linux-kernel - Re: Dealing with the NMI mess

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Date:	Fri, 24 Jul 2015 15:21:28 +0200
From:	Willy Tarreau <w@....eu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: Dealing with the NMI mess

On Fri, Jul 24, 2015 at 09:03:42AM -0400, Steven Rostedt wrote:
> On Fri, 24 Jul 2015 14:43:04 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
>  
> > > I'm not too familiar with how to use hw breakpoints, but I'm guessing
> > > (correct me if I'm wrong) that breakpoints on code that trigger when
> > > executed, but watchpoints on data trigger when accessed. Then
> > > copy_from_user_inatomic() would only trigger on watchpoints (it's not
> > > executing that code, at least I hope it isn't!), and those wont bother
> > > us.
> > 
> > These things can be: RW, W, X.
> > 
> > Sure, hitting a user X watchpoint is going to be 'interesting', but its
> > fairly easy to hit a RW one.
> 
> But do we care if we do hit one? The return from the #DB handler can
> use a RET. Right?

My understanding is that by using RET we can't set the RF flag and #DB
will immediately strike again when the operation is attempted again. Thus
we have to completely disable the breakpoints on leaving after the first
one strikes, resulting in some userland breakpoints being missed. Maybe
it can be accepted as a limitation when perf is running. I don't know if
the output of perf is that relevant when a debugger is present BTW.

Willy

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