[PATCH] asan: Don't fold some strlens with -fsanitize=address [PR110676]

[Jakub Jelinek <jakub@redhat.com>]

Hi!

The UB on the following testcase isn't diagnosed by -fsanitize=address,
because we see that the array has a single element and optimize the
strlen to 0.  I think it is fine to assume e.g. for range purposes the
lower bound for the strlen as long as we don't try to optimize
strlen (str)
where we know that it returns [26, 42] to
26 + strlen (str + 26), but for the upper bound we really want to punt
on optimizing that for -fsanitize=address to read all the bytes of the
string and diagnose if we run to object end etc.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-02-06  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/110676
	* gimple-fold.cc (gimple_fold_builtin_strlen): For -fsanitize=address
	reset maxlen to sizetype maximum.

	* gcc.dg/asan/pr110676.c: New test.

--- gcc/gimple-fold.cc.jj	2024-01-31 12:24:51.714239628 +0100
+++ gcc/gimple-fold.cc	2024-02-05 21:38:03.829964904 +0100
@@ -4019,6 +4019,11 @@ gimple_fold_builtin_strlen (gimple_stmt_
       maxlen = wi::to_wide (max_object_size (), prec) - 2;
     }
 
+  /* For -fsanitize=address, don't optimize the upper bound of the
+     length to be able to diagnose UB on non-zero terminated arrays.  */
+  if (sanitize_flags_p (SANITIZE_ADDRESS))
+    maxlen = wi::max_value (TYPE_PRECISION (sizetype), UNSIGNED);
+
   if (minlen == maxlen)
     {
       /* Fold the strlen call to a constant.  */
--- gcc/testsuite/gcc.dg/asan/pr110676.c.jj	2024-02-05 21:42:43.657104536 +0100
+++ gcc/testsuite/gcc.dg/asan/pr110676.c	2024-02-05 21:42:39.091167524 +0100
@@ -0,0 +1,14 @@
+/* PR sanitizer/110676 */
+/* { dg-do run } */
+/* { dg-skip-if "" { *-*-* } { "*" } { "-O0" } } */
+/* { dg-shouldfail "asan" } */
+
+int
+main ()
+{
+  char s[1] = "A";
+  return __builtin_strlen (s);
+}
+
+/* { dg-output "ERROR: AddressSanitizer: stack-buffer-overflow on address.*(\n|\r\n|\r)" } */
+/* { dg-output "READ of size.*" } */

	Jakub