Re: [PATCH] kmemcheck: move hook before preempt_conditional_sti()

[Ingo Molnar <mingo@elte.hu>]

* Vegard Nossum <vegard.nossum@gmail.com> wrote:

> 2009/6/7 Ingo Molnar <mingo@elte.hu>:
> >
> > * Vegard Nossum <vegard.nossum@gmail.com> wrote:
> >
> >> There are actually two problems here:
> >>
> >> 1. We absolutely cannot enable IRQs in case the fault was caused by
> >>    kmemcheck.
> >>
> >> 2. We cannot enable preemption and then return from the debug handler
> >>    without disabling preemption afterwards.
> >>
> >> The problem seems to be a merge fallout between three commits:
> >>
> >> commit 3d2a71a596bd9c761c8487a2178e95f8a61da083
> >> Author: Alexander van Heukelum <heukelum@fastmail.fm>
> >> Date:   Tue Sep 30 18:41:37 2008 +0200
> >>
> >>     x86, traps: converge do_debug handlers
> >>
> >> commit 08d68323d1f0c34452e614263b212ca556dae47f
> >> Author: K.Prasad <prasad@linux.vnet.ibm.com>
> >> Date:   Mon Jun 1 23:44:08 2009 +0530
> >>
> >>     hw-breakpoints: modifying generic debug exception to use thread-specific deb
> >>
> >> commit 787ecfaa503dc63ff1831ddc74b15dad49bace1d
> >> Author: Vegard Nossum <vegard.nossum@gmail.com>
> >> Date:   Fri Apr 4 00:53:23 2008 +0200
> >>
> >>     x86: add hooks for kmemcheck
> >>
> >> I encourage the kprobe developers to check whether their code is correct
> >> as it stands in current tip/master. Also, comments on this particular
> >> change is welcome.
> >>
> >> Reported-by: Ingo Molnar <mingo@elte.hu>
> >> Cc: Alexander van Heukelum <heukelum@fastmail.fm>
> >> Cc: K.Prasad <prasad@linux.vnet.ibm.com>
> >> Cc: Alan Stern <stern@rowland.harvard.edu>
> >> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> >> Cc: Pekka Enberg <penberg@cs.helsinki.fi>
> >> Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
> >> ---
> >>  arch/x86/kernel/traps.c |    8 ++++----
> >>  1 files changed, 4 insertions(+), 4 deletions(-)
> >
> >>
> >> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> >> index c8a7f87..a898c6b 100644
> >> --- a/arch/x86/kernel/traps.c
> >> +++ b/arch/x86/kernel/traps.c
> >> @@ -550,6 +550,10 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
> >>                                                       SIGTRAP) == NOTIFY_STOP)
> >>               return;
> >>
> >> +     /* Catch kmemcheck conditions first of all! */
> >> +     if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
> >> +             return;
> >> +
> >>       /* It's safe to allow irq's after DR6 has been saved */
> >>       preempt_conditional_sti(regs);
> >>
> >> @@ -559,10 +563,6 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
> >>               return;
> >>       }
> >>
> >> -     /* Catch kmemcheck conditions first of all! */
> >> -     if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
> >> -             return;
> >> -
> >>       /*
> >>        * Single-stepping through system calls: ignore any exceptions in
> >>        * kernel space, but re-enable TF when returning to user mode.
> >
> > Yeah - this could solve the crash i saw. Mind sending a pull request
> > too?
> 
> It did solve it, I tested it :-D
> 
> This patch was against tip/master; tip/kmemcheck does not have the
> problem. I think it might have been introduced in this merge:
> 
> commit 85b9b2801e46a147330b8a0f321bc40342ff5b4c
> Merge: bf8d9b3... 7387400...
> Author: Ingo Molnar <mingo@elte.hu>
> Date:   Thu Jun 4 13:56:43 2009 +0200
> 
>     Merge branch 'tracing/hw-breakpoints'
> 
>     Conflicts:
>         arch/x86/Kconfig
>         arch/x86/kernel/traps.c
>         kernel/Makefile

Ah. kmemcheck + hw-breakpoints interaction. Fun.

	Ingo