linux-kernel - Re: [PATCH v3 10/10] LoongArch/ftrace: Fix unwind state when option func_stack

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Message-ID: <CAEr6+EBHrShLC9oWb0kVpy44gcHoQ-bnLoEN-bA0WR2VN0CLuw@mail.gmail.com>
Date:   Wed, 7 Sep 2022 22:00:54 +0800
From:   Jeff Xie <xiehuan09@...il.com>
To:     Qing Zhang <zhangqing@...ngson.cn>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Jiaxun Yang <jiaxun.yang@...goat.com>, hejinyang@...ngson.cn
Subject: Re: [PATCH v3 10/10] LoongArch/ftrace: Fix unwind state when option func_stack_trace

Hi Qing,

On Wed, Sep 7, 2022 at 7:56 PM Qing Zhang <zhangqing@...ngson.cn> wrote:
>
> Ftrace plays like function head exception, prologue analysis will stop soon
> because PC is at entry.
>
>    90000000004c5a54 <callee>:
>    90000000004c5a54:    03400000  andi   $zero, $zero, 0x0   ==>move  t0, ra
>    90000000004c5a58:    03400000  andi   $zero, $zero, 0x0   ==>bl callsite
> ==>90000000004c5a5c:    02fcc063  addi.d $sp, $sp, -208(0xf30)
>    ...
>
> When encountering ftrace_call, save trace function ra at PT_ERA, save parent ra
> at PT_R1, At this time, pc is the position after the two nops of callee. There is
> no conventional prologue operation between this position and the function entry,
> so we need to resetthe first flag to make the caller continue to unwind.
>
> testing method:
>   echo path_openat > ./set_ftrace_filter
>   echo 1 > ./options/func_stack_trace
>   echo function > ./current_tracer
>
> Signed-off-by: Qing Zhang <zhangqing@...ngson.cn>
> ---
>  arch/loongarch/include/asm/unwind.h     |  2 +-
>  arch/loongarch/kernel/unwind_prologue.c | 33 +++++++++++++++++++++----
>  2 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/arch/loongarch/include/asm/unwind.h b/arch/loongarch/include/asm/unwind.h
> index f66b07c3e6a1..f2b52b9ea93d 100644
> --- a/arch/loongarch/include/asm/unwind.h
> +++ b/arch/loongarch/include/asm/unwind.h
> @@ -20,7 +20,7 @@ struct unwind_state {
>         char type; /* UNWINDER_XXX */
>         struct stack_info stack_info;
>         struct task_struct *task;
> -       bool first, error;
> +       bool first, error, is_ftrace;
>         int graph_idx;
>         unsigned long sp, pc, ra;
>  };
> diff --git a/arch/loongarch/kernel/unwind_prologue.c b/arch/loongarch/kernel/unwind_prologue.c
> index 03f8b31a90cc..0bb19d3ae8ba 100644
> --- a/arch/loongarch/kernel/unwind_prologue.c
> +++ b/arch/loongarch/kernel/unwind_prologue.c
> @@ -14,9 +14,7 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
>
>         if (unwind_done(state))
>                 return 0;
> -       else if (state->type)
> -               return state->pc;
> -       else if (state->first)
> +       else if (state->type || state->first)
>                 return state->pc;
>
>         return *(unsigned long *)(state->sp);
> @@ -42,16 +40,39 @@ static bool unwind_by_guess(struct unwind_state *state)
>         return false;
>  }
>
> +static void unwind_state_fixup(struct unwind_state *state)
> +{
> +       static unsigned long ftrace_case = (unsigned long)ftrace_call + 4;
> +
> +       if (state->pc == ftrace_case)
> +               state->is_ftrace = true;
> +}
> +
>  static bool unwind_by_prologue(struct unwind_state *state)
>  {
>         struct stack_info *info = &state->stack_info;
>         union loongarch_instruction *ip, *ip_end;
>         unsigned long frame_size = 0, frame_ra = -1;
>         unsigned long size, offset, pc = state->pc;
> +       struct pt_regs *regs;
>
>         if (state->sp >= info->end || state->sp < info->begin)
>                 return false;
>
> +       if (state->is_ftrace) {
> +               /*
> +                * As we meet ftrace_regs_entry, reset first flag like first doing
> +                * tracing, Prologue analysis will stop soon because PC is at entry.
> +                */
> +               regs = (struct pt_regs *)state->sp;
> +               state->pc = regs->csr_era;
> +               state->ra = regs->regs[1];
> +               state->sp = regs->regs[3];
> +               state->first = true;
> +               state->is_ftrace = false;
> +               return true;
> +       }
> +
>         if (!kallsyms_lookup_size_offset(pc, &size, &offset))
>                 return false;
>
> @@ -97,7 +118,7 @@ static bool unwind_by_prologue(struct unwind_state *state)
>
>         state->pc = *(unsigned long *)(state->sp + frame_ra);
>         state->sp = state->sp + frame_size;
> -       return !!__kernel_text_address(state->pc);
> +       goto out;
>
>  first:
>         state->first = false;
> @@ -106,7 +127,9 @@ static bool unwind_by_prologue(struct unwind_state *state)
>
>         state->pc = state->ra;
>
> -       return !!__kernel_text_address(state->ra);
> +out:
> +       unwind_state_fixup(state);
> +       return !!__kernel_text_address(state->pc);
>  }
>
>  void unwind_start(struct unwind_state *state, struct task_struct *task,
> --
> 2.20.1
>

The patch looks good.

Tested-by: Jeff Xie <xiehuan09@...il.com>

-- 
Thanks,
JeffXie