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

From: Andy Lutomirski
Date: Fri Apr 10 2015 - 19:22:12 EST

On Fri, Apr 10, 2015 at 4:18 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> On Fri, Apr 10, 2015 at 04:15:39PM -0700, Andy Lutomirski wrote:
>> On Fri, Apr 10, 2015 at 4:05 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> 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.
> Yes we need it for (old style) kprobes in those paths.
> For user space it shouldn't matter.

For entries directly from userspace, we're not using the IST stack any more.


> -Andi
> --
> ak@xxxxxxxxxxxxxxx -- Speaking for myself only.

Andy Lutomirski
AMA Capital Management, LLC
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