Re: [PATCH v15 4/6] arm64: Introduce stack trace reliability checks in the unwinder

Mark Rutland
2022-06-26T08:33:31Z

On Fri, Jun 17, 2022 at 04:07:15PM -0500, madvenka@linux.microsoft.com wrote:
> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>
> There are some kernel features and conditions that make a stack trace
> unreliable. Callers may require the unwinder to detect these cases.
> E.g., livepatch.
>
> Introduce a new function called unwind_check_reliability() that will
> detect these cases and set a flag in the stack frame. Call
> unwind_check_reliability() for every frame in unwind().
>
> Introduce the first reliability check in unwind_check_reliability() - If
> a return PC is not a valid kernel text address, consider the stack
> trace unreliable. It could be some generated code. Other reliability checks
> will be added in the future.
>
> Let unwind() return a boolean to indicate if the stack trace is
> reliable.
>
> Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> Reviewed-by: Mark Brown <broonie@kernel.org>
> ---
> arch/arm64/kernel/stacktrace.c | 31 +++++++++++++++++++++++++++++--
> 1 file changed, 29 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index c749129aba5a..5ef2ce217324 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -44,6 +44,8 @@
> * @final_fp: Pointer to the final frame.
> *
> * @failed: Unwind failed.
> + *
> + * @reliable: Stack trace is reliable.
> */

I would strongly prefer if we could have something like an
unwind_state_is_reliable() helper, and just use that directly, rather than
storing that into the state.

That way, we can opt-into any expensive checks in the reliable unwinder (e.g.
__kernel_text_address), and can use them elsewhere for informative purposes
(e.g. when dumping a stacktrace out to the console).

> struct unwind_state {
> unsigned long fp;
> @@ -57,6 +59,7 @@ struct unwind_state {
> struct task_struct *task;
> unsigned long final_fp;
> bool failed;
> + bool reliable;
> };
>
> static void unwind_init_common(struct unwind_state *state,
> @@ -80,6 +83,7 @@ static void unwind_init_common(struct unwind_state *state,
> state->prev_fp = 0;
> state->prev_type = STACK_TYPE_UNKNOWN;
> state->failed = false;
> + state->reliable = true;
>
> /* Stack trace terminates here. */
> state->final_fp = (unsigned long)task_pt_regs(task)->stackframe;
> @@ -242,11 +246,34 @@ static void notrace unwind_next(struct unwind_state *state)
> }
> NOKPROBE_SYMBOL(unwind_next);
>
> -static void notrace unwind(struct unwind_state *state,
> +/*
> + * Check the stack frame for conditions that make further unwinding unreliable.
> + */
> +static void unwind_check_reliability(struct unwind_state *state)
> +{
> + if (state->fp == state->final_fp) {
> + /* Final frame; no more unwind, no need to check reliability */
> + return;
> + }
> +
> + /*
> + * If the PC is not a known kernel text address, then we cannot
> + * be sure that a subsequent unwind will be reliable, as we
> + * don't know that the code follows our unwind requirements.
> + */
> + if (!__kernel_text_address(state->pc))
> + state->reliable = false;
> +}

I'd strongly prefer that we split this into two helpers, e.g.

static inline bool unwind_state_is_final(struct unwind_state *state)
{
return state->fp == state->final_fp;
}

static inline bool unwind_state_is_reliable(struct unwind_state *state)
{
return __kernel_text_address(state->pc);
}

> +
> +static bool notrace unwind(struct unwind_state *state,
> stack_trace_consume_fn consume_entry, void *cookie)
> {
> - while (unwind_continue(state, consume_entry, cookie))
> + unwind_check_reliability(state);
> + while (unwind_continue(state, consume_entry, cookie)) {
> unwind_next(state);
> + unwind_check_reliability(state);

This is going to slow down regular unwinds even when the reliablity value is
not consumed (e.g. for KASAN traces on alloc and free), so I don't think this
should live here, and should be intreoduced with arch_stack_walk_reliable().

Thanks,
Mark.

> + }
> + return !state->failed && state->reliable;
> }
> NOKPROBE_SYMBOL(unwind);
>
> --
> 2.25.1