Re: [PATCH v2] tools: Fix use-after-free for realloc(..., 0)

[Justin Forbes <jmforbes@linuxtx.org>]

On Sun, Feb 13, 2022 at 10:24:43AM -0800, Kees Cook wrote:
> GCC 12 was correctly reporting a potential use-after-free condition in
> the xrealloc helper. Fix the warning by avoiding an implicit "free(ptr)"
> when size == 0:
> 
> In file included from help.c:12:
> In function 'xrealloc',
>     inlined from 'add_cmdname' at help.c:24:2: subcmd-util.h:56:23: error: pointer may be used after 'realloc' [-Werror=use-after-free]
>    56 |                 ret = realloc(ptr, size);
>       |                       ^~~~~~~~~~~~~~~~~~
> subcmd-util.h:52:21: note: call to 'realloc' here
>    52 |         void *ret = realloc(ptr, size);
>       |                     ^~~~~~~~~~~~~~~~~~
> subcmd-util.h:58:31: error: pointer may be used after 'realloc' [-Werror=use-after-free]
>    58 |                         ret = realloc(ptr, 1);
>       |                               ^~~~~~~~~~~~~~~
> subcmd-util.h:52:21: note: call to 'realloc' here
>    52 |         void *ret = realloc(ptr, size);
>       |                     ^~~~~~~~~~~~~~~~~~
> 
> Reported-by: "Valdis Klētnieks" <valdis.kletnieks@vt.edu>
> Fixes: 2f4ce5ec1d44 ("perf tools: Finalize subcmd independence")
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Tested this patch with the Fedora rawhide builds, and this does indeed
fix the issue.

Tested-by: Justin M. Forbes <jforbes@fedoraproject.org>