fold-const: Fix up a buffer overflow in native_encode_initializer [PR98407]

For flexible array members we need to incrementally clear just from
ptr + total_bytes up to new ptr + total_bytes, but memset has been called
with the length from ptr, so was missing - total_bytes.  Additionally,
in this code off is guaranteed to be -1 and thus o 0, so don't bother pretending
we could handle anything else, it would be more complicated than that.

2020-12-21  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/98407
	* fold-const.c (native_encode_initializer): When handling flexible
	array members, fix up computation of length for memset.  Also remove
	" - o" as o is always guaranteed to be 0 in this code path.

	* gcc.c-torture/compile/pr98407.c: New test.

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 1694ba4554bf2f8c6f9b118f5e353040ce8d96cf..3a0f39a85b8c99d33cee9253f536c6f05898c2f2 100644 (file)
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -8280,9 +8280,9 @@ native_encode_initializer (tree init, unsigned char *ptr, int len,
                    return 0;
                  if (pos + fieldsize > total_bytes)
                    {
-                     if (ptr != NULL && total_bytes - o < len)
-                       memset (ptr + (total_bytes - o),
-                               '\0', MIN (pos + fieldsize - o, len));
+                     if (ptr != NULL && total_bytes < len)
+                       memset (ptr + total_bytes, '\0',
+                               MIN (pos + fieldsize, len) - total_bytes);
                      total_bytes = pos + fieldsize;
                    }
                }

diff --git a/gcc/testsuite/gcc.c-torture/compile/pr98407.c b/gcc/testsuite/gcc.c-torture/compile/pr98407.c
new file mode 100644 (file)
index 0000000..29eb080
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr98407.c
@@ -0,0 +1,10 @@
+/* PR tree-optimization/98407 */
+
+struct S { int a; int b[]; };
+const struct S c = { 0, { 0 } }, d = { 0, { 0 } };
+
+int
+foo (void)
+{
+  return __builtin_memcmp (&c, &d, sizeof d);
+}