libcasper: Neuter false positive -Wuse-after-free warnings from GCC 13

[jhb]

Summary

GCC 13 incorrectly thinks a call to free after a failed realloc is a
use after free.

lib/libcasper/services/cap_grp/cap_grp.c: In function 'group_resize':
lib/libcasper/services/cap_grp/cap_grp.c:65:17: error: pointer 'buf' may be used after 'realloc' [-Werror=use-after-free]

65 |                 free(buf);
   |                 ^~~~~~~~~

lib/libcasper/services/cap_grp/cap_grp.c:63:19: note: call to 'realloc' here

63 |         gbuffer = realloc(buf, gbufsize);
   |                   ^~~~~~~~~~~~~~~~~~~~~~

[imp]

I thought this is becoming UB behavior...
But for our implementation it's well defined.

[jhb]

In D42576#971550, @imp wrote:

I thought this is becoming UB behavior...
But for our implementation it's well defined.

If realloc fails and returns NULL, the original pointer is always valid, that's how realloc is defined in C. There is some discussion about making realloc() with a new size of 0 as UB to permit varying implementations, but that isn't relevant here.

[imp]

In D42576#971578, @jhb wrote:

In D42576#971550, @imp wrote:

I thought this is becoming UB behavior...
But for our implementation it's well defined.

If realloc fails and returns NULL, the original pointer is always valid, that's how realloc is defined in C. There is some discussion about making realloc() with a new size of 0 as UB to permit varying implementations, but that isn't relevant here.

realloc(p, 0) is slated to be undefined behavior in C23. You are correct... I thought wrong. This all looks good.