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

[Kees Cook]

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@xxxxxx>
Fixes: 2f4ce5ec1d44 ("perf tools: Finalize subcmd independence")
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 tools/lib/subcmd/subcmd-util.h | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/tools/lib/subcmd/subcmd-util.h b/tools/lib/subcmd/subcmd-util.h
index 794a375dad36..de392fc5fd3a 100644
--- a/tools/lib/subcmd/subcmd-util.h
+++ b/tools/lib/subcmd/subcmd-util.h
@@ -49,15 +49,24 @@ static NORETURN inline void die(const char *err, ...)
 
 static inline void *xrealloc(void *ptr, size_t size)
 {
-	void *ret = realloc(ptr, size);
-	if (!ret && !size)
-		ret = realloc(ptr, 1);
+	void *ret;
+
+	/*
+	 * Convert a zero-sized allocation into 1 byte, since
+	 * realloc(ptr, 0) means free(ptr), but we don't want
+	 * to release the memory. For a new allocation (when
+	 * ptr == NULL), avoid triggering NULL-checking error
+	 * conditions for zero-sized allocations.
+	 */
+	if (!size)
+		size = 1;
+	ret = realloc(ptr, size);
 	if (!ret) {
-		ret = realloc(ptr, size);
-		if (!ret && !size)
-			ret = realloc(ptr, 1);
-		if (!ret)
-			die("Out of memory, realloc failed");
+		/*
+		 * If realloc() fails, the original block is left untouched;
+		 * it is not freed or moved.
+		 */
+		die("Out of memory, realloc failed");
 	}
 	return ret;
 }
-- 
2.30.2