[PATCH v2 4/4] reftable/basics: return NULL on zero-sized allocations

From: Patrick Steinhardt <ps@pks.im>
Date: Dec 22, 2024, 07:24 UTC
Message-ID: <20241222-b4-pks-reftable-oom-fix-without-readers-v2-4-19550090d15a@pks.im>
In-Reply-To: <20241222-b4-pks-reftable-oom-fix-without-readers-v2-0-19550090d15a@pks.im>

In the preceding commits we have fixed a couple of issues when allocating zero-sized objects. These issues were masked by implementation-defined behaviour. Quoting malloc(3p):

  If size is 0, either:

    * A null pointer shall be returned and errno may be set to an
      implementation-defined value, or

    * A pointer to the allocated space shall be returned. The
      application shall ensure that the pointer is not used to access an
      object.

So it is perfectly valid that implementations of this function may or may not return a NULL pointer in such a case.

Adapt both `reftable_malloc()` and `reftable_realloc()` so that they return NULL pointers on zero-sized allocations. This should remove any implementation-defined behaviour in our allocators and thus allows us to detect such platform-specific issues more easily going forward.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 reftable/basics.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/reftable/basics.c b/reftable/basics.c
index 7d84a5d62dead1cf1a60698b1bb12fe6ac41c090..70b1091d1495bb5b4c8aae63bd9213dc704aecde 100644
--- a/reftable/basics.c
+++ b/reftable/basics.c
@@ -17,6 +17,8 @@ static void (*reftable_free_ptr)(void *);
 
 void *reftable_malloc(size_t sz)
 {
+	if (!sz)
+		return NULL;
 	if (reftable_malloc_ptr)
 		return (*reftable_malloc_ptr)(sz);
 	return malloc(sz);
@@ -24,6 +26,11 @@ void *reftable_malloc(size_t sz)
 
 void *reftable_realloc(void *p, size_t sz)
 {
+	if (!sz) {
+		reftable_free(p);
+		return NULL;
+	}
+
 	if (reftable_realloc_ptr)
 		return (*reftable_realloc_ptr)(p, sz);
 	return realloc(p, sz);

-- 
2.48.0.rc0.184.g0fc57dec57.dirty