[PATCH] Fix -fsanitize=address with > 32 byte aligned vars (PR sanitizer/88619)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

The prev_offset we push into the vector is supposed to be the end of
the red zone for the variable, it needs to be ASAN_MIN_RED_ZONE_SIZE
aligned, but by aligning it more we can end up with red zone with negative
size.  E.g. on the testcase, we have initial frame_offset -8 (because of
-fstack-protector guard), we align it to -32 first (required that the
start and end of the var block is 32 byte aligned (4 byte aligned in shadow
memory)).  The first variable needs 64 byte alignment, but is just 4 bytes,
so the next after
  offset = alloc_stack_frame_space (size, alignment);
we end up with frame_offset (== offset) -64 and the start of red zone is
after that 4 byte variable, i.e. at -60.  But the align_base tweak from
-32 with 64 byte alignment gives -64.

Now, the stack vars are sorted first by large vs. small alignment (not
relevant to x86), then by size and only afterwards by alignment, so similar
problems could appear if we have some large variable followed by > 32 byte
aligned very small variable.

Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for
trunk?

2019-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/88619
	* cfgexpand.c (expand_stack_vars): Only align prev_offset to
	ASAN_MIN_RED_ZONE_SIZE, not to maximum of that and alignb.

	* c-c++-common/asan/pr88619.c: New test.

--- gcc/cfgexpand.c.jj	2019-01-01 12:37:17.328972145 +0100
+++ gcc/cfgexpand.c	2019-01-03 14:11:16.209366858 +0100
@@ -1130,7 +1130,7 @@ expand_stack_vars (bool (*pred) (size_t)
 		  prev_offset = frame_offset.to_constant ();
 		}
 	      prev_offset = align_base (prev_offset,
-					MAX (alignb, ASAN_MIN_RED_ZONE_SIZE),
+					ASAN_MIN_RED_ZONE_SIZE,
 					!FRAME_GROWS_DOWNWARD);
 	      tree repr_decl = NULL_TREE;
 	      unsigned HOST_WIDE_INT size
--- gcc/testsuite/c-c++-common/asan/pr88619.c.jj	2019-01-03 14:16:00.779659964 +0100
+++ gcc/testsuite/c-c++-common/asan/pr88619.c	2019-01-03 14:15:40.568994254 +0100
@@ -0,0 +1,14 @@
+/* PR sanitizer/88619 */
+/* { dg-do compile { target fstack_protector } } */
+/* { dg-options "-fstack-protector-strong -fsanitize=address" } */
+
+typedef int A __attribute__((aligned (64)));
+
+int
+main ()
+{
+  A b;
+  int *p = &b;
+  *(p - 1) = 123;
+  __builtin_alloca (b);
+}

	Jakub