Re: [PATCH v10 00/18] Enable FSGSBASE instructions

[Sasha Levin]

On Sun, May 10, 2020 at 12:15:34PM +0200, Thomas Gleixner wrote:
> Vegard Nossum <vegard.nossum@xxxxxxxxxx> writes:
> > On 5/10/20 10:09 AM, Vegard Nossum wrote:
> >
> > I spoke a few minutes too soon. Just hit this, if anybody wants to have
> > a look:
> >
> > [ 6402.786418] ------------[ cut here ]------------
> > [ 6402.787769] WARNING: CPU: 0 PID: 13802 at arch/x86/kernel/traps.c:811
> > do_debug+0x16c/0x210
>
> > [ 6402.820353] Call Trace:
> > [ 6402.821043]  <#DB>
> > [ 6402.821622]  debug+0x37/0x70
> > [ 6402.822449] RIP: 0010:arch_stack_walk_user+0x79/0x110
>
> That's a cute way to trigger that WARN_ON in the #DB handler.
>
> > [ 6402.816468] DR0: 0000000000000001 DR1: 0000000040006070 DR2: 00007ffff7ffd000
> > [ 6402.818406] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000003b3062a
>
> #DB recursion
>
>  [ 6402.832288] RDX: 0000000040006073
>
> 27:	48 8b 1a      mov    (%rdx),%rbx
>
> Breakpoint on user space stack, #DB triggers and the low level ASM
> irqflags tracepoint has stacktrace enabled which unwinds into the user
> stack and triggers #DB again.
>
> Bah. I know why I want to ban all that tracing muck from low level entry code.
>
> > It might not be related to the patch set, mind.
>
> It's unrelated.

Thanks for testing Vegard!

--
Thanks,
Sasha