Re: [PATCH] ARM: hw_breakpoint: Handle inexact watchpoint addresses

Hi,

On Wed, Nov 20, 2019 at 11:18 AM Will Deacon [off-list ref] wrote:

On Sat, Oct 19, 2019 at 11:12:26AM -0700, Douglas Anderson wrote:

quoted

This is commit fdfeff0f9e3d ("arm64: hw_breakpoint: Handle inexact
watchpoint addresses") but ported to arm32, which has the same
problem.

This problem was found by Android CTS tests, notably the
"watchpoint_imprecise" test [1].  I tested locally against a copycat
(simplified) version of the test though.

[1] https://android.googlesource.com/platform/bionic/+/master/tests/sys_ptrace_test.cpp

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

 arch/arm/kernel/hw_breakpoint.c | 96 ++++++++++++++++++++++++---------
 1 file changed, 70 insertions(+), 26 deletions(-)

Sorry for taking so long to look at this. After wrapping my head around the
logic again

Yeah.  It was a little weird and (unfortunately) arbitrarily different
in some places compared to the arm64 code.

I think it looks fine, so please put it into the patch system
with my Ack:

Acked-by: Will Deacon <will@kernel.org>

Thanks!  Submitted as:

https://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=8944/1

One interesting difference between the implementation here and the arm64
code is that I think if you have multiple watchpoints, all of which fire
with a distance != 0, then arm32 will actually report them all whereas
you'd only get one on arm64.

Are you sure about that?  The "/* No exact match found. */" code is
outside the for loop so it should only be able to trigger for exactly
one breakpoint, no?

-Doug

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