linux-kernel - Re: [PATCH 0/2] Don't show PF_IO

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Date:   Thu, 25 Mar 2021 15:50:11 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     io-uring <io-uring@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Stefan Metzmacher <metze@...ba.org>
Subject: Re: [PATCH 0/2] Don't show PF_IO_WORKER in /proc/<pid>/task/

On 3/25/21 2:43 PM, Eric W. Biederman wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
> 
>> On Thu, Mar 25, 2021 at 12:42 PM Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>>>
>>> On Thu, Mar 25, 2021 at 12:38 PM Linus Torvalds
>>> <torvalds@...ux-foundation.org> wrote:
>>>>
>>>> I don't know what the gdb logic is, but maybe there's some other
>>>> option that makes gdb not react to them?
>>>
>>> .. maybe we could have a different name for them under the task/
>>> subdirectory, for example (not  just the pid)? Although that probably
>>> messes up 'ps' too..
>>
>> Actually, maybe the right model is to simply make all the io threads
>> take signals, and get rid of all the special cases.
>>
>> Sure, the signals will never be delivered to user space, but if we
>>
>>  - just made the thread loop do "get_signal()" when there are pending signals
>>
>>  - allowed ptrace_attach on them
>>
>> they'd look pretty much like regular threads that just never do the
>> user-space part of signal handling.
>>
>> The whole "signals are very special for IO threads" thing has caused
>> so many problems, that maybe the solution is simply to _not_ make them
>> special?
> 
> The special case in check_kill_permission is certainly unnecessary.
> Having the signal blocked is enough to prevent signal_pending() from
> being true. 
> 
> 
> The most straight forward thing I can see is to allow ptrace_attach and
> to modify ptrace_check_attach to always return -ESRCH for io workers
> unless ignore_state is set causing none of the other ptrace operations
> to work.
> 
> That is what a long running in-kernel thread would do today so
> user-space aka gdb may actually cope with it.
> 
> 
> We might be able to support if io workers start supporting SIGSTOP but I
> am not at all certain.

See patch just send out as a POC, mostly, not fully sanitized yet. But
I did try to return -ESRCH from ptrace_check_attach() if it's an IO
thread and ignore_state isn't set:

if (!ignore_state && child->flags & PF_IO_WORKER)
	return -ESRCH;

and that causes gdb to abort at that thread. For the same test case
as in the previous email, you get:

Attaching to process 358
[New LWP 359]
[New LWP 360]
[New LWP 361]
Couldn't get CS register: No such process.
(gdb) 0x00007ffa58537125 in ?? ()

(gdb) bt
#0  0x00007ffa58537125 in ?? ()
#1  0x0000000000000000 in ?? ()
(gdb) info threads
  Id   Target Id             Frame 
* 1    LWP 358 "io_uring"    0x00007ffa58537125 in ?? ()
  2    LWP 359 "iou-mgr-358" Couldn't get registers: No such process.
(gdb) q
A debugging session is active.

	Inferior 1 [process 358] will be detached.

Quit anyway? (y or n) y
Couldn't write debug register: No such process.

where 360 here is a regular pthread created thread, and 361 is another
iou-mgr-x task. While gdb behaves better in this case, it does still
prevent you from inspecting thread 3 which would be totally valid.

-- 
Jens Axboe