linux-kernel - Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

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Message-ID: <CALCETrXyYWBt5C+kUBk8f2Y3RG6fVjLu9n++UH9+Ax-12E8bHw@mail.gmail.com>
Date:	Fri, 10 Apr 2015 16:15:39 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andi Kleen <ak@...ux.intel.com>, X86 ML <x86@...nel.org>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>, Jan Beulich <JBeulich@...e.com>
Subject: Re: [PATCH 4/8] x86: Add support for rd/wr fs/gs base

On Fri, Apr 10, 2015 at 4:05 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> One might argue that this code serves no purpose, but it's there, so
>> we had better keep our per-invocation usage of DEBUG_STACK within 4k.
>
> Only if you run NKLD. I doubt KDB or GDB support nesting.
> We can ask Jan if he still uses it.

You can trigger these things in various ways with kprobes,
single-stepping, and user-space watchpoints.  These things can
definitely nest in interesting ways.  Whether or not the IST shift
mechanism is involved if they nest is a complicated question, because
there's also the "debug" IDT in play.

OMG this code is overcomplicated.

--Andy
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