[LTP] [PATCH] fcntl.2: F_OFD_XXX needs flock64

[Michael Kerrisk (man-pages)]

[Jeff, can you comment?]

Hi Cyril,

On 08/16/2016 11:55 PM, Cyril Hrubis wrote:
> If we pass struct flock to the F_OFD_XXX fcntl() it will fail with
> EINVAL with a 32bit binary. That is because glibc uses fcntl64() by
> default but the struct flock uses 32bit off_t for 32bit binaries (unless
> _FILE_OFFSET_BITS=64) and kernel always expect flock64 for F_OFD_XXX in
> fcntl64(). Hence kernel will read some garbage that is a few bytes after
> the 32bit flock structure in this case which will likely end up with the
> syscall returning EINVAL.

Okay -- I confirm the problem you report. I'm just not sure that the
patch below is the best fix. So, to summarize:

* On 64-bit, flock{} and flock64{} are the same structure.
* On 32-bit, flock{} and flock64{} are different.
* On 32-bit, F_OFD operations require flock64{}, but the traditional
  F_* lock operations do not.
* To use flock64{} with F_OFD operations, we can either explicitly use
  flock64{} or we can compile with -D_FILE_OFFSET_BITS=64

One solution would be your patch below, but it feels wrong: on 64-bit
flock{} suffices, and is consistent with the traditional F_* operations.
An alternative would be a note in the man page that says something along
the lines that on 32-bit, one must compile with -D_FILE_OFFSET_BITS=64
when using the F_OFD operations.

Your thoughts?

Cheers,

Michael

> Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
> CC: Yuriy Kolerov <Yuriy.Kolerov@synopsys.com>
> ---
>  man2/fcntl.2 | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/man2/fcntl.2 b/man2/fcntl.2
> index f0c1acf..4606709 100644
> --- a/man2/fcntl.2
> +++ b/man2/fcntl.2
> @@ -533,7 +533,7 @@ As with traditional advisory locks, the third argument to
>  .BR fcntl (),
>  .IR lock ,
>  is a pointer to an
> -.IR flock
> +.IR flock64
>  structure.
>  By contrast with traditional record locks, the
>  .I l_pid
> @@ -543,7 +543,7 @@ when using the commands described below.
>  The commands for working with open file description locks are analogous
>  to those used with traditional locks:
>  .TP
> -.BR F_OFD_SETLK " (\fIstruct flock *\fP)"
> +.BR F_OFD_SETLK " (\fIstruct flock64 *\fP)"
>  Acquire an open file description lock (when
>  .I l_type
>  is
> @@ -564,7 +564,7 @@ this call returns \-1 and sets
>  to
>  .BR EAGAIN .
>  .TP
> -.BR F_OFD_SETLKW " (\fIstruct flock *\fP)"
> +.BR F_OFD_SETLKW " (\fIstruct flock64 *\fP)"
>  As for
>  .BR F_OFD_SETLK ,
>  but if a conflicting lock is held on the file, then wait for that lock to be
> @@ -578,7 +578,7 @@ set to
>  see
>  .BR signal (7)).
>  .TP
> -.BR F_OFD_GETLK " (\fIstruct flock *\fP)"
> +.BR F_OFD_GETLK " (\fIstruct flock64 *\fP)"
>  On input to this call,
>  .I lock
>  describes an open file description lock we would like to place on the file.
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/