[PATCH v3 6/7] riscv: Add KPROBES_ON_FTRACE supported

[Masami Hiramatsu]

On Wed, 15 Jul 2020 00:24:54 +0800
Guo Ren <guoren at kernel.org> wrote:

> Thx Masami,
> 
> On Tue, Jul 14, 2020 at 7:38 PM Masami Hiramatsu <mhiramat at kernel.org> wrote:
> >
> > On Mon, 13 Jul 2020 23:39:21 +0000
> > guoren at kernel.org wrote:
> >
> > > From: Guo Ren <guoren at 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?

On x86, the fentry replaces the mcount with just one call instruction, without
saving any arguments. This means all probes which are puts on the address of
target symbol, are automatically using ftrace. IOW, all probes on _do_fork+0
will use ftrace. We don't need any disassembling.

I think if RISC-V already support "-fpatchable-function-entry=2" option on
GCC, you can easily enable it as same as arm64. See https://lkml.org/lkml/2019/6/18/648

Thank you,

-- 
Masami Hiramatsu <mhiramat at kernel.org>