Building (and using) Glibc with GCC 11 triggers warnings like the one below for calls to setjmp() and sigsetjmp():

In file included from ../include/setjmp.h:2,
                 from ../nptl/descr.h:24,
                 from ../sysdeps/x86_64/nptl/tls.h:130,
                 from ../include/link.h:51,
                 from ../include/dlfcn.h:4,
                 from ../sysdeps/generic/ldsodefs.h:32,
                 from ../sysdeps/x86/ldsodefs.h:65,
                 from ../sysdeps/gnu/ldsodefs.h:46,
                 from ../sysdeps/unix/sysv/linux/ldsodefs.h:25,
                 from ../sysdeps/x86/libc-start.c:22:
../csu/libc-start.c: In function ‘__libc_start_main’:
../setjmp/setjmp.h:49:25: warning: ‘_setjmp’ accessing 200 bytes in a region of size 72 [-Wstringop-overflow=]
   49 | #define setjmp(env)     _setjmp (env)
      |                         ^~~~~~~~~~~~~
../csu/libc-start.c:301:20: note: in expansion of macro ‘setjmp’
  301 |   not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
      |                    ^~~~~~
../setjmp/setjmp.h:49:25: note: referencing argument 1 of type ‘struct __jmp_buf_tag *’
   49 | #define setjmp(env)     _setjmp (env)
      |                         ^~~~~~~~~~~~~
../csu/libc-start.c:301:20: note: in expansion of macro ‘setjmp’
  301 |   not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
      |                    ^~~~~~
In file included from <command-line>:
../include/setjmp.h:27:20: note: in a call to function ‘_setjmp’
   27 | libc_hidden_proto (_setjmp)
      |                    ^~~~~~~
./../include/libc-symbols.h:605:33: note: in definition of macro ‘__hidden_proto’
  605 |   extern thread __typeof (name) name __hidden_proto_hiddenattr (attrs);
      |                                 ^~~~
./../include/libc-symbols.h:624:44: note: in expansion of macro ‘hidden_proto’
  624 | # define libc_hidden_proto(name, attrs...) hidden_proto (name, ##attrs)
      |                                            ^~~~~~~~~~~~
../include/setjmp.h:27:1: note: in expansion of macro ‘libc_hidden_proto’
   27 | libc_hidden_proto (_setjmp)
      | ^~~~~~~~~~~~~~~~~


The problem seems to be caused by the following declarations:

typedef long int __jmp_buf[8];   // 64 bytes

typedef struct
{
  struct
  {
    __jmp_buf __cancel_jmp_buf;   // 64 bytes
    int __mask_was_saved;         // 4 bytes
  } __cancel_jmp_buf[1];          // 72 bytes (68 + 4 padding)
  void *__pad[4];
} __pthread_unwind_buf_t __attribute__ ((__aligned__));

typedef struct
{
  unsigned long int __val[(1024 / (8 * sizeof (unsigned long int)))];
} __sigset_t;                     // 128 bytes

struct __jmp_buf_tag              // 200 bytes (196 + 4 padding)
  {
    __jmp_buf __jmpbuf;           // 64 bytes
    int __mask_was_saved;         // 4 bytes
    __sigset_t __saved_mask;      // 128 bytes
  };

extern int _setjmp (struct __jmp_buf_tag __env[1]) __attribute__ ((__nothrow__));

and calls like the one below:

   ... _setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);

_setjmp() is declared to take an array of at least one __jmp_buf_tag element, and unwind_buf.cancel_jmp_buf is an array of a single element of the unnamed struct in __pthread_unwind_buf_t.  __jmp_buf_tag is a much larger type than the unnamed struct, so the warning is justified.

I'm sure the code must be safe so perhaps the way to avoid the warning is to change the declaration of setjmp() to take a pointer to long instead.