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

[Andrea della Porta <andrea.porta@suse.com>]

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