Re: [PATCH v3 1/2] arm64: Implement arch_stack_walk_reliable

Andrea della Porta
2025-03-31T09:05:35Z

Hi song,

On 10:15 Thu 20 Mar , Song Liu wrote:
> With proper exception boundary detection, it is possible to implment
> arch_stack_walk_reliable without sframe.
>
> Note that, arch_stack_walk_reliable does not guarantee getting reliable
> stack in all scenarios. Instead, it can reliably detect when the stack
> trace is not reliable, which is enough to provide reliable livepatching.
>
> Signed-off-by: Song Liu <song@kernel.org>
> ---
> arch/arm64/Kconfig | 2 +-
> arch/arm64/kernel/stacktrace.c | 66 +++++++++++++++++++++++++---------
> 2 files changed, 51 insertions(+), 17 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 701d980ea921..31d5e1ee6089 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -276,6 +276,7 @@ config ARM64
> select HAVE_SOFTIRQ_ON_OWN_STACK
> select USER_STACKTRACE_SUPPORT
> select VDSO_GETRANDOM
> + select HAVE_RELIABLE_STACKTRACE
> help
> ARM 64-bit (AArch64) Linux support.
>
> @@ -2500,4 +2501,3 @@ endmenu # "CPU Power Management"
> source "drivers/acpi/Kconfig"
>
> source "arch/arm64/kvm/Kconfig"
> -
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 1d9d51d7627f..7e07911d8694 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -56,6 +56,7 @@ struct kunwind_state {
> enum kunwind_source source;
> union unwind_flags flags;
> struct pt_regs *regs;
> + bool end_on_unreliable;
> };
>
> static __always_inline void
> @@ -230,8 +231,26 @@ kunwind_next_frame_record(struct kunwind_state *state)
> new_fp = READ_ONCE(record->fp);
> new_pc = READ_ONCE(record->lr);
>
> - if (!new_fp && !new_pc)
> - return kunwind_next_frame_record_meta(state);
> + if (!new_fp && !new_pc) {
> + int ret;
> +
> + ret = kunwind_next_frame_record_meta(state);
> + if (ret < 0) {
> + /*
> + * This covers two different conditions:
> + * 1. ret == -ENOENT, unwinding is done.
> + * 2. ret == -EINVAL, unwinding hit error.
> + */
> + return ret;
> + }
> + /*
> + * Searching across exception boundaries. The stack is now
> + * unreliable.
> + */
> + if (state->end_on_unreliable)
> + return -EINVAL;
> + return 0;
> + }
>
> unwind_consume_stack(&state->common, info, fp, sizeof(*record));
>
> @@ -277,21 +296,24 @@ kunwind_next(struct kunwind_state *state)
>
> typedef bool (*kunwind_consume_fn)(const struct kunwind_state *state, void *cookie);
>
> -static __always_inline void
> +static __always_inline int
> do_kunwind(struct kunwind_state *state, kunwind_consume_fn consume_state,
> void *cookie)
> {
> - if (kunwind_recover_return_address(state))
> - return;
> + int ret;
>
> - while (1) {
> - int ret;
> + ret = kunwind_recover_return_address(state);
> + if (ret)
> + return ret;
>
> + while (1) {
> if (!consume_state(state, cookie))
> - break;
> + return -EINVAL;
> ret = kunwind_next(state);
> + if (ret == -ENOENT)
> + return 0;
> if (ret < 0)
> - break;
> + return ret;
> }
> }
>
> @@ -324,10 +346,10 @@ do_kunwind(struct kunwind_state *state, kunwind_consume_fn consume_state,
> : stackinfo_get_unknown(); \
> })
>
> -static __always_inline void
> +static __always_inline int
> kunwind_stack_walk(kunwind_consume_fn consume_state,
> void *cookie, struct task_struct *task,
> - struct pt_regs *regs)
> + struct pt_regs *regs, bool end_on_unreliable)
> {
> struct stack_info stacks[] = {
> stackinfo_get_task(task),
> @@ -348,11 +370,12 @@ kunwind_stack_walk(kunwind_consume_fn consume_state,
> .stacks = stacks,
> .nr_stacks = ARRAY_SIZE(stacks),
> },
> + .end_on_unreliable = end_on_unreliable,
> };
>
> if (regs) {
> if (task != current)
> - return;
> + return -EINVAL;
> kunwind_init_from_regs(&state, regs);
> } else if (task == current) {
> kunwind_init_from_caller(&state);
> @@ -360,7 +383,7 @@ kunwind_stack_walk(kunwind_consume_fn consume_state,
> kunwind_init_from_task(&state, task);
> }
>
> - do_kunwind(&state, consume_state, cookie);
> + return do_kunwind(&state, consume_state, cookie);
> }
>
> struct kunwind_consume_entry_data {
> @@ -384,7 +407,18 @@ noinline noinstr void arch_stack_walk(stack_trace_consume_fn consume_entry,
> .cookie = cookie,
> };
>
> - kunwind_stack_walk(arch_kunwind_consume_entry, &data, task, regs);
> + kunwind_stack_walk(arch_kunwind_consume_entry, &data, task, regs, false);
> +}
> +
> +noinline noinstr int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
> + void *cookie, struct task_struct *task)
> +{
> + struct kunwind_consume_entry_data data = {
> + .consume_entry = consume_entry,
> + .cookie = cookie,
> + };
> +
> + return kunwind_stack_walk(arch_kunwind_consume_entry, &data, task, NULL, true);
> }
>
> struct bpf_unwind_consume_entry_data {
> @@ -409,7 +443,7 @@ noinline noinstr void arch_bpf_stack_walk(bool (*consume_entry)(void *cookie, u6
> .cookie = cookie,
> };
>
> - kunwind_stack_walk(arch_bpf_unwind_consume_entry, &data, current, NULL);
> + kunwind_stack_walk(arch_bpf_unwind_consume_entry, &data, current, NULL, false);
> }
>
> static const char *state_source_string(const struct kunwind_state *state)
> @@ -456,7 +490,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
> return;
>
> printk("%sCall trace:\n", loglvl);
> - kunwind_stack_walk(dump_backtrace_entry, (void *)loglvl, tsk, regs);
> + kunwind_stack_walk(dump_backtrace_entry, (void *)loglvl, tsk, regs, false);
>
> put_task_stack(tsk);
> }
> --
> 2.47.1
>

Tested-by: Andrea della Porta <andrea.porta@suse.com>

Thanks,
Andrea