[BUG] arm64/hw_breakpoint: trigger breakpoint exception infinitely

[Will Deacon]

On Tue, Apr 26, 2016 at 04:23:19PM +0800, Li Bin wrote:
> This bug can be reproduced by compiling the hw_breakpoint sample
> module that is in the kernel tree, when loading it with parameter
> ksym=nfsdstats, and running nfsstat:
> 
> $ insmod data_breakpoint.ko ksym=nfsdstats
> $ nfsstat
> 
> It will trigger breakpoint exception infinitely:
> 
> nfsdstats value is changed
> ...
> Call trace:
> ...
> Dump stack from sample_hbp_handler
> nfsdstats value is changed
> ...
> Call trace:
> ...
> Dump stack from sample_hbp_handler
> ......
> ......
> 
> This is because that the overflow_handler is set to the callback
> in the module, and in watchpoint_handler, it will not disable the
> breakpoint (and set single step that reenable it), so when exception
> return, it will trigger the breakpoint exception again immediately...

As I explained before, GDB/ptrace *relies* on this behaviour. If you
register an breakpoint overflow handler on arm64, then you need to handle
the step.

Will