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

[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?

	Ingo