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From:   Marco Elver <elver@google.com>
Date:   Wed, 1 Sep 2021 19:51:06 +0200
Message-ID: <CANpmjNOJN4yG8g+Qz0icppRK6coVdN5yKMhyTCQeBRFq7_YpfA@mail.gmail.com>
Subject: Re: [PATCH] s390/unwind: use current_frame_address() to unwind
 current task
To:     Vasily Gorbik <gor@linux.ibm.com>
Cc:     Heiko Carstens <hca@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev, qemu-s390x <qemu-s390x@nongnu.org>
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On Wed, 1 Sept 2021 at 16:06, Vasily Gorbik <gor@linux.ibm.com> wrote:
> current_stack_pointer() simply returns current value of %r15. If
> current_stack_pointer() caller allocates stack (which is the case in
> unwind code) %r15 points to a stack frame allocated for callees, meaning
> current_stack_pointer() caller (e.g. stack_trace_save) will end up in
> the stacktrace. This is not expected by stack_trace_save*() callers and
> causes problems.
>
> current_frame_address() on the other hand returns function stack frame
> address, which matches %r15 upon function invocation. Using it in
> get_stack_pointer() makes it more aligned with x86 implementation
> (according to BACKTRACE_SELF_TEST output) and meets stack_trace_save*()
> caller's expectations, notably KCSAN.
>
> Also make sure unwind_start is always inlined.
>
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Suggested-by: Marco Elver <elver@google.com>
> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Tested-by: Marco Elver <elver@google.com>

Thanks!

> ---
>  arch/s390/include/asm/stacktrace.h | 20 ++++++++++----------
>  arch/s390/include/asm/unwind.h     |  8 ++++----
>  2 files changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/arch/s390/include/asm/stacktrace.h b/arch/s390/include/asm/stacktrace.h
> index 3d8a4b94c620..22c41d7fd95c 100644
> --- a/arch/s390/include/asm/stacktrace.h
> +++ b/arch/s390/include/asm/stacktrace.h
> @@ -34,16 +34,6 @@ static inline bool on_stack(struct stack_info *info,
>         return addr >= info->begin && addr + len <= info->end;
>  }
>
> -static __always_inline unsigned long get_stack_pointer(struct task_struct *task,
> -                                                      struct pt_regs *regs)
> -{
> -       if (regs)
> -               return (unsigned long) kernel_stack_pointer(regs);
> -       if (task == current)
> -               return current_stack_pointer();
> -       return (unsigned long) task->thread.ksp;
> -}
> -
>  /*
>   * Stack layout of a C stack frame.
>   */
> @@ -74,6 +64,16 @@ struct stack_frame {
>         ((unsigned long)__builtin_frame_address(0) -                    \
>          offsetof(struct stack_frame, back_chain))
>
> +static __always_inline unsigned long get_stack_pointer(struct task_struct *task,
> +                                                      struct pt_regs *regs)
> +{
> +       if (regs)
> +               return (unsigned long) kernel_stack_pointer(regs);
> +       if (task == current)
> +               return current_frame_address();
> +       return (unsigned long) task->thread.ksp;
> +}
> +
>  /*
>   * To keep this simple mark register 2-6 as being changed (volatile)
>   * by the called function, even though register 6 is saved/nonvolatile.
> diff --git a/arch/s390/include/asm/unwind.h b/arch/s390/include/asm/unwind.h
> index de9006b0cfeb..5ebf534ef753 100644
> --- a/arch/s390/include/asm/unwind.h
> +++ b/arch/s390/include/asm/unwind.h
> @@ -55,10 +55,10 @@ static inline bool unwind_error(struct unwind_state *state)
>         return state->error;
>  }
>
> -static inline void unwind_start(struct unwind_state *state,
> -                               struct task_struct *task,
> -                               struct pt_regs *regs,
> -                               unsigned long first_frame)
> +static __always_inline void unwind_start(struct unwind_state *state,
> +                                        struct task_struct *task,
> +                                        struct pt_regs *regs,
> +                                        unsigned long first_frame)
>  {
>         task = task ?: current;
>         first_frame = first_frame ?: get_stack_pointer(task, regs);
> --
> 2.25.4