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From:   Song Shuai <suagrfillet@gmail.com>
Date:   Tue, 22 Nov 2022 09:51:11 +0000
Message-ID: <CAAYs2=gBfKPBMBVpkOYfixavkANhBnArSdCdHDgv8219sR4BzA@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 09:29=E5=86=99=E9=81=93=EF=BC=9A
>
>
>
> On Tue, Nov 22, 2022 at 5:11 PM Song Shuai <suagrfillet@gmail.com> wrote:
>>
>> 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> wro=
te:
>> > >
>> > > 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
>
>
> Thx, I would fold the patch into (riscv: ftrace: Reduce the detour code s=
ize to half). And add Co-developed-by: Song Shuai <suagrfillet@gmail.com> t=
ag on that.
>
> I would send the v3 series involving your patches.
>
That's an honor for me.

Thanks,
Song
>>
>> ```
>> > >
>> > > The following is the ftrace bug log.
>> > >
>> > > ```
>> > > [  419.632855] 00000000f8776803: expected (ffe00297 1a4282e7) but go=
t (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 =
function
>> > > [  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, u=
nsigned 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
>
>
>
> --
> Best Regards
>  Guo Ren