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SubjectRe: [PATCH] riscv/ftrace: fix ftrace_modify_call bug
Guo Ren <guoren@kernel.org> 于2022年11月22日周二 08:57写道:
>
> 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 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, unsigned long old_addr,
> > unsigned long addr)
> > {
> > unsigned int call[2];
> > - unsigned long caller = rec->ip + 4;
> > + unsigned long caller = rec->ip;
> > int ret;
> >
> > make_call_t0(caller, old_addr, call);
> > --
> > 2.20.1
> >
>
>
> --
> Best Regards
> Guo Ren
Thanks,
Song

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