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

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.

1 files changed, 41 insertions, 11 deletions

diff --git a/sysdeps/unix/sysv/linux/kernel-posix-timers.h b/sysdeps/unix/sysv/linux/kernel-posix-timers.h
index 4f8d97d3de..6d22b2b0b3 100644
--- a/sysdeps/unix/sysv/linux/kernel-posix-timers.h
+++ b/sysdeps/unix/sysv/linux/kernel-posix-timers.h
@@ -43,21 +43,11 @@ extern pthread_mutex_t __active_timer_sigev_thread_lock attribute_hidden;
 /* Type of timers in the kernel.  */
 typedef int kernel_timer_t;
 
-
-/* Internal representation of timer.  */
+/* Internal representation of SIGEV_THREAD timer.  */
 struct timer
 {
-  /* Notification mechanism.  */
-  int sigev_notify;
-
-  /* Timer ID returned by the kernel.  */
   kernel_timer_t ktimerid;
 
-  /* All new elements must be added after ktimerid.  And if the thrfunc
-     element is not the third element anymore the memory allocation in
-     timer_create needs to be changed.  */
-
-  /* Parameters for the thread to be started for SIGEV_THREAD.  */
   void (*thrfunc) (sigval_t);
   sigval_t sival;
   pthread_attr_t attr;
@@ -65,3 +55,43 @@ struct timer
   /* Next element in list of active SIGEV_THREAD timers.  */
   struct timer *next;
 };
+
+
+/* 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 the MSB bit of the returned
+   'timer_t' to indicate the timer handles a SIGEV_THREAD.  */
+
+static inline timer_t
+kernel_timer_to_timerid (kernel_timer_t ktimerid)
+{
+  return (timer_t) ((intptr_t) ktimerid);
+}
+
+static inline timer_t
+timer_to_timerid (struct timer *ptr)
+{
+  return (timer_t) (INTPTR_MIN | (uintptr_t) ptr >> 1);
+}
+
+static inline bool
+timer_is_sigev_thread (timer_t timerid)
+{
+  return (intptr_t) timerid < 0;
+}
+
+static inline struct timer *
+timerid_to_timer (timer_t timerid)
+{
+  return (struct timer *)((uintptr_t) timerid << 1);
+}
+
+static inline kernel_timer_t
+timerid_to_kernel_timer (timer_t timerid)
+{
+  if (timer_is_sigev_thread (timerid))
+    return timerid_to_timer (timerid)->ktimerid;
+  else
+    return (kernel_timer_t) ((uintptr_t) timerid);
+}