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

[Michael Ellerman]

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 :}

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.

cheers