netdev - Re: [PATCH] x86/asm: Don't use rbp as temp register in csum_partial_copy

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Message-ID: <20170504161129.syz46qyxge3nupki@treble>
Date:   Thu, 4 May 2017 11:11:29 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        "David S . Miller" <davem@...emloft.net>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [PATCH] x86/asm: Don't use rbp as temp register in
 csum_partial_copy_generic()

On Thu, May 04, 2017 at 03:56:49PM +0000, David Laight wrote:
> From: Josh Poimboeuf
> > Sent: 04 May 2017 15:52
> > Andrey Konovalov reported the following warning while fuzzing the kernel
> > with syzkaller:
> > 
> >   WARNING: kernel stack regs at ffff8800686869f8 in a.out:4933 has bad 'bp' value c3fc855a10167ec0
> > 
> > The unwinder dump revealed that rbp had a bad value when an interrupt
> > occurred in csum_partial_copy_generic().
> > 
> > That function saves rbp on the stack and then overwrites it, using it as
> > a scratch register.  That's problematic because it breaks stack traces
> > if an interrupt occurs in the middle of the function.
> 
> Does gcc guarantee not to use bp as a scratch register in leaf functions?

At least in practice, gcc doesn't touch rbp in leaf functions.  (I don't
know about guarantees.)

-- 
Josh