Abort on C23 UB zero sized realloc

h_malloc.c

@@ -1513,6 +1513,11 @@ EXPORT void *h_calloc(size_t nmemb, size_t size) {
 }
 
 EXPORT void *h_realloc(void *old, size_t size) {
+    // deprecated in C17, UB since C23
+    if (unlikely(old != NULL && size == 0)) {
+        fatal_error("invalid zero sized realloc");
+    }
+
     size = adjust_size_for_canary(size);
     if (old == NULL) {
         return alloc(size);

test/.gitignore

@@ -41,4 +41,5 @@ overflow_small_8_byte
 uninitialized_read_large
 uninitialized_read_small
 realloc_init
+realloc_c23_undefined_behaviour
 __pycache__/

test/Makefile

@@ -67,7 +67,8 @@ EXECUTABLES := \
     invalid_malloc_object_size_small \
     invalid_malloc_object_size_small_quarantine \
     impossibly_large_malloc \
-    realloc_init
+    realloc_init \
+    realloc_c23_undefined_behaviour
 
 all: $(EXECUTABLES)
 

test/realloc_c23_undefined_behaviour.c

@@ -0,0 +1,16 @@
+#include <stdlib.h>
+
+#include "test_util.h"
+
+OPTNONE int main(void) {
+    void *p, *q;
+
+    p = malloc(16);
+    if (!p) {
+        return -1;
+    }
+
+    q = realloc(p, 0);
+
+    return 0;
+}

test/test_smc.py

@@ -169,6 +169,12 @@ class TestSimpleMemoryCorruption(unittest.TestCase):
         self.assertEqual(stderr.decode("utf-8"),
                          "fatal allocator error: invalid realloc\n")
 
+    def test_realloc_c23_undefined_behaviour(self):
+        _stdout, stderr, returncode = self.run_test("realloc_c23_undefined_behaviour")
+        self.assertEqual(returncode, -6)
+        self.assertEqual(stderr.decode("utf-8"),
+                         "fatal allocator error: invalid zero sized realloc\n")
+
     def test_write_after_free_large_reuse(self):
         _stdout, _stderr, returncode = self.run_test(
             "write_after_free_large_reuse")