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From:   Song Shuai <suagrfillet@gmail.com>
Date:   Tue, 22 Nov 2022 09:11:34 +0000
Message-ID: <CAAYs2=iSTTuthLhwiyw08yG2q=yLrdKPmvbG2rS7uibzhe-wXQ@mail.gmail.com>
Subject: Re: [PATCH] riscv/ftrace: fix ftrace_modify_call bug
To:     Guo Ren <guoren@kernel.org>
Cc:     rostedt@goodmis.org, mhiramat@kernel.org, mark.rutland@arm.com,
        paul.walmsley@sifive.com, palmer@dabbelt.com,
        aou@eecs.berkeley.edu, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org
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Guo Ren <guoren@kernel.org> =E4=BA=8E2022=E5=B9=B411=E6=9C=8822=E6=97=A5=E5=
=91=A8=E4=BA=8C 08:57=E5=86=99=E9=81=93=EF=BC=9A
>
> On Tue, Nov 22, 2022 at 3:54 PM Song Shuai <suagrfillet@gmail.com> wrote:
> >
> > With this commit (riscv: ftrace: Reduce the detour code size to half)
> > patched, ftrace bug occurred When hosting kprobe and function tracer
> > at the same function.
> >
> > Obviously, the variable caller in ftrace_modify_call was assigned by
> > rec->ip with 4 offset failing the code replacing at function entry.
> > And the caller should be assigned by rec->ip directly to indicate
> > the function entry.
> Thank you, it's my fault, but I think the problem is:
>
> Before (riscv: ftrace: Reduce the detour code size to half)
>      0: REG_S  ra, -SZREG(sp)
>      4: auipc  ra, ?          <- We need "rec->ip + 4" here
>      8: jalr   ?(ra)
>     12: REG_L  ra, -SZREG(sp)
>
> After (riscv: ftrace: Reduce the detour code size to half)
>      0: auipc  t0, ?  <- We needn't "rec->ip + 4" anymore
>      4: jalr   t0, ?(t0)
>
> I copied rec->ip + 4 blindly, then caused the bug. Right?
>
Yes, you're right.

Here's my simple test case for your convenience.
```
echo kernel_read > set_ftrace_filter
echo function > current_tracer
echo 'p:myp kernel_read' > kprobe_events
echo 1 > events/kprobes/myp/enable # ftrace bug
```
> >
> > The following is the ftrace bug log.
> >
> > ```
> > [  419.632855] 00000000f8776803: expected (ffe00297 1a4282e7) but got (=
1a8282e7 f0227179)
> > [  419.633390] ------------[ ftrace bug ]------------
> > [  419.633553] ftrace failed to modify
> > [  419.633569] [<ffffffff802091cc>] kernel_read+0x0/0x52
> > [  419.633863]  actual:   97:02:e0:ff:e7:82:82:1a
> > [  419.634087] Updating ftrace call site to call a different ftrace fun=
ction
> > [  419.634279] ftrace record flags: e0000002
> > [  419.634487]  (2) R
> > [  419.634487]  expected tramp: ffffffff800093cc
> > [  419.634935] ------------[ cut here ]------------
> > ```
> >
> > Signed-off-by: Song Shuai <suagrfillet@gmail.com>
> > ---
> >  arch/riscv/kernel/ftrace.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> > index 8c77f236fc71..61b24d767e2e 100644
> > --- a/arch/riscv/kernel/ftrace.c
> > +++ b/arch/riscv/kernel/ftrace.c
> > @@ -132,7 +132,7 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsi=
gned long old_addr,
> >                        unsigned long addr)
> >  {
> >         unsigned int call[2];
> > -       unsigned long caller =3D rec->ip + 4;
> > +       unsigned long caller =3D rec->ip;
> >         int ret;
> >
> >         make_call_t0(caller, old_addr, call);
> > --
> > 2.20.1
> >
>
>
> --
> Best Regards
>  Guo Ren
Thanks,
Song