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

* 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


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