Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads

From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: Simon Marchi <simon.marchi@polymtl.ca>,
	Stefan Metzmacher <metze@samba.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jens Axboe <axboe@kernel.dk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	io-uring <io-uring@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	linux-toolchains@vger.kernel.org
Subject: Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es' registers for io_threads
Date: Thu, 6 May 2021 01:12:20 +0200	[thread overview]
Message-ID: <YJMmVHGn33W2n2Ux@zn.tnic> (raw)
In-Reply-To: <CALCETrUF5M+Qw+RfY8subR7nzmpMyFsE3NHSAPoMVWMz6_hr-w@mail.gmail.com>

On Wed, May 05, 2021 at 03:11:18PM -0700, Andy Lutomirski wrote:
> Since I'm not holding my breath, please at least keep in mind that
> anything you do here is merely a heuristic, cannot be fully correct,
> and then whenever gdb determines that a thread group or a thread is
> "32-bit", gdb is actually deciding to operate in a degraded mode for
> that task, is not accurately representing the task state, and is at
> risk of crashing, malfunctioning, or crashing the inferior due to its
> incorrect assumptions.  If you have ever attached gdb to QEMU's
> gdbserver and tried to debug the early boot process of a 64-bit Linux
> kernel, you may have encountered this class of bugs.  gdb works very,
> very poorly for this use case.

So we were talking about this with toolchain folks today and they gave
me this example:

Imagine you've stopped the target this way:

	<insn><-- stopped here
	<insn>
	<mode changing insn>
	<insn>
	<insn>
	...

now, if you dump rIP and say, rIP + the 10 following insns at the place
you've stopped it, gdb cannot know that 2 insns further into the stream
a

<mode changing insn>

is coming and it should change the disassembly of the insns after that
<mode changing insn> to the new mode. Unless it goes and inspects all
further instructions and disassembles them and analyzes the flow...

So what you can do is 

(gdb) set arch ...

at the <mode changing insn> to the mode you're changing to.

Dunno, maybe I'm missing something but this sounds like without user
help gdb can only assume things.

Good night and good luck.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette