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

[Alexander Potapenko <glider@google.com>]

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?
>
> > +             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?
>
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-- 
Alexander Potapenko
Software Engineer

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