Re: [PATCH v10 08/11] arm64: Rename unwinder functions, prevent them from being traced and kprobed

Mark Rutland
2021-10-27T17:53:35Z

On Thu, Oct 14, 2021 at 09:58:44PM -0500, madvenka@linux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>
> Rename unwinder functions for consistency and better naming.
>
> - Rename start_backtrace() to unwind_start().
> - Rename unwind_frame() to unwind_next().
> - Rename walk_stackframe() to unwind().

This looks good to me.

Could we split this from the krpbes/tracing changes? I think this stands
on it's own, and (as below) the kprobes/tracing changes need some more
explanation, and would make sense as a separate patch.

> Prevent the following unwinder functions from being traced:
>
> - unwind_start()
> - unwind_next()
>
> unwind() is already prevented from being traced.

This could do with an explanation in the commis message as to why we
need to do this. If this is fixing a latent issue, it should be in a
preparatory patch that we can backport.

I dug into this a bit, and from taking a look, we prohibited ftrace in commit:

0c32706dac1b0a72 ("arm64: stacktrace: avoid tracing arch_stack_walk()")

... which is just one special case of graph return stack unbalancing,
and should be addressed by using HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, so
with the patch making us use HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, that's
no longer necessary.

So we no longer seem to have a specific reason to prohibit ftrace
here.

> Prevent the following unwinder functions from being kprobed:
>
> - unwind_start()
>
> unwind_next() and unwind() are already prevented from being kprobed.

Likewise, I think this needs some explanation. From diggin, we
prohibited kprobes in commit:

ee07b93e7721ccd5 ("arm64: unwind: Prohibit probing on return_address()")

... and the commit message says we need to do this because this is
(transitively) called by trace_hardirqs_off(), which is kprobes
blacklisted, but doesn't explain the actual problem this results in.

AFAICT x86 directly uses __builtin_return_address() here, but that won't
recover rewritten addresses, which seems like a bug (or at least a
limitation) on x86, assuming I've read that correctly.

Thanks,
Mark.

> Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> ---
> arch/arm64/kernel/stacktrace.c | 34 +++++++++++++++++++---------------
> 1 file changed, 19 insertions(+), 15 deletions(-)
>
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 7d32cee9ef4b..f4f3575f71fd 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -33,8 +33,8 @@
> */
>
>
> -static void start_backtrace(struct stackframe *frame, unsigned long fp,
> - unsigned long pc)
> +static void notrace unwind_start(struct stackframe *frame, unsigned long fp,
> + unsigned long pc)
> {
> frame->fp = fp;
> frame->pc = pc;
> @@ -45,7 +45,7 @@ static void start_backtrace(struct stackframe *frame, unsigned long fp,
> /*
> * Prime the first unwind.
> *
> - * In unwind_frame() we'll check that the FP points to a valid stack,
> + * In unwind_next() we'll check that the FP points to a valid stack,
> * which can't be STACK_TYPE_UNKNOWN, and the first unwind will be
> * treated as a transition to whichever stack that happens to be. The
> * prev_fp value won't be used, but we set it to 0 such that it is
> @@ -56,6 +56,8 @@ static void start_backtrace(struct stackframe *frame, unsigned long fp,
> frame->prev_type = STACK_TYPE_UNKNOWN;
> }
>
> +NOKPROBE_SYMBOL(unwind_start);
> +
> /*
> * Unwind from one frame record (A) to the next frame record (B).
> *
> @@ -63,8 +65,8 @@ static void start_backtrace(struct stackframe *frame, unsigned long fp,
> * records (e.g. a cycle), determined based on the location and fp value of A
> * and the location (but not the fp value) of B.
> */
> -static int notrace unwind_frame(struct task_struct *tsk,
> - struct stackframe *frame)
> +static int notrace unwind_next(struct task_struct *tsk,
> + struct stackframe *frame)
> {
> unsigned long fp = frame->fp;
> struct stack_info info;
> @@ -104,7 +106,7 @@ static int notrace unwind_frame(struct task_struct *tsk,
>
> /*
> * Record this frame record's values and location. The prev_fp and
> - * prev_type are only meaningful to the next unwind_frame() invocation.
> + * prev_type are only meaningful to the next unwind_next() invocation.
> */
> frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp));
> frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8));
> @@ -132,28 +134,30 @@ static int notrace unwind_frame(struct task_struct *tsk,
>
> return 0;
> }
> -NOKPROBE_SYMBOL(unwind_frame);
>
> -static void notrace walk_stackframe(struct task_struct *tsk,
> - unsigned long fp, unsigned long pc,
> - bool (*fn)(void *, unsigned long),
> - void *data)
> +NOKPROBE_SYMBOL(unwind_next);
> +
> +static void notrace unwind(struct task_struct *tsk,
> + unsigned long fp, unsigned long pc,
> + bool (*fn)(void *, unsigned long),
> + void *data)
> {
> struct stackframe frame;
>
> - start_backtrace(&frame, fp, pc);
> + unwind_start(&frame, fp, pc);
>
> while (1) {
> int ret;
>
> if (!fn(data, frame.pc))
> break;
> - ret = unwind_frame(tsk, &frame);
> + ret = unwind_next(tsk, &frame);
> if (ret < 0)
> break;
> }
> }
> -NOKPROBE_SYMBOL(walk_stackframe);
> +
> +NOKPROBE_SYMBOL(unwind);
>
> static bool dump_backtrace_entry(void *arg, unsigned long where)
> {
> @@ -208,7 +212,7 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
> fp = thread_saved_fp(task);
> pc = thread_saved_pc(task);
> }
> - walk_stackframe(task, fp, pc, consume_entry, cookie);
> + unwind(task, fp, pc, consume_entry, cookie);
>
> }
>
> --
> 2.25.1
>