sysdeps: sem_open: Clear O_CREAT when semaphore file is expected to exist [BZ #30789]

  When invoking sem_open with O_CREAT as one of its flags, we'll end up
in the second part of sem_open's "if ((oflag & O_CREAT) == 0 || (oflag
& O_EXCL) == 0)", which means that we don't expect the semaphore file
to exist.

In that part, open_flags is initialized as "O_RDWR | O_CREAT | O_EXCL
| O_CLOEXEC" and there's an attempt to open(2) the file, which will
likely fail because it won't exist.  After that first (expected)
failure, some cleanup is done and we go back to the label "try_again",
which lives in the first part of the aforementioned "if".

The problem is that, in that part of the code, we expect the semaphore
file to exist, and as such O_CREAT (this time the flag we pass to
open(2)) needs to be cleaned from open_flags, otherwise we'll see
another failure (this time unexpected) when trying to open the file,
which will lead the call to sem_open to fail as well.

This can cause very strange bugs, especially with OpenMPI, which makes
extensive use of semaphores.

Fix the bug by simplifying the logic when choosing open(2) flags and
making sure O_CREAT is not set when the semaphore file is expected to
exist.

A regression test for this issue would require a complex and cpu time
consuming logic, since to trigger the wrong code path is not
straightforward due the racy condition.  There is a somewhat reliable
reproducer in the bug, but it requires using OpenMPI.

This resolves BZ #30789.

See also: https://bugs.launchpad.net/ubuntu/+source/h5py/+bug/2031912

Signed-off-by: Sergio Durigan Junior <sergiodj@sergiodj.net>
Co-Authored-By: Simon Chopin <simon.chopin@canonical.com>
Co-Authored-By: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Fixes: 533deafbdf189f5fbb280c28562dd43ace2f4b0f ("Use O_CLOEXEC in more places (BZ #15722)")
---
 sysdeps/pthread/sem_open.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)
  

On 01/11/23 19:15, Sergio Durigan Junior wrote:
> When invoking sem_open with O_CREAT as one of its flags, we'll end up
> in the second part of sem_open's "if ((oflag & O_CREAT) == 0 || (oflag
> & O_EXCL) == 0)", which means that we don't expect the semaphore file
> to exist.
> 
> In that part, open_flags is initialized as "O_RDWR | O_CREAT | O_EXCL
> | O_CLOEXEC" and there's an attempt to open(2) the file, which will
> likely fail because it won't exist.  After that first (expected)
> failure, some cleanup is done and we go back to the label "try_again",
> which lives in the first part of the aforementioned "if".
> 
> The problem is that, in that part of the code, we expect the semaphore
> file to exist, and as such O_CREAT (this time the flag we pass to
> open(2)) needs to be cleaned from open_flags, otherwise we'll see
> another failure (this time unexpected) when trying to open the file,
> which will lead the call to sem_open to fail as well.
> 
> This can cause very strange bugs, especially with OpenMPI, which makes
> extensive use of semaphores.
> 
> Fix the bug by simplifying the logic when choosing open(2) flags and
> making sure O_CREAT is not set when the semaphore file is expected to
> exist.
> 
> A regression test for this issue would require a complex and cpu time
> consuming logic, since to trigger the wrong code path is not
> straightforward due the racy condition.  There is a somewhat reliable
> reproducer in the bug, but it requires using OpenMPI.
> 
> This resolves BZ #30789.
> 
> See also: https://bugs.launchpad.net/ubuntu/+source/h5py/+bug/2031912
> 
> Signed-off-by: Sergio Durigan Junior <sergiodj@sergiodj.net>
> Co-Authored-By: Simon Chopin <simon.chopin@canonical.com>
> Co-Authored-By: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
> Fixes: 533deafbdf189f5fbb280c28562dd43ace2f4b0f ("Use O_CLOEXEC in more places (BZ #15722)")

LGTM, thanks.  I will install this shortly.

> ---
>  sysdeps/pthread/sem_open.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/sysdeps/pthread/sem_open.c b/sysdeps/pthread/sem_open.c
> index e5db929d20..0e331a7445 100644
> --- a/sysdeps/pthread/sem_open.c
> +++ b/sysdeps/pthread/sem_open.c
> @@ -32,11 +32,12 @@
>  # define __unlink unlink
>  #endif
>  
> +#define SEM_OPEN_FLAGS (O_RDWR | O_NOFOLLOW | O_CLOEXEC)
> +
>  sem_t *
>  __sem_open (const char *name, int oflag, ...)
>  {
>    int fd;
> -  int open_flags;
>    sem_t *result;
>  
>    /* Check that shared futexes are supported.  */
> @@ -65,10 +66,8 @@ __sem_open (const char *name, int oflag, ...)
>    /* If the semaphore object has to exist simply open it.  */
>    if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
>      {
> -      open_flags = O_RDWR | O_NOFOLLOW | O_CLOEXEC;
> -      open_flags |= (oflag & ~(O_CREAT|O_ACCMODE));
>      try_again:
> -      fd = __open (dirname.name, open_flags);
> +      fd = __open (dirname.name, (oflag & O_EXCL) | SEM_OPEN_FLAGS);
>  
>        if (fd == -1)
>  	{
> @@ -135,8 +134,7 @@ __sem_open (const char *name, int oflag, ...)
>  	    }
>  
>  	  /* Open the file.  Make sure we do not overwrite anything.  */
> -	  open_flags = O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC;
> -	  fd = __open (tmpfname, open_flags, mode);
> +	  fd = __open (tmpfname, O_CREAT | O_EXCL | SEM_OPEN_FLAGS, mode);
>  	  if (fd == -1)
>  	    {
>  	      if (errno == EEXIST)