Re: [PATCH 1/2] x86: Ignore trap bits on single step exceptions

[Michael Stefaniuc]

Hello Frederic,

On 11/13/2010 10:37 PM, Frederic Weisbecker wrote:
> When a single step exception fires, the trap bits, used to
> signal hardware breakpoints, are in a random state.
>
> These trap bits might be set if another exception will follow,
> like a breakpoint in the next instruction, or a watchpoint in the
> previous one. Or there can be any junk there.
>
> So if we handle these trap bits during the single step exception,
> we are going to handle an exception twice, or we are going to
> handle junk.
>
> Just ignore them in this case.
>
> This fixes https://bugzilla.kernel.org/show_bug.cgi?id=21332
sorry for the delay in testing this.

I have cherry-picked this patch on top of v2.6.37-rc3-102-gea49b16 and 
the ntdll/exception tests pass now.

Many thanks
bye
	michael

> Reported-by: Michael Stefaniuc<mstefani@xxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker<fweisbec@xxxxxxxxx>
> Cc: Rafael J. Wysocki<rjw@xxxxxxx>
> Cc: Maciej Rutecki<maciej.rutecki@xxxxxxxxx>
> Cc: Alexandre Julliard<julliard@xxxxxxxxxx>
> Cc: Jason Wessel<jason.wessel@xxxxxxxxxxxxx>
> Cc: All since 2.6.33.x<stable@xxxxxxxxxx>
> ---
>   arch/x86/kernel/hw_breakpoint.c |    4 ++++
>   1 files changed, 4 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> index ff15c9d..42c5942 100644
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -433,6 +433,10 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
>   	dr6_p = (unsigned long *)ERR_PTR(args->err);
>   	dr6 = *dr6_p;
>
> +	/* If it's a single step, TRAP bits are random */
> +	if (dr6&  DR_STEP)
> +		return NOTIFY_DONE;
> +
>   	/* Do an early return if no trap bits are set in DR6 */
>   	if ((dr6&  DR_TRAP_BITS) == 0)
>   		return NOTIFY_DONE;


--
Michael Stefaniuc                           Tel.: +49-711-96437-199
Consulting Communications Engineer          Fax.: +49-711-96437-111
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