[PATCH] arm64: kprobes: Use BRK instead of single-step when executing instructions out-of-line

[Jean-Philippe Brucker]

On Fri, Oct 30, 2020 at 04:06:24PM +0000, Will Deacon wrote:
> On Thu, Oct 29, 2020 at 06:34:42PM +0100, Jean-Philippe Brucker wrote:
> > Commit 36dadef23fcc ("kprobes: Init kprobes in early_initcall") enabled
> > using kprobes from early_initcall. Unfortunately at this point the
> > hardware debug infrastructure is not operational. The OS lock may still
> > be locked, and the hardware watchpoints may have unknown values when
> > kprobe enables debug monitors to single-step instructions.
> > 
> > Rather than using hardware single-step, append a BRK instruction after
> > the instruction to be executed out-of-line.
> > 
> > Fixes: 36dadef23fcc ("kprobes: Init kprobes in early_initcall")
> > Suggested-by: Will Deacon <will at kernel.org>
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe at linaro.org>
> > ---
> > 
> > An alternative to fix [1]. I haven't done uprobes yet since they don't
> > suffer the same problem, but could add it to the bottom of my list.
> > Lightly tested with kprobe smoke test and the BPF selftests.
> > Interestingly on Seattle when running the "overhead" BPF selftest, that
> > triggers a kprobe a bunch of times, I see a 20-30% improvement with this
> > patch. I'm guessing it's the cost of touching the debug sysregs?
> > 
> > [1] https://lore.kernel.org/linux-arm-kernel/20201026172907.1468294-1-jean-philippe@linaro.org/
> > ---
> >  arch/arm64/include/asm/brk-imm.h        |  2 +
> >  arch/arm64/include/asm/debug-monitors.h |  1 +
> >  arch/arm64/include/asm/kprobes.h        |  2 +-
> >  arch/arm64/kernel/probes/kprobes.c      | 56 +++++++++----------------
> >  4 files changed, 24 insertions(+), 37 deletions(-)
> 
> Thanks! I'm about to send fixes for -rc2, but I'd like this to get a little
> bit of time in linux-next, so I aim to queue it next week for -rc3. Just
> wanted to make sure you don't think I'm ignored you.

No worries, I'm more confortable having this sit in linux-next

Thanks,
Jean