linux-kernel - Re: [PATCH] Kconfig.debug: disable CONFIG_FRAME_WARN for KASAN_STACK && CC_IS

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Date:   Fri, 21 Apr 2023 16:10:50 +0100
From:   Tudor Ambarus <tudor.ambarus@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     joneslee@...gle.com, Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        zhaoyang.huang@...soc.com,
        "Liam R. Howlett" <liam.howlett@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Mark Brown <broonie@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, nogikh@...gle.com
Subject: Re: [PATCH] Kconfig.debug: disable CONFIG_FRAME_WARN for KASAN_STACK
 && CC_IS_CLANG by default

Hi, Arnd,

On 4/21/23 15:30, Arnd Bergmann wrote:
> On Fri, Apr 21, 2023, at 15:01, Tudor Ambarus wrote:
> 
>> The conclusion is that when KASAN is enabled the stack usage increases a
>> bit, but nothing unmanageable ~30-70 bytes, whereas when enabling
>> KASAN_STACK the stack usage is excessive, from ~1.7K to ~5.8K for these
>> cases.
>>
>> Disable CONFIG_FRAME_WARN for KASAN_STACK && CC_IS_CLANG by default.
>> Adventurers can still override the default value by input prompt or
>> explicit values in defconfigs in case they feel that some real warnings
>> are missed.
>>
>> Signed-off-by: Tudor Ambarus <tudor.ambarus@...aro.org>
> 
> I think we are still better off with the warning enabled. When someone
> really wants to run a kernel with KASAN_STACK enabled, they should have
> a chance to see what they are getting into and not report any runtime
> bugs that come from stack overflow. 
> 

Are such stack overflows warnings reliable when we know that stack
instrumentation causes excessive stack usage? Until clang is fixed one
shall hunt frame-larger-than warnings with KASAN_STACK disabled.

Cheers,
ta