linux-kernel - Re: [PATCH 02/10] exit: Add and use make_task

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Date:   Wed, 05 Jan 2022 14:46:10 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Gladkov <legion@...nel.org>,
        Kyle Huey <me@...ehuey.com>, Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH 02/10] exit: Add and use make_task_dead.

Al Viro <viro@...iv.linux.org.uk> writes:

> On Wed, Dec 08, 2021 at 02:25:24PM -0600, Eric W. Biederman wrote:
>> There are two big uses of do_exit.  The first is it's design use to be
>> the guts of the exit(2) system call.  The second use is to terminate
>> a task after something catastrophic has happened like a NULL pointer
>> in kernel code.
>> 
>> Add a function make_task_dead that is initialy exactly the same as
>> do_exit to cover the cases where do_exit is called to handle
>> catastrophic failure.  In time this can probably be reduced to just a
>> light wrapper around do_task_dead. For now keep it exactly the same so
>> that there will be no behavioral differences introducing this new
>> concept.
>> 
>> Replace all of the uses of do_exit that use it for catastraphic
>> task cleanup with make_task_dead to make it clear what the code
>> is doing.
>> 
>> As part of this rename rewind_stack_do_exit
>> rewind_stack_and_make_dead.
>
> Umm...   What about .Linvalid_mask: in arch/xtensa/kernel/entry.S?
> That's an obvious case for your make_task_dead().

Good catch.

Being in assembly it did not have anything after the name do_exit so it
hid from my regex "[^A-Za-z0-9_]do_exit[^A-Za-z0-9]".  Thank you for
finding that.

Skimming the surrounding code it looks like Linvalid_mask can only be
reached by buggy hardware or buggy kernel code.    If userspace could
trigger the condition it would be a candidate for force_exit_sig.

I am a bit puzzled why die is not called, instead of die being
handrolled there.

xtensa folks any thoughts?

If not I will queue up a minimal patch to replace do_exit with
make_task_dead.

Eric