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

[Guo Ren]

Hi Masami,

Current riscv ftrace_caller utilize fp(s0) - 8 in stack to get ra of
function, eg:
foo:
   2bb0:       7119                    addi    sp,sp,-128
    2bb2:       f8a2                    sd      s0,112(sp)
    2bb4:       fc86                    sd      ra,120(sp)
...
    2bc4:       0100                    addi    s0,sp,128
...
0000000000002bca <.LVL828>:
    2bca:       00000097                auipc   ra,0x0
    2bce:       000080e7                jalr    ra # 2bca <.LVL828> //_mcount

So just put two nops before prologue of function isn't enough, because
riscv don't like arm64 which could use x9-x18 reserved regs to pass
ra(x30).
    | mov       x9, x30
    | bl        <ftrace-entry>
If the benefit is just making a kprobe on function symbol address to
prevent disassembling, I'll delay this feature.


I also have a look at HAVE_FENTRY & HAVE_NOP_MCOUNT. Seems it just
avoid using scripts/recordmcount.pl script and directly generate nops
for _mcount.
It's different from -fpatchable-function-entry=2 which generating nops
before function prologue in arm64, isn't it?

On Tue, Jul 21, 2020 at 9:27 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Wed, 15 Jul 2020 00:24:54 +0800
> Guo Ren <guoren@xxxxxxxxxx> wrote:
>
> > Thx Masami,
> >
> > On Tue, Jul 14, 2020 at 7:38 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> > >
> > > On Mon, 13 Jul 2020 23:39:21 +0000
> > > guoren@xxxxxxxxxx wrote:
> > >
> > > > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > > >
> > > > 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 & âï90 use fentryïcan 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@xxxxxxxxxx>



-- 
Best Regards
 Guo Ren

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