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SubjectRe: [PATCH v3 6/7] riscv: Add KPROBES_ON_FTRACE supported
Thx Masami,

On Tue, Jul 14, 2020 at 7:38 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> On Mon, 13 Jul 2020 23:39:21 +0000
> guoren@kernel.org wrote:
>
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > This patch adds support for kprobes on ftrace call sites to avoids
> > much of the overhead with regular kprobes. Try it with simple
> > steps:
> >
> > 1. Get _do_fork ftrace call site.
> > Dump of assembler code for function _do_fork:
> > 0xffffffe00020af64 <+0>: addi sp,sp,-128
> > 0xffffffe00020af66 <+2>: sd s0,112(sp)
> > 0xffffffe00020af68 <+4>: sd ra,120(sp)
> > 0xffffffe00020af6a <+6>: addi s0,sp,128
> > 0xffffffe00020af6c <+8>: sd s1,104(sp)
> > 0xffffffe00020af6e <+10>: sd s2,96(sp)
> > 0xffffffe00020af70 <+12>: sd s3,88(sp)
> > 0xffffffe00020af72 <+14>: sd s4,80(sp)
> > 0xffffffe00020af74 <+16>: sd s5,72(sp)
> > 0xffffffe00020af76 <+18>: sd s6,64(sp)
> > 0xffffffe00020af78 <+20>: sd s7,56(sp)
> > 0xffffffe00020af7a <+22>: mv s4,a0
> > 0xffffffe00020af7c <+24>: mv a0,ra
> > 0xffffffe00020af7e <+26>: nop <<<<<<<< here!
> > 0xffffffe00020af82 <+30>: nop
> > 0xffffffe00020af86 <+34>: ld s3,0(s4)
> >
> > 2. Set _do_fork+26 as the kprobe.
> > echo 'p:myprobe _do_fork+26 dfd=%a0 filename=%a1 flags=%a2 mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
> > echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> > cat /sys/kernel/debug/tracing/trace
> > tracer: nop
> >
> > entries-in-buffer/entries-written: 3/3 #P:1
> >
> > _-----=> irqs-off
> > / _----=> need-resched
> > | / _---=> hardirq/softirq
> > || / _--=> preempt-depth
> > ||| / delay
> > TASK-PID CPU# |||| TIMESTAMP FUNCTION
> > | | | |||| | |
> > sh-87 [000] .... 551.557031: myprobe: (_do_fork+0x1a/0x2e6) dfd=0xffffffe00020af7e filename=0xffffffe00020b34e flags=0xffffffe00101e7c0 mode=0x20af86ffffffe0
> >
> > cat /sys/kernel/debug/kprobes/list
> > ffffffe00020af7e k _do_fork+0x1a [FTRACE]
> > ^^^^^^
>
> Hmm, this seems fentry is not supported on RISC-V yet. But anyway,
> it will be useful for users (if they can find the offset).

Seems only x86 & ⬆️90 use fentry,can you elaborate more about fentry's
benefit and how the user could set kprobe on ftrace call site without
disassemble?

--
Best Regards
Guo Ren

ML: https://lore.kernel.org/linux-csky/

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