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

On Mon, Dec 02, 2019 at 08:36:19AM -0800, Doug Anderson wrote:

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

quoted

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.

quoted

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

quoted

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?

I didn't test it, but I think that we'll convert the first watchpoint into a
mismatch breakpoint on arm32 and then when we resume execution, we'll hit
the subsequent watchpoint and so on until we actually manage to "step" the
instruction. On arm64, we'll use hardware step directly and therefore
disable all watchpoints prior to performing the step.

Will

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