linux-kernel - Re: [PATCH 2/9] ARM: traps: use get_kernel_nofault instead of set

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Message-ID: <CAK8P3a0HhfcM+bovy2pA4omhC5w6POqvf2Yx61_hQSwika2AEA@mail.gmail.com>
Date:   Fri, 18 Sep 2020 14:36:23 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux- <kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] ARM: traps: use get_kernel_nofault instead of set_fs()

On Fri, Sep 18, 2020 at 9:42 AM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Thu, Sep 17, 2020 at 07:29:37PM +0200, Arnd Bergmann wrote:
> > On Tue, Sep 8, 2020 at 8:15 AM Christoph Hellwig <hch@....de> wrote:
> >
> > I looked through the history now and the only code path I could
> > find that would arrive here this way is from bad_mode(), indicating
> > that there is probably a hardware bug or the contents of *regs are
> > corrupted.
>
> Yes, that's correct.  It isn't something entirely theoretical, although
> we never see it now, it used to happen in the distant past due to saved
> regs corruption.  If bad_mode() ever gets called, all bets are off and
> we're irrecoverably crashing.
>
> Note that in that case, while user_mode(regs) may return true or false,
> regs->ARM_sp and regs->ARM_lr are always the SVC mode stack and return
> address after regs has been stacked, and not the expected values for
> the parent context (which we have most likely long since destroyed.)

Ok, I have rewritten the patch and my changelog text accordingly, sending
an updated version now.

Thanks,

      Arnd