linux-kernel - Re: [PATCH] arm64: lib: use C string functions with KASAN enabled.

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Message-ID: <20180910125303.dlysg3jba4wv7bhg@lakrids.cambridge.arm.com>
Date:   Mon, 10 Sep 2018 13:53:03 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kyeongdon Kim <kyeongdon.kim@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: lib: use C string functions with KASAN enabled.

On Mon, Sep 10, 2018 at 12:33:22PM +0100, Mark Rutland wrote:
> On Fri, Sep 07, 2018 at 06:48:10PM +0300, Andrey Ryabinin wrote:
> > On 09/07/2018 05:56 PM, Will Deacon wrote:
> > > I don't understand this bit: efistub uses the __pi_ prefixed
> > > versions of the routines, so why do we need to declare them as weak?
> > 
> > Weak needed because we can't have two non-weak functions with the same
> > name.
> > 
> > Alternative approach would be to never use e.g. "strlen" name for asm
> > implementation of strlen() under CONFIG_KASAN=y.  But that would
> > require adding some special ENDPIPROC_KASAN() macro since we want
> > __pi_strlen() to point to the asm_strlen().
> 
> Somehow, what we have today works with CONFIG_FORTIFY_SOURCE, which
> AFAICT would suffer from texactly the same problem with things like
> memcpy.
> 
> So either we're getting away with that by chance already (and should fix
> that regardless of this patch), or this is not actually a problem.

I now see those functions are marked weak in the assembly
implementation; sorry for the noise.

Regardless, I still think it's preferable to avoid weak wherever
possible.

I have a couple of local patches to do that for KASAN, though it's not
clear to me how that should interact with FORTIFY_SOURCE.

Thanks,
Mark.