[PATCH] powerpc: Clear branch trap (MSR.BE) before delivering SIGTRAP

Matt Evans matt at ozlabs.org
Wed Mar 28 21:36:03 AEDT 2018

Howdy Michael,

> On 28 Mar 2018, at 06:54, Michael Ellerman <mpe at ellerman.id.au> wrote:
> Matt Evans <matt at ozlabs.org> writes:
>> When using SIG_DBG_BRANCH_TRACING, MSR.BE is left enabled in the
>> user context when single_step_exception() prepares the SIGTRAP
>> delivery.  The resulting branch-trap-within-the-SIGTRAP-handler
>> isn't healthy.
>> Commit 2538c2d08f46141550a1e68819efa8fe31c6e3dc broke this, by
>> replacing an MSR mask operation of ~(MSR_SE | MSR_BE) with a call
>> to clear_single_step() which only clears MSR_SE.
>> This patch adds a new helper, clear_br_trace(), which clears the
>> debug trap before invoking the signal handler.  This helper is a
>> NOP for BookE as SIG_DBG_BRANCH_TRACING isn't supported on BookE.
>> Signed-off-by: Matt Evans <matt at ozlabs.org>
> Hi Matt!
> It seems we might not be regularly testing this code :}

I know, rite? ;-)

> How did you hit/find the bug? And do you have a test case by any chance?
> I found the test code at the bottom of:
>  https://lwn.net/Articles/114587/
> But it didn't immediately work.

I'm using this feature as part of a debug harness I wrote to log a program’s control flow (to create a “known good” pattern to compare a PPC interpreter against).  So at least the feature has /one/ user.  ;-)

The symptoms of the bug are that if you use single-stepping you get a sequence of SIGTRAPs representing each instruction completion (good), but if you use branch tracing the process just dies with SIGTRAP (looks like it’s never caught by the signal handler).  What’s really happening is that there /is/ a signal delivered to the handler, but (because branch tracing is left on) that then causes a second debug exception from the handler itself, i.e. whilst SIGTRAP’s masked.

OK, let me have a dig to reduce my program to something very basic and I’ll post something — sorry, I should’ve got a PoC ready before.  (I did start out inspired by that post you linked to, but IIRC I don’t think it worked out of the box for me either.)



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