linux-kernel - Re: [GIT PULL v4.5] Fix INT1 recursion with unregistered breakpoints

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Date:	Mon, 11 Jan 2016 18:30:29 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [GIT PULL v4.5] Fix INT1 recursion with unregistered breakpoints

On 1/11/16, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Jan 11, 2016 at 4:44 PM, Jeff Merkey <linux.mdb@...il.com> wrote:
>> Hi Thomas,
>>
>> I agree with #2, we should clear the breakpoint.  As for #1, if
>> there's an execute breakpoint it MUST be cleared or it will just fire
>> off again when it sees the iretd from the int1 exception handler.  I
>> do use the breakpoint API Thomas, this showed up while debugging and
>> testing the API with "lazy debug register switching".
>>
>> So do you want me to expand the patch and clear the breakpoint?  Just
>> give the word and I'll get busy and GIT -R- DONE.
>
> It seems to me that you're papering over some issue instead of fixing
> the root cause.  If you're using the API, then either you're doing it
> wrong or the API is broken.  Can you figure out which and fix it?
>
> --Andy
>

Andy,

Linux should not crash because someone triggered a breakpoint or one
got triggered due to a program leaving some bits lying in a read only
register (DR6) which for some strange reason someone in the linux
world decided could be used as local storage and to pass arguments
between subsystems - a register intel designed to be read from for
status.    I did not design what's in that API, I have to live with
it.  So all I am asking is that we fix this issue.  It does not matter
to my debugger is this is fixed or not in Linux, since I carry the fix
in my patch, but it does matter to the overall robustness of Linux.

If someone triggers a breakpoint or one gets triggered due to the API,
Linux won't crash, just print a log message.  So what's the big deal
about this small and modest change to prevent that.  It makes Linux
more robust.

If you have suggestions on what items you want to see added to this
commit or possibly other commits to address these other areas, please
let me know, and I am happy to code them.

Jeff