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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 23 Oct 2020 09:32:54 -0700
Message-ID: <CAHk-=whGbM1E0BbSVvxGRj5nBaNRXXD-oKcgrM40s4gvYV_C+w@mail.gmail.com>
Subject: Re: [LTP] mmstress[1309]: segfault at 7f3d71a36ee8 ip
 00007f3d77132bdf sp 00007f3d71a36ee8 error 4 in libc-2.27.so[7f3d77058000+1aa000]
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        =?UTF-8?B?RGFuaWVsIETDrWF6?= <daniel.diaz@linaro.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        zenglg.jy@cn.fujitsu.com,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        X86 ML <x86@kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        lkft-triage@lists.linaro.org,
        "Eric W. Biederman" <ebiederm@xmission.com>,
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        Linux-Next Mailing List <linux-next@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Ingo Molnar <mingo@redhat.com>, LTP List <ltp@lists.linux.it>,
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On Fri, Oct 23, 2020 at 8:54 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Oct 23, 2020 at 12:14 AM Rasmus Villemoes
> <linux@rasmusvillemoes.dk> wrote:
> >
> > That's certainly garbage. Now, I don't know if it's a sufficient fix (or
> > could break something else), but the obvious first step of rearranging
> > so that the ptr argument is evaluated before the assignment to __val_pu
>
> Ack. We could do that.
>
> I'm more inclined to just bite the bullet and go back to the ugly
> conditional on the size that I had hoped to avoid, but if that turns
> out too ugly, mind signing off on your patch and I'll have that as a
> fallback?

Actually, looking at that code, and the fact that we've used the
"register asm()" format forever for the get_user() side, I think your
approach is the right one.

I'd rename the internal ptr variable to "__ptr_pu", and make sure the
assignments happen just before the asm call (with the __val_pu
assignment being the final thing).

lso, it needs to be

        void __user *__ptr_pu;

instead of

        __typeof__(ptr) __ptr = (ptr);

because "ptr" may actually be an array, and we need to have the usual
C "array to pointer" conversions happen, rather than try to make
__ptr_pu be an array too.

So the patch would become something like the appended instead, but I'd
still like your sign-off (and I'd put you as author of the fix).

Narest, can you confirm that this patch fixes the issue for you?

                  Linus

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