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posix: Fix -Warray-bounds instances building timer_create [BZ #26687]

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GCC 11 -Warray-bounds triggers invalid warnings when building
Linux timer_create.c:

../sysdeps/unix/sysv/linux/timer_create.c: In function '__timer_create_new':
../sysdeps/unix/sysv/linux/timer_create.c:83:17: warning: array subscript 'struct timer[0]' is partly outside array bounds of 'unsigned char[8]' [-Warray-bounds]
   83 |             newp->sigev_notify = (evp != NULL
      |                 ^~
../sysdeps/unix/sysv/linux/timer_create.c:59:47: note: referencing an object of size 8 allocated by 'malloc'
   59 |         struct timer *newp = (struct timer *) malloc (offsetof (struct timer,
      |                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   60 |                                                                 thrfunc));
      |                                                                 ~~~~~~~~~

The struct allocated for !SIGEV_THREAD timers only requires two 'int'
fields (sigev_notify and ktimerid) and the offsetof trick tries minimize
the memory usage by only allocation the required size.  However,
although the resulting size is suffice for !SIGEV_THREAD time, accessing
the partially allocated object is error-prone and UB.

This patch fixes both issues by embedding the information whether
the timer if a SIGEV_THREAD in the returned 'timer_t'.  For
!SIGEV_THREAD, the resulting 'timer_t' is the returned kernel timer
identifer (kernel_timer_t), while for SIGEV_THREAD it uses the fact
malloc returns at least _Alignof (max_align_t) pointers plus that
valid kernel_timer_t are always positive to set MSB bit of the returned
'timer_t' to indicate the timer handles a SIGEV_THREAD.

It allows to remove the memory allocation for !SIGEV_THREAD and also
remove the 'sigev_notify' field from 'struct timer'.

Checked on x86_64-linux-gnu and i686-linux-gnu.
This commit is contained in:
Adhemerval Zanella
2020-10-05 17:30:05 -03:00
parent 862897d2ad
commit 7a887dd537
8 changed files with 83 additions and 89 deletions
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8
sysdeps/unix/sysv/linux/timer_getoverr.c
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@ -31,10 +31,6 @@ int
timer_getoverrun (timer_t timerid)
{
#undef timer_getoverrun
struct timer *kt = (struct timer *) timerid;
/* Get the information from the kernel. */
int res = INLINE_SYSCALL (timer_getoverrun, 1, kt->ktimerid);
return res;
kernel_timer_t ktimerid = timerid_to_kernel_timer (timerid);
return INLINE_SYSCALL_CALL (timer_getoverrun, ktimerid);
}