[PATCH] linux: Clear mode_t padding bits (BZ#25623)

[Adhemerval Zanella]

On 03/03/2020 18:51, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>> diff --git a/sysdeps/unix/sysv/linux/msgctl.c b/sysdeps/unix/sysv/linux/msgctl.c
>> index 27879e76cd..48801a3fba 100644
>> --- a/sysdeps/unix/sysv/linux/msgctl.c
>> +++ b/sysdeps/unix/sysv/linux/msgctl.c
> 
>> +#else
>> +	  /* Old Linux kernel versions might not clear the mode padding.  */
>> +	  if (sizeof ((struct msqid_ds){0}.msg_perm.mode
>> +	      != sizeof ((struct __old_ipc_perm){0}.mode)))
>> +	    buf->msg_perm.mode &= 0xFFFF;
>> +#endif
> 
> The condition always evaluates to true becaduse the first sizeof is
> applied to the result of the != operator.

Sigh, indeed (I should have verified if x86_64 resulting code would be
constant). Is is ok with the suppose fix (below)?

          if (sizeof ((struct msqid_ds){0}.msg_perm.mode)
              != sizeof ((struct __old_ipc_perm){0}.mode))
            buf->msg_perm.mode &= 0xFFFF;