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    SubjectRe: [PATCH 1/2] x86: Ignore trap bits on single step exceptions
    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@redhat.com>
    > Signed-off-by: Frederic Weisbecker<fweisbec@gmail.com>
    > Cc: Rafael J. Wysocki<rjw@sisk.pl>
    > Cc: Maciej Rutecki<maciej.rutecki@gmail.com>
    > Cc: Alexandre Julliard<julliard@winehq.org>
    > Cc: Jason Wessel<jason.wessel@windriver.com>
    > Cc: All since 2.6.33.x<stable@kernel.org>
    > ---
    > 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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