arm64/hw_breakpoint: trigger breakpoint exception infinitely

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...

Quoted 22 lines · Li Bin

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
......
......

Quoted 4 lines

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

.