Avoid -Wstringop-overflow warning in pthread_cleanup_push macros

GCC 11 introduces a -Wstringop-overflow warning for calls to functions
with an array argument passed as a pointer to memory not large enough
for that array.  This includes the __sigsetjmp calls from
pthread_cleanup_push macros, because those use a structure in
__pthread_unwind_buf_t, which has a common initial subsequence with
jmp_buf but does not include the saved signal mask; this is OK in this
case because the second argument to __sigsetjmp is 0 so the signal
mask is not accessed.

To avoid this warning, use a function alias __sigsetjmp_cancel with
first argument an array of exactly the type used in the calls to the
function, if using GCC 11 or later.  With older compilers, continue to
use __sigsetjmp with a cast, to avoid any issues with compilers
predating the returns_twice attribute not applying the same special
handling to __sigsetjmp_cancel as to __sigsetjmp.

Tested with build-many-glibcs.py for arm-linux-gnueabi that this fixes
the testsuite build failures.

misc/sys/cdefs.h

@@ -563,4 +563,12 @@ _Static_assert (0, "IEEE 128-bits long double requires redirection on this platf
 #  define __attr_access(x)
 #endif
 
+/* Specify that a function such as setjmp or vfork may return
+   twice.  */
+#if __GNUC_PREREQ (4, 1)
+# define __attribute_returns_twice__ __attribute__ ((__returns_twice__))
+#else
+# define __attribute_returns_twice__ /* Ignore.  */
+#endif
+
 #endif	 /* sys/cdefs.h */

sysdeps/nptl/pthread.h

@@ -512,13 +512,15 @@ extern void pthread_testcancel (void);
 
 /* Cancellation handling with integration into exception handling.  */
 
+struct __cancel_jmp_buf_tag
+{
+  __jmp_buf __cancel_jmp_buf;
+  int __mask_was_saved;
+};
+
 typedef struct
 {
-  struct
-  {
-    __jmp_buf __cancel_jmp_buf;
-    int __mask_was_saved;
-  } __cancel_jmp_buf[1];
+  struct __cancel_jmp_buf_tag __cancel_jmp_buf[1];
   void *__pad[4];
 } __pthread_unwind_buf_t __attribute__ ((__aligned__));
 
@@ -658,8 +660,8 @@ __pthread_cleanup_routine (struct __pthread_cleanup_frame *__frame)
     __pthread_unwind_buf_t __cancel_buf;				      \
     void (*__cancel_routine) (void *) = (routine);			      \
     void *__cancel_arg = (arg);						      \
-    int __not_first_call = __sigsetjmp ((struct __jmp_buf_tag *) (void *)     \
-					__cancel_buf.__cancel_jmp_buf, 0);    \
+    int __not_first_call = __sigsetjmp_cancel (__cancel_buf.__cancel_jmp_buf, \
+					       0);			      \
     if (__glibc_unlikely (__not_first_call))				      \
       {									      \
 	__cancel_routine (__cancel_arg);				      \
@@ -693,8 +695,8 @@ extern void __pthread_unregister_cancel (__pthread_unwind_buf_t *__buf)
     __pthread_unwind_buf_t __cancel_buf;				      \
     void (*__cancel_routine) (void *) = (routine);			      \
     void *__cancel_arg = (arg);						      \
-    int __not_first_call = __sigsetjmp ((struct __jmp_buf_tag *) (void *)     \
-					__cancel_buf.__cancel_jmp_buf, 0);    \
+    int __not_first_call = __sigsetjmp_cancel (__cancel_buf.__cancel_jmp_buf, \
+					       0);			      \
     if (__glibc_unlikely (__not_first_call))				      \
       {									      \
 	__cancel_routine (__cancel_arg);				      \
@@ -730,9 +732,24 @@ extern void __pthread_unwind_next (__pthread_unwind_buf_t *__buf)
      ;
 #endif
 
-/* Function used in the macros.  */
+/* Function used in the macros.  Calling __sigsetjmp, with its first
+   argument declared as an array, results in a -Wstringop-overflow
+   warning from GCC 11 because struct pthread_unwind_buf is smaller
+   than jmp_buf.  The calls from the macros have __SAVEMASK set to 0,
+   so nothing beyond the common prefix is used and this warning is a
+   false positive.  Use an alias with its first argument declared to
+   use the type in the macros if possible to avoid this warning.  */
+#if __GNUC_PREREQ (11, 0)
+extern int __REDIRECT_NTHNL (__sigsetjmp_cancel,
+			     (struct __cancel_jmp_buf_tag __env[1],
+			      int __savemask),
+			     __sigsetjmp) __attribute_returns_twice__;
+#else
+# define __sigsetjmp_cancel(env, savemask) \
+  __sigsetjmp ((struct __jmp_buf_tag *) (void *) (env), (savemask))
 extern int __sigsetjmp (struct __jmp_buf_tag __env[1],
 			int __savemask) __THROWNL;
+#endif
 
 
 /* Mutex handling.  */