Re: [PATCH] Do not overflow string buffer (PR objc/85476).

[Martin Sebor <msebor at gmail dot com>]

On 04/20/2018 03:44 AM, Martin Liška wrote:
> Hi.
>
> Quite obvious package that causes an ASAN error described in the PR.
>
> Patch can bootstrap on ppc64le-redhat-linux and survives regression tests.

As an aside, I went and looked at the rest of code to see if
the overflow could be detected at compile time and if it could
be why it's not.  Here's what the code boils down to:

  void f (char*);

  void g (const char *s)
  {
     unsigned n = strlen (s);
     char *d = alloca (n);
     strcpy (d, s);
     f (d);
  }

Even though the off-by-one error is obvious it's not detected
either with _FORTIFY_SOURCE or without.  Both fail because
compute_builtin_object_size() only detects constant sizes.

But the strlen pass tracks both the size of allocations and
the lengths of even non-constant strings (computed by strlen)
so detecting the overflow there should be straightforward.
In the test case above the pass sees the following:

  _1 = __builtin_strlen (s_4(D));
  _9 = _1 & 4294967295;
  d_6 = __builtin_alloca (_9);
  __builtin_strcpy (d_6, s_4(D));

I've raised bug 85484 to try to implement this in GCC 9.

(Another way to handle this would be to enhance builtin-object
size to track non-constant sizes but that would require bigger
changes).

Martin

> Ready to be installed?
> Martin
>
> gcc/objc/ChangeLog:
>
> 2018-04-20  Martin Liska  <mliska@suse.cz>
>
> 	PR objc/85476
> 	* objc-act.c (finish_class): Do not overflow string buffer.
> ---
>  gcc/objc/objc-act.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)