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References: <20190520154751.84763-1-elver@google.com> <ebec4325-f91b-b392-55ed-95dbd36bbb8e@virtuozzo.com>
In-Reply-To: <ebec4325-f91b-b392-55ed-95dbd36bbb8e@virtuozzo.com>
From:   Alexander Potapenko <glider@google.com>
Date:   Tue, 21 May 2019 17:52:55 +0200
Message-ID: <CAG_fn=W+_Ft=g06wtOBgKnpD4UswE_XMXd61jw5ekOH_zeUVOQ@mail.gmail.com>
Subject: Re: [PATCH v2] mm/kasan: Print frame description for stack bugs
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc:     Marco Elver <elver@google.com>,
        Dmitriy Vyukov <dvyukov@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Tue, May 21, 2019 at 5:43 PM Andrey Ryabinin <aryabinin@virtuozzo.com> w=
rote:
>
>
>
> 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 =3D=3D 1 ? "object" : "objects");
> > +
> > +     while (num_objects--) {
> > +             unsigned long offset;
> > +             unsigned long size;
> > +
> > +             /* access offset */
> > +             if (!tokenize_frame_descr(&frame_descr, token, sizeof(tok=
en),
> > +                                       &offset))
> > +                     return;
> > +             /* access size */
> > +             if (!tokenize_frame_descr(&frame_descr, token, sizeof(tok=
en),
> > +                                       &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(tok=
en),
> > +                                       NULL))
> > +                     return;
> > +
> > +             /* Strip line number, if it exists. */
>
>    Why?
>
> > +             strreplace(token, ':', '\0');
> > +
>
> ...
>
> > +
> > +     aligned_addr =3D round_down((unsigned long)addr, sizeof(long));
> > +     mem_ptr =3D round_down(aligned_addr, KASAN_SHADOW_SCALE_SIZE);
> > +     shadow_ptr =3D kasan_mem_to_shadow((void *)aligned_addr);
> > +     shadow_bottom =3D kasan_mem_to_shadow(end_of_stack(current));
> > +
> > +     while (shadow_ptr >=3D shadow_bottom && *shadow_ptr !=3D KASAN_ST=
ACK_LEFT) {
> > +             shadow_ptr--;
> > +             mem_ptr -=3D KASAN_SHADOW_SCALE_SIZE;
> > +     }
> > +
> > +     while (shadow_ptr >=3D shadow_bottom && *shadow_ptr =3D=3D KASAN_=
STACK_LEFT) {
> > +             shadow_ptr--;
> > +             mem_ptr -=3D KASAN_SHADOW_SCALE_SIZE;
> > +     }
> > +
>
> I suppose this won't work if stack grows up, which is fine because it gro=
ws 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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--=20
Alexander Potapenko
Software Engineer

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