[PATCH RFC 2/6] arm64: Kprobes with single stepping support

[will.deacon@arm.com (Will Deacon)]

On Wed, Nov 13, 2013 at 03:55:42PM +0000, Sandeepa Prabhu wrote:
> On 13 November 2013 20:01, Will Deacon <will.deacon@arm.com> wrote:
> > On Wed, Nov 13, 2013 at 06:55:33AM +0000, Sandeepa Prabhu wrote:
> >> 1.  Placing watchpoint ( attr.bp_type = HW_BREAKPOINT_W |
> >> HW_BREAKPOINT_R) upon vfs_symlink symbol, but seems watch-point is not
> >> triggering at all.
> >
> > vfs_symlink is a function. Why would you expect to write it?
> This is generic test module (samples/hw_breakpoint/data_breakpoint.ko)
> which places watchpoint for bothe read/write.
> Atleast watchpt should have triggered for Read right? I also tried
> with othe functions like do_fork, vfs_read etc but no hit.

You'd need to place something for exec if you want to see anything on the
instruction side. A read by the instruction fetcher does not trigger a read
watchpoint on ARM.

> >> 2.  Placing text breakpoint (modified sample module with attr.bp_type
> >> = HW_BREAKPOINT_X) upon vfs_symlink, and run "ln -s /dev/null
> >> /tmp/foo".  This time, breakpoint hit but exception is re-cursing
> >> infinitely!
> >
> > The problem here is that we expect the overflow handler to deal with the
> > stepping (like GDB does via ptrace). If you don't register a handler, the
> > kernel will do the step (like you would get if you used perf stat -e
> > mem:0xNNNN:x).
> [This test was done on upstream branch, without kprobes patches.]
> Hmm, then this is expected with test breakpoint right? is this
> handling to be done by perf and ptrace?

perf stat doesn't register an overflow handler, so the hw_breakpoint
backend will handle the step. ptrace registers a handler which sends a
SIGTRAP to the debugger (e.g. gdb), which handles the step manually
(probably using a PTRACE_SINGLESTEP request).

> I did not see arm64 support in linux/tools/perf/,  there are multiple
> patches in mailing list though. Are you aware of any version of perf
> that work with arm64?

The perf tool should work fine on arm64 using mainline. Are you seeing
problems?

Will