Messages in this thread | | | Subject | Re: [PATCH] ftrace: enable trampoline when rec count decrement to one | From | "chengjian (D)" <> | Date | Wed, 8 May 2019 23:02:33 +0800 |
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Hi, Steven
On 2019/5/6 3:34, Steven Rostedt wrote: > > Thanks for the patch. There was some race condition that prevented me > from doing this in the first place, but unfortunately, I don't remember > what that was :-/ > > I'll have to think about this before applying this patch. > > Maybe there isn't a race condition, and I was just playing it safe, as > there was a race condition between switching from regs caller back to > non regs caller. > > -- Steve > .
function tracer uses ftrace_caller() and livepatch uses ftrace_regs_caller().
I can modify my testcase to trigger this race condition.
#enable livepatch insmod klp_unshare_files.ko cat /sys/kernel/debug/tracing/enabled_functions unshare_files (1) R I tramp: 0xffffffffc0008000 (klp_ftrace_handler+0x0/0xa0) ->ftrace_ops_assist_func+0x0/0xf0 [NOW, the rec caller is ftracer_regs_caller TRAMPOLINE]
#function tracer echo unshare_files > /sys/kernel/debug/tracing/set_ftrace_filter echo function > /sys/kernel/debug/tracing/current_tracer cat /sys/kernel/debug/tracing/enabled_functions unshare_files (2) R I ->ftrace_ops_list_func+0x0/0x170 [NOW, the rec caller is ftracer_regs_caller]
# disable livepatch echo 0 > /sys/kernel/livepatch/klp_unshare_files/enabled rmmod klp_unshare_files
cat /sys/kernel/debug/tracing/enabled_functions unshare_files (1) tramp: 0xffffffffc0005000 (function_trace_call+0x0/0x120) ->function_trace_call+0x0/0x120 [NOW, the rec caller is ftrace_caller TRAMPOLINE]
So, the caller switch from regs caller back to non regs caller when disable the livepatch. Could this testcase cause the race condition ? BUT, Nothing happened here.
What will happen when the race triggers ? What can I do.
Thanks.
Cheng Jian.
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