Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Michael Schmitz <schmitzmic@gmail.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        alpha <linux-alpha@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Arnd Bergmann <arnd@kernel.org>,
        Ley Foon Tan <ley.foon.tan@intel.com>,
        Tejun Heo <tj@kernel.org>, Kees Cook <keescook@chromium.org>
References: <CAHk-=wgdO5VwSUFjfF9g=DAQNYmVxzTq73NtdisYErzdZKqDGg@mail.gmail.com>
        <87sg1lwhvm.fsf@disp2133>
        <CAHk-=wgsnMTr0V-0F4FOk30Q1h7CeT8wLvR1MSnjack7EpyWtQ@mail.gmail.com>
        <6e47eff8-d0a4-8390-1222-e975bfbf3a65@gmail.com>
        <924ec53c-2fd9-2e1c-bbb1-3fda49809be4@gmail.com>
        <87eed4v2dc.fsf@disp2133>
        <5929e116-fa61-b211-342a-c706dcb834ca@gmail.com>
        <87fsxjorgs.fsf@disp2133>
        <CAHk-=wj5cJjpjAmDptmP9u4__6p3Y93SCQHG8Ef4+h=cnLiCsA@mail.gmail.com>
        <YNCaMDQVYB04bk3j@zeniv-ca.linux.org.uk>
        <YNCfNWC1UMvuE5d5@zeniv-ca.linux.org.uk>
Date:   Mon, 21 Jun 2021 11:50:56 -0500
In-Reply-To: <YNCfNWC1UMvuE5d5@zeniv-ca.linux.org.uk> (Al Viro's message of
        "Mon, 21 Jun 2021 14:16:21 +0000")
Message-ID: <87czsfi2kv.fsf@disp2133>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads
Precedence: bulk

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

> On Mon, Jun 21, 2021 at 01:54:56PM +0000, Al Viro wrote:
>> On Tue, Jun 15, 2021 at 02:58:12PM -0700, Linus Torvalds wrote:
>> 
>> > And I think our horrible "kernel threads return to user space when
>> > done" is absolutely horrifically nasty. Maybe of the clever sort, but
>> > mostly of the historical horror sort.
>> 
>> How would you prefer to handle that, then?  Separate magical path from
>> kernel_execve() to switch to userland?  We used to have something of
>> that sort, and that had been a real horror...
>> 
>> As it is, it's "kernel thread is spawned at the point similar to
>> ret_from_fork(), runs the payload (which almost never returns) and
>> then proceeds out to userland, same way fork(2) would've done."
>> That way kernel_execve() doesn't have to do anything magical.
>> 
>> Al, digging through the old notes and current call graph...
>
> 	FWIW, the major assumption back then had been that get_signal(),
> signal_delivered() and all associated machinery (including coredumps)
> runs *only* from SIGPENDING/NOTIFY_SIGNAL handling.
>
> 	And "has complete registers on stack" is only a part of that;
> there was other fun stuff in the area ;-/  Do we want coredumps for
> those, and if we do, will the de_thread stuff work there?

Do we want coredumps from processes that use io_uring? yes
Exactly what we want from io_uring threads is less clear.  We can't
really give much that is meaningful beyond the thread ids of the
io_uring threads.

What problems do are you seeing beyond the missing registers on the
stack for kernel threads?

I don't immediately see the connection between coredumps and de_thread.

The function de_thread arranges for the fatal_signal_pending to be true,
and that should work just fine for io_uring threads.  The io_uring
threads process the fatal_signal with get_signal and then proceed to
exit eventually calling do_exit.

Eric