On Sat, Feb 25, 2017 at 01:34:15AM +0900, Masami Hiramatsu wrote: > > Ah, but that is only #PF, we also use __ex_table on other faults/traps, > > like #GP which would need help in do_general_protection(), > > #GP is also handled via kprobe_exceptions_notify().
Ah! that's where its hidden :-) Thanks! > > It looks like it rewrites regs->ip, which would make return from fault > > return to the wrong place, no? > > Hmm, when regs->ip is reset to the original place, kprobe_fault_handler() > returns 0 and normal #PF handler fixup pages etc. and retry from the > original place. This might kick kprobes again and do singlestep. > > So, yes, it may not enough for other faults if those will not only check > regs->ip, but read the instruction pointed by regs->ip (as your patch). > In that case you need to use recover_probed_instruction() instead of > probe_kernel_address(). (BTW, recover_probed_instruction() uses memcpy() > without checking kernel_text, it should use probe_kernel_address().) > > One more complication with __ex_table and optimized kprobes is that we > > need to be careful not to clobber __ex_table[].fixup. It would be very > > bad if the optimized probe were to clobber the address we let the fixup > > return to -- or that needs fixups too, _after_ running > > __ex_table[].handler(). > > can_optimize() takes care about that case. If the probe target function > (not only probed address) includes an exception address, it rejects > optimizing probes. Ah, so it does search_exception_tables(), which avoids clobbering actual exception instructions, and most fixups live in .text.fixup and I cannot actually find if that is excluded. In any case, thanks for the pointers, I'll see if I can spot any actual holes.
