Re: [PATCH v2] mm/kasan: Print frame description for stack bugs

[Marco Elver <elver@google.com>]

On Tue, 21 May 2019 at 17:53, Alexander Potapenko <glider@google.com> wrote:
>
> On Tue, May 21, 2019 at 5:43 PM Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> >
> > On 5/20/19 6:47 PM, Marco Elver wrote:
> >
> > > +static void print_decoded_frame_descr(const char *frame_descr)
> > > +{
> > > +     /*
> > > +      * We need to parse the following string:
> > > +      *    "n alloc_1 alloc_2 ... alloc_n"
> > > +      * where alloc_i looks like
> > > +      *    "offset size len name"
> > > +      * or "offset size len name:line".
> > > +      */
> > > +
> > > +     char token[64];
> > > +     unsigned long num_objects;
> > > +
> > > +     if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
> > > +                               &num_objects))
> > > +             return;
> > > +
> > > +     pr_err("\n");
> > > +     pr_err("this frame has %lu %s:\n", num_objects,
> > > +            num_objects == 1 ? "object" : "objects");
> > > +
> > > +     while (num_objects--) {
> > > +             unsigned long offset;
> > > +             unsigned long size;
> > > +
> > > +             /* access offset */
> > > +             if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
> > > +                                       &offset))
> > > +                     return;
> > > +             /* access size */
> > > +             if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
> > > +                                       &size))
> > > +                     return;
> > > +             /* name length (unused) */
> > > +             if (!tokenize_frame_descr(&frame_descr, NULL, 0, NULL))
> > > +                     return;
> > > +             /* object name */
> > > +             if (!tokenize_frame_descr(&frame_descr, token, sizeof(token),
> > > +                                       NULL))
> > > +                     return;
> > > +
> > > +             /* Strip line number, if it exists. */
> >
> >    Why?

The filename is not included, and I don't think it adds much in terms
of ability to debug; nor is the line number included with all
descriptions. I think, the added complexity of separating the line
number and parsing is not worthwhile here. Alternatively, I could not
pay attention to the line number at all, and leave it as is -- in that
case, some variable names will display as "foo:123".

> >
> > > +             strreplace(token, ':', '\0');
> > > +
> >
> > ...
> >
> > > +
> > > +     aligned_addr = round_down((unsigned long)addr, sizeof(long));
> > > +     mem_ptr = round_down(aligned_addr, KASAN_SHADOW_SCALE_SIZE);
> > > +     shadow_ptr = kasan_mem_to_shadow((void *)aligned_addr);
> > > +     shadow_bottom = kasan_mem_to_shadow(end_of_stack(current));
> > > +
> > > +     while (shadow_ptr >= shadow_bottom && *shadow_ptr != KASAN_STACK_LEFT) {
> > > +             shadow_ptr--;
> > > +             mem_ptr -= KASAN_SHADOW_SCALE_SIZE;
> > > +     }
> > > +
> > > +     while (shadow_ptr >= shadow_bottom && *shadow_ptr == KASAN_STACK_LEFT) {
> > > +             shadow_ptr--;
> > > +             mem_ptr -= KASAN_SHADOW_SCALE_SIZE;
> > > +     }
> > > +
> >
> > I suppose this won't work if stack grows up, which is fine because it grows up only on parisc arch.
> > But "BUILD_BUG_ON(IS_ENABLED(CONFIG_STACK_GROUWSUP))" somewhere wouldn't hurt.
> Note that KASAN was broken on parisc from day 1 because of other
> assumptions on the stack growth direction hardcoded into KASAN
> (e.g. __kasan_unpoison_stack() and __asan_allocas_unpoison()).
> So maybe this BUILD_BUG_ON can be added in a separate patch as it's
> not specific to what Marco is doing here?

Happy to send a follow-up patch, or add here. Let me know what you prefer.

Thanks,
-- Marco