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From:   Kees Cook <keescook@chromium.org>
Date:   Fri, 26 Oct 2018 20:42:12 +0100
Message-ID: <CAGXu5j+w+3VPMPHWNQi8UbTCRgbP1YSpjP2xct8hJEdyXdKYRw@mail.gmail.com>
Subject: Re: [RFC 6/6] Revert "pstore/ram_core: Do not reset restored zone's
 position and size"
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, kernel-team@android.com,
        Anton Vorontsov <anton@enomsg.org>,
        Colin Cross <ccross@android.com>,
        Tony Luck <tony.luck@intel.com>
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On Fri, Oct 26, 2018 at 7:22 PM, Joel Fernandes <joel@joelfernandes.org> wrote:
> On Fri, Oct 26, 2018 at 07:16:28PM +0100, Kees Cook wrote:
>> On Fri, Oct 26, 2018 at 7:00 PM, Joel Fernandes (Google)
>> <joel@joelfernandes.org> wrote:
>> > This reverts commit 25b63da64708212985c06c7f8b089d356efdd9cf.
>> >
>> > Due to the commit which is being reverted here, it is not possible to
>> > know if pstore's messages were from a previous boot, or from very old
>> > boots. This creates an awkard situation where its unclear if crash or
>> > other logs are from the previous boot or from very old boots. Also
>> > typically we dump the pstore buffers after one reboot and are interested
>> > in only the previous boot's crash so let us reset the buffer after we
>> > save them.
>> >
>> > Lastly, if we don't zap them, then I think it is possible that part of
>> > the buffer will be from this boot and the other parts will be from
>> > previous boots. So this revert fixes all of this by calling
>> > persistent_ram_zap always.
>>
>> I like the other patches (comments coming), but not this one: it's
>> very intentional to keep all crashes around until they're explicitly
>> unlinked from the pstore filesystem from userspace. Especially true
>> for catching chains of kernel crashes, or a failed log collection,
>> etc. Surviving multiple reboots is the expected behavior on Chrome OS
>> too.
>
> Oh, ok. Hence the RFC tag ;-) We can drop this one then. I forgot that
> unlinking was another way to clear the logs.

In another thread I discovered that the "single prz" ones actually
_are_ zapped at boot. I didn't realize, but it explains why pmsg would
vanish on me sometimes. ;) I always thought I was just doing something
wrong with it. (And I wonder if it's actually a bug that pmsg is
zapped -- console doesn't matter: it's overwritten every boot by
design.)

-- 
Kees Cook