linux-kernel - Re: [LTP] mmstress[1309]: segfault at 7f3d71a36ee8 ip 00007f3d77132bdf sp 00007f3d71a36ee8 error 4 in libc-2.27.so[7f3d77058000+1aa000]

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Message-ID: <CAHk-=wjBjUvjN_Mw2Oo5XMUJFSREi3d9AHTSFmgp-a42ZH_K_w@mail.gmail.com>
Date:   Fri, 23 Oct 2020 08:52:50 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Daniel Díaz <daniel.diaz@...aro.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        zenglg.jy@...fujitsu.com,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        X86 ML <x86@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        lkft-triage@...ts.linaro.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-mm <linux-mm@...ck.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Ingo Molnar <mingo@...hat.com>, LTP List <ltp@...ts.linux.it>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [LTP] mmstress[1309]: segfault at 7f3d71a36ee8 ip
 00007f3d77132bdf sp 00007f3d71a36ee8 error 4 in libc-2.27.so[7f3d77058000+1aa000]

On Thu, Oct 22, 2020 at 10:02 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> I haven't reproduced the crash, but I did find a smoking gun that confirms the
> "register shenanigans are evil shenanigans" theory.  I ran into a similar thing
> recently where a seemingly innocuous line of code after loading a value into a
> register variable wreaked havoc because it clobbered the input register.

Yup, that certainly looks like the smoking gun.

Thanks for finding an example of this, clearly I'll have to either go
back to the "conditionally use 'A' or 'a' depending on size" model, or
perhaps try Rasmus' patch.

              Linus