linux-kernel - Re: [PATCH] riscv/ftrace: fix ftrace_modify

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Date:   Tue, 22 Nov 2022 16:57:07 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Song Shuai <suagrfillet@...il.com>
Cc:     rostedt@...dmis.org, mhiramat@...nel.org, mark.rutland@....com,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv/ftrace: fix ftrace_modify_call bug

On Tue, Nov 22, 2022 at 3:54 PM Song Shuai <suagrfillet@...il.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?

>
> 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@...il.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