-10      GNU C Library: Shared libraries
ii  netbase                      4.32        Basic TCP/IP networking system
ii  samba-common                 1:3.0.28a-3 Samba common files used by both th

smbfs recommends no packages.

- -- no debconf information

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