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From: Andy Lutomirski <[email protected]>
To: Thomas Gleixner <[email protected]>
Cc: Linus Torvalds <[email protected]>,
	Andy Lutomirski <[email protected]>, Jens Axboe <[email protected]>,
	Stefan Metzmacher <[email protected]>,
	Linux Kernel Mailing List <[email protected]>,
	io-uring <[email protected]>,
	the arch/x86 maintainers <[email protected]>
Subject: Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads
Date: Mon, 3 May 2021 16:15:06 -0700	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>



> On May 3, 2021, at 3:56 PM, Thomas Gleixner <[email protected]> wrote:
> 
> On Mon, May 03 2021 at 15:08, Linus Torvalds wrote:
>>> On Mon, May 3, 2021 at 2:49 PM Andy Lutomirski <[email protected]> wrote:
>>> 
>>> To be clear, I'm suggesting that we -EINVAL the PTRACE_GETREGS calls
>>> and such, not the ATTACH.  I have no idea what gdb will do if this
>>> happens, though.
>> 
>> I feel like the likelihood that it will make gdb work any better is
>> basically zero.
>> 
>> I think we should just do Stefan's patch - I assume it generates
>> something like four instructions (two loads, two stores) on x86-64,
>> and it "just works".
>> 
>> Yeah, yeah, it presumably generates 8 instructions on 32-bit x86, and
>> we could fix that by just using the constant __USER_CS/DS instead (no
>> loads necessary) since 32-bit doesn't have any compat issues.
>> 
>> But is it worth complicating the patch for a couple of instructions in
>> a non-critical path?
>> 
>> And I don't see anybody stepping up to say "yes, I will do the patch
>> for gdb", so I really think the least pain is to just take the very
>> straightforward and tested kernel patch.
>> 
>> Yes, yes, that also means admitting to ourselves that the gdb
>> situation isn't likely going to improve, but hey, if nobody in this
>> thread is willing to work on the gdb side to fix the known issues
>> there, isn't that the honest thing to do anyway?
> 
> GDB is one thing. But is this setup actually correct under all
> circumstances?
> 
> It's all fine that we have lots of blurb about GDB, but there is no
> reasoning why this does not affect regular kernel threads which take the
> same code path.
> 
> Neither is there an answer what happens in case of a signal delivered to
> this thread and what any other GDB/ptraced induced poking might cause.
> 
> This is a half setup user space thread which is assumed to behave like a
> regular kernel thread, but is this assumption actually true?
> 
> 

I’m personally concerned about FPU state. No one ever imagined when writing and reviewing the FPU state code that we were going to let ptrace poke the state on a kernel thread.

Now admittedly kernel_execve() magically turns kernel threads into user threads, but, again, I see no evidence that anyone has thought through all the implications of letting ptrace go to town before doing so.

(Is the io_uring thread a kthread style kernel thread?  kthread does horrible, horrible things with the thread stack.)

  reply	other threads:[~2021-05-03 23:15 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <[email protected]>
2021-05-03 16:05 ` [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads Andy Lutomirski
2021-05-03 19:14   ` Linus Torvalds
2021-05-03 20:15     ` Andy Lutomirski
2021-05-03 20:21       ` Andy Lutomirski
2021-05-03 20:37       ` Linus Torvalds
2021-05-03 21:26         ` Jens Axboe
2021-05-03 21:49           ` Andy Lutomirski
2021-05-03 22:08             ` Linus Torvalds
2021-05-03 22:56               ` Thomas Gleixner
2021-05-03 23:15                 ` Andy Lutomirski [this message]
2021-05-03 23:16                 ` Linus Torvalds
2021-05-03 23:19                   ` Linus Torvalds
2021-05-03 23:27                   ` Stefan Metzmacher
2021-05-03 23:48                     ` Linus Torvalds
2021-05-04  2:50                       ` Jens Axboe
2021-05-04 11:39                         ` Stefan Metzmacher
2021-05-04 15:53                           ` Linus Torvalds
2021-05-12  4:24                         ` Olivier Langlois
2021-05-12 17:44                           ` Linus Torvalds
2021-05-12 20:55                             ` Jens Axboe
2021-05-20  4:13                               ` Olivier Langlois
2021-05-21  7:31                                 ` Olivier Langlois
2021-05-25 19:39                                   ` Olivier Langlois
2021-05-25 19:45                                     ` Olivier Langlois
2021-05-25 19:52                                     ` Jens Axboe
2021-05-25 20:23                                     ` Linus Torvalds
2021-05-04  8:22                       ` David Laight
2021-05-04  0:01                   ` Andy Lutomirski
2021-05-04  8:39     ` Peter Zijlstra
2021-05-04 15:35       ` Borislav Petkov
2021-05-04 15:55         ` Simon Marchi
2021-05-05 11:29           ` Stefan Metzmacher
2021-05-05 21:59             ` Simon Marchi
2021-05-05 22:11               ` Andy Lutomirski
2021-05-05 23:12                 ` Borislav Petkov
2021-05-05 23:22                   ` Andy Lutomirski
2021-05-06  1:04                 ` Simon Marchi
2021-05-06 15:11                   ` Andy Lutomirski
2021-05-06  9:47                 ` David Laight
2021-05-06  9:53                   ` David Laight
2021-05-05 22:21               ` Stefan Metzmacher
2021-05-05 23:15                 ` Simon Marchi
2021-04-11 15:27 Stefan Metzmacher
2021-04-14 21:28 ` Stefan Metzmacher

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