linux-kernel - RE: [PATCH] x86/asm: Don't use rbp as temp register in csum_partial_copy

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Date:   Thu, 4 May 2017 15:56:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Josh Poimboeuf' <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
CC:     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()

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?

	David