Re: [PATCH v4 4/9] fprobe: rethook: Use ftrace_regs in fprobe exit handler and rethook

Masami Hiramatsu (Google) [off-list ref] writes:

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IOW, it is ftrace save regs/restore regs code issue. I need to check how the
function_graph implements it.

gpr2-gpr14 are always saved in ftrace_caller/ftrace_regs_caller(),
regardless of the FTRACE_WITH_REGS flags. The only difference is that
without the FTRACE_WITH_REGS flag the program status word (psw) is not
saved because collecting that is a rather expensive operation.

Thanks for checking that! So s390 will recover those saved registers
even if FTRACE_WITH_REGS flag is not set? (I wonder what is the requirement
of the ftrace_regs when returning from ftrace_call() without
FTRACE_WITH_REGS?)

Yes, it will recover these in all cases.

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I used the following commands to test rethook (is that the correct
testcase?)

#!/bin/bash
cd /sys/kernel/tracing

echo 'r:icmp_rcv icmp_rcv' >kprobe_events
echo 1 >events/kprobes/icmp_rcv/enable
ping -c 1 127.0.0.1
cat trace

No, the kprobe will path pt_regs to rethook.
Cna you run

echo "f:icmp_rcv%return icmp_rcv" >> dynamic_events

Ah, ok. Seems to work as well:

  ping-481     [001] ..s2.    53.918480: icmp_rcv: (ip_protocol_deliver_rcu+0x42/0x218 <- icmp_rcv)
  ping-481     [001] ..s2.    53.918491: icmp_rcv: (ip_protocol_deliver_rcu+0x42/0x218 <- icmp_rcv)