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To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jens Axboe <axboe@kernel.dk>, io-uring <io-uring@vger.kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>,
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 <8efd9977-003b-be65-8ae2-4b04d8dd1224@samba.org>
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 <CAHk-=whEObPkZBe4766DmR46-=5QTUiatWbSOaD468eTgYc1tg@mail.gmail.com>
From:   Stefan Metzmacher <metze@samba.org>
Subject: Re: [PATCH 0/6] Allow signals for IO threads
Message-ID: <2d8a73ef-2f18-6872-bad1-a34deb20f641@samba.org>
Date:   Thu, 1 Apr 2021 18:00:39 +0200
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Am 01.04.21 um 17:39 schrieb Linus Torvalds:
> On Thu, Apr 1, 2021 at 7:58 AM Stefan Metzmacher <metze@samba.org> wrote:
>>
>>>
>>> Ok, the following makes gdb happy again:
>>>
>>> --- a/arch/x86/kernel/process.c
>>> +++ b/arch/x86/kernel/process.c
>>> @@ -163,6 +163,8 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
>>>         /* Kernel thread ? */
>>>         if (unlikely(p->flags & (PF_KTHREAD | PF_IO_WORKER))) {
>>>                 memset(childregs, 0, sizeof(struct pt_regs));
>>> +               if (p->flags & PF_IO_WORKER)
>>> +                       childregs->cs = current_pt_regs()->cs;
>>>                 kthread_frame_init(frame, sp, arg);
>>>                 return 0;
>>>         }
>>
>> Would it be possible to fix this remaining problem before 5.12 final?
> 
> Please not that way.
> 
> But doing something like
> 
>         childregs->cs = __USER_CS;
>         childregs->ss = __USER_DS;
>         childregs->ds = __USER_DS;
>         childregs->es = __USER_DS;
> 
> might make sense (just do it unconditionally, rather than making it
> special to PF_IO_WORKER).
> 
> Does that make gdb happy too?

I haven't tried it, but it seems gdb tries to use PTRACE_PEEKUSR
against the last thread tid listed under /proc/<pid>/tasks/ in order to
get the architecture for the userspace application, so my naive assumption
would be that it wouldn't allow the detection of a 32-bit application
using a 64-bit kernel.

metze