[gcc r11-6281] fold-const: Fix up a buffer overflow in native_encode_initializer [PR98407]

[gcc r11-6281] fold-const: Fix up a buffer overflow in native_encode_initializer [PR98407]

[Jakub Jelinek]

https://gcc.gnu.org/g:d8aeee11af715507e61464d390f14e4f4fde61b0

commit r11-6281-gd8aeee11af715507e61464d390f14e4f4fde61b0
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Dec 21 10:14:46 2020 +0100

    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:
---
 gcc/fold-const.c                              |  6 +++---
 gcc/testsuite/gcc.c-torture/compile/pr98407.c | 10 ++++++++++
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 1694ba4554b..3a0f39a85b8 100644
--- 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
index 00000000000..29eb0803944
--- /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);
+}