linux-kernel - Re: [PATCH 24/26] ocfs2: reduce stack size with KASAN

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Date:   Thu, 2 Mar 2017 23:22:26 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Joe Perches <joe@...ches.com>
Cc:     kasan-dev <kasan-dev@...glegroups.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-media@...r.kernel.org,
        linux-wireless <linux-wireless@...r.kernel.org>,
        kernel-build-reports@...ts.linaro.org,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH 24/26] ocfs2: reduce stack size with KASAN

On Thu, Mar 2, 2017 at 6:46 PM, Joe Perches <joe@...ches.com> wrote:
> On Thu, 2017-03-02 at 17:38 +0100, Arnd Bergmann wrote:
>> The internal logging infrastructure in ocfs2 causes special warning code to be
>> used with KASAN, which produces rather large stack frames:
>
>> fs/ocfs2/super.c: In function 'ocfs2_fill_super':
>> fs/ocfs2/super.c:1219:1: error: the frame size of 3264 bytes is larger than 3072 bytes [-Werror=frame-larger-than=]
>
> At least by default it doesn't seem to.
>
> gcc 6.2 allyesconfig, CONFIG_KASAN=y
> with either CONFIG_KASAN_INLINE or CONFIG_KASAN_OUTLINE
>
> gcc doesn't emit a stack warning

The warning is disabled until patch 26/26. which picks the 3072 default.
The 3264 number was with gcc-7, which is worse than gcc-6 since it enables
an extra check.

>> By simply passing the mask by value instead of reference, we can avoid the
>> problem completely.
>
> Any idea why that's so?

With KASAN, every time we inline the function, the compiler has to allocate
space for another copy of the variable plus a redzone to detect whether
passing it by reference into another function causes an overflow at runtime.

>>  On 64-bit architectures, this is also more efficient,
>
> Efficient true, but the same overall stack no?

Here is what I see with CONFIG_FRAME_WARN=300 and x86_64-linux-gcc-6.3.1:

before:
fs/ocfs2/super.c: In function 'ocfs2_parse_options.isra.3':
fs/ocfs2/super.c:1508:1: error: the frame size of 352 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_enable_quotas':
fs/ocfs2/super.c:974:1: error: the frame size of 344 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_fill_super':
fs/ocfs2/super.c:1219:1: error: the frame size of 552 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]

after:
fs/ocfs2/super.c: In function 'ocfs2_fill_super':
fs/ocfs2/super.c:1219:1: error: the frame size of 472 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]

and with gcc-7.0.1 (including -fsanitize-address-use-after-scope), before:
fs/ocfs2/super.c: In function 'ocfs2_check_volume':
fs/ocfs2/super.c:2512:1: error: the frame size of 768 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_statfs':
fs/ocfs2/super.c:1717:1: error: the frame size of 320 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_parse_options.isra.3':
fs/ocfs2/super.c:1508:1: error: the frame size of 464 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_enable_quotas':
fs/ocfs2/super.c:974:1: error: the frame size of 320 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_remount':
fs/ocfs2/super.c:752:1: error: the frame size of 568 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_initialize_super.isra.8':
fs/ocfs2/super.c:2339:1: error: the frame size of 1712 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]
fs/ocfs2/super.c: In function 'ocfs2_fill_super':
fs/ocfs2/super.c:1219:1: error: the frame size of 3264 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]

after:
fs/ocfs2/super.c: In function 'ocfs2_fill_super':
fs/ocfs2/super.c:1219:1: error: the frame size of 704 bytes is larger
than 300 bytes [-Werror=frame-larger-than=]

     Arnd