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

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

On Wed, Nov 13, 2013 at 06:55:33AM +0000, Sandeepa Prabhu wrote:
> >>> I'm unsure about arm64's debug feature behavior, what does happen when
> >>> it performs a single-step on sw-breakpoint?
> >>>
> >>>> Sandeepa: I think you need to retry Masami's test on the arm64 model, since
> >>>> I'm fairly sure it won't work as expected without some additional code.
> >>>
> >>> OK, anyway, for testing same one, we need to port ftrace first. So the next
> >
> > Sorry for confusion, s/next/fallback is what I meant. Making a kprobe module
> > can be done without ftrace port.
> >
> >>> plan is to make a kprobe module to put a probe (which just printk something)
> >>> on a specific function (e.g. vfs_symlink), and run perf record with
> >>> hw-breakpoint as below
> >>>
> >>> $ perf record -e "mem:0xXXXXXX:k" ln -s /dev/null /tmp/foo
> >>>
> >>> Note that 0xXXXXXX is the address of vfs_symlink.
> >>>
> >>> After that, you can see the message in dmesg and also check the perf result
> >>> with "sudo perf script --dump" (you can find a PERF_RECORD_SAMPLE entry if
> >>> it works)
> Hi Will, Masami,
> 
> I am not sure of 'perf' right now (my minimal rootfs doesn't have) but
> I tried to test hardware breakpoints using sample modules
> "samples/hw_breakpoint/" on arm64 upstream branch. This should use
> same kernel api as perf I believe.
> 
> 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?

> 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).

Will