reftable: fix allocation count on realloc error

When realloc(3) fails, it returns NULL and keeps the original allocation
intact.  REFTABLE_ALLOC_GROW overwrites both the original pointer and
the allocation count variable in that case, simultaneously leaking the
original allocation and misrepresenting the number of storable items.

parse_names() avoids the leak by keeping the original pointer if
reallocation fails, but still increase the allocation count in such a
case as if it succeeded.  That's OK, because the error handling code
just frees everything and doesn't look at names_cap anymore.

reftable_buf_add() does the same, but here it is a problem as it leaves
the reftable_buf in a broken state, with ->alloc being roughly twice as
big as the actually allocated memory, allowing out-of-bounds writes in
subsequent calls.

Reimplement REFTABLE_ALLOC_GROW to avoid leaks, keep allocation counts
in sync and still signal failures to callers while avoiding code
duplication in callers.  Make it an expression that evaluates to 0 if no
reallocation is needed or it succeeded and 1 on failure while keeping
the original pointer and allocation counter values.

Adjust REFTABLE_ALLOC_GROW_OR_NULL to the new calling convention for
REFTABLE_ALLOC_GROW, but keep its support for non-size_t alloc variables
for now.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/basics.c

@@ -124,11 +124,8 @@ int reftable_buf_add(struct reftable_buf *buf, const void *data, size_t len)
 	size_t newlen = buf->len + len;
 
 	if (newlen + 1 > buf->alloc) {
-		char *reallocated = buf->buf;
-		REFTABLE_ALLOC_GROW(reallocated, newlen + 1, buf->alloc);
-		if (!reallocated)
+		if (REFTABLE_ALLOC_GROW(buf->buf, newlen + 1, buf->alloc))
 			return REFTABLE_OUT_OF_MEMORY_ERROR;
-		buf->buf = reallocated;
 	}
 
 	memcpy(buf->buf + buf->len, data, len);
@@ -233,11 +230,9 @@ char **parse_names(char *buf, int size)
 			next = end;
 		}
 		if (p < next) {
-			char **names_grown = names;
-			REFTABLE_ALLOC_GROW(names_grown, names_len + 1, names_cap);
-			if (!names_grown)
+			if (REFTABLE_ALLOC_GROW(names, names_len + 1,
+						names_cap))
 				goto err;
-			names = names_grown;
 
 			names[names_len] = reftable_strdup(p);
 			if (!names[names_len++])

reftable/basics.h

@@ -120,22 +120,35 @@ char *reftable_strdup(const char *str);
 #define REFTABLE_ALLOC_ARRAY(x, alloc) (x) = reftable_malloc(st_mult(sizeof(*(x)), (alloc)))
 #define REFTABLE_CALLOC_ARRAY(x, alloc) (x) = reftable_calloc((alloc), sizeof(*(x)))
 #define REFTABLE_REALLOC_ARRAY(x, alloc) (x) = reftable_realloc((x), st_mult(sizeof(*(x)), (alloc)))
-#define REFTABLE_ALLOC_GROW(x, nr, alloc) \
-	do { \
-		if ((nr) > alloc) { \
-			alloc = 2 * (alloc) + 1; \
-			if (alloc < (nr)) \
-				alloc = (nr); \
-			REFTABLE_REALLOC_ARRAY(x, alloc); \
-		} \
-	} while (0)
+
+static inline void *reftable_alloc_grow(void *p, size_t nelem, size_t elsize,
+					size_t *allocp)
+{
+	void *new_p;
+	size_t alloc = *allocp * 2 + 1;
+	if (alloc < nelem)
+		alloc = nelem;
+	new_p = reftable_realloc(p, st_mult(elsize, alloc));
+	if (!new_p)
+		return p;
+	*allocp = alloc;
+	return new_p;
+}
+
+#define REFTABLE_ALLOC_GROW(x, nr, alloc) ( \
+	(nr) > (alloc) && ( \
+		(x) = reftable_alloc_grow((x), (nr), sizeof(*(x)), &(alloc)), \
+		(nr) > (alloc) \
+	) \
+)
 
 #define REFTABLE_ALLOC_GROW_OR_NULL(x, nr, alloc) do { \
-	void *reftable_alloc_grow_or_null_orig_ptr = (x); \
-	REFTABLE_ALLOC_GROW((x), (nr), (alloc)); \
-	if (!(x)) { \
-		reftable_free(reftable_alloc_grow_or_null_orig_ptr); \
+	size_t reftable_alloc_grow_or_null_alloc = alloc; \
+	if (REFTABLE_ALLOC_GROW((x), (nr), reftable_alloc_grow_or_null_alloc)) { \
+		REFTABLE_FREE_AND_NULL(x); \
 		alloc = 0; \
+	} else { \
+		alloc = reftable_alloc_grow_or_null_alloc; \
 	} \
 } while (0)
 

t/unit-tests/t-reftable-basics.c

@@ -146,6 +146,32 @@ int cmd_main(int argc UNUSED, const char *argv[] UNUSED)
 		check_int(in, ==, out);
 	}
 
+	if_test ("REFTABLE_ALLOC_GROW works") {
+		int *arr = NULL, *old_arr;
+		size_t alloc = 0, old_alloc;
+
+		check(!REFTABLE_ALLOC_GROW(arr, 1, alloc));
+		check(arr != NULL);
+		check_uint(alloc, >=, 1);
+		arr[0] = 42;
+
+		old_alloc = alloc;
+		old_arr = arr;
+		reftable_set_alloc(malloc, realloc_stub, free);
+		check(REFTABLE_ALLOC_GROW(arr, old_alloc + 1, alloc));
+		check(arr == old_arr);
+		check_uint(alloc, ==, old_alloc);
+
+		old_alloc = alloc;
+		reftable_set_alloc(malloc, realloc, free);
+		check(!REFTABLE_ALLOC_GROW(arr, old_alloc + 1, alloc));
+		check(arr != NULL);
+		check_uint(alloc, >, old_alloc);
+		arr[alloc - 1] = 42;
+
+		reftable_free(arr);
+	}
+
 	if_test ("REFTABLE_ALLOC_GROW_OR_NULL works") {
 		int *arr = NULL;
 		size_t alloc = 0, old_alloc;