On 2017.01.20 at 15:27 +0100, Jakub Jelinek wrote:
> On Fri, Jan 20, 2017 at 03:08:21PM +0100, Martin Liška wrote:
> > Unfortunately this way would not work as clobber marks content of the
> > memory as uninitialize
> > is different behavior that just marking a memory can be used (and maybe
> > already contains a value).
> >
> > This shows the problem:
> >
> > #include <string.h>
> >
> > char cc;
> > char ptr[] = "sparta2";
> >
> > void get(char **x)
> > {
> > *x = ptr;
> > }
> >
> > int main()
> > {
> > char *here = &cc;
> >
> > for (;;)
> > {
> > next_line:
> > if (here == NULL)
> > __builtin_abort();
> > get (&here);
> > if (strcmp (here, "sparta") == 0)
> > goto next_line;
> > else if (strcmp (here, "sparta2") == 0)
> > break;
> > }
> > }
> >
> > With the patch, DSE would optimize out '*here = &cc;' and thus aborts. The
> > problem is definitely
> > related to goto magic, where we are more defensive in placement of
> > ASAN_MARK(UNPOISON,...).
> > Hope your optimization is still valid for situations w/o artificial
> > ASAN_MARK(UNPOISON,...) placed due
> > to goto magic.
> >
> > Do we still want to do it now, or postponing to GCC 8 would be better
> > option?
>
> I'd still like to resolve it for GCC 7 if at all possible, I think otherwise
> -fsanitize=address is by default unnecessarily slower (so it is a regression
> anyway).
Another possibility would be to disable use-after-scope for gcc-7 (like
LLVM) and re-enable it for gcc-8.
diff --git a/gcc/opts.c b/gcc/opts.c
index 5f573a16ff15..2664b54133e4 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -993,7 +993,7 @@ finish_options (struct gcc_options *opts, struct
gcc_options *opts_set,
enabled. */
if ((opts->x_flag_sanitize & SANITIZE_USER_ADDRESS)
&& !opts_set->x_flag_sanitize_address_use_after_scope)
- opts->x_flag_sanitize_address_use_after_scope = true;
+ opts->x_flag_sanitize_address_use_after_scope = false;
/* Force -fstack-reuse=none in case -fsanitize-address-use-after-scope
is enabled. */
--
Markus
