Re: [PATCH 5/8] arm64: Recover kretprobe modified return address in stacktrace

[Will Deacon]

On Fri, Oct 08, 2021 at 09:28:58PM +0900, Masami Hiramatsu wrote:
> Since the kretprobe replaces the function return address with
> the kretprobe_trampoline on the stack, stack unwinder shows it
> instead of the correct return address.
> 
> This checks whether the next return address is the
> __kretprobe_trampoline(), and if so, try to find the correct
> return address from the kretprobe instance list.
> 
> With this fix, now arm64 can enable
> CONFIG_ARCH_CORRECT_STACKTRACE_ON_KRETPROBE, and pass the
> kprobe self tests.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>  arch/arm64/Kconfig                  |    1 +
>  arch/arm64/include/asm/stacktrace.h |    2 ++
>  arch/arm64/kernel/stacktrace.c      |    3 +++
>  3 files changed, 6 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 5c7ae4c3954b..edde5171ffb2 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -11,6 +11,7 @@ config ARM64
>  	select ACPI_PPTT if ACPI
>  	select ARCH_HAS_DEBUG_WX
>  	select ARCH_BINFMT_ELF_STATE
> +	select ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
>  	select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION
>  	select ARCH_ENABLE_MEMORY_HOTPLUG
>  	select ARCH_ENABLE_MEMORY_HOTREMOVE
> diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
> index 8aebc00c1718..8f997a602651 100644
> --- a/arch/arm64/include/asm/stacktrace.h
> +++ b/arch/arm64/include/asm/stacktrace.h
> @@ -9,6 +9,7 @@
>  #include <linux/sched.h>
>  #include <linux/sched/task_stack.h>
>  #include <linux/types.h>
> +#include <linux/llist.h>
>  
>  #include <asm/memory.h>
>  #include <asm/ptrace.h>
> @@ -59,6 +60,7 @@ struct stackframe {
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  	int graph;
>  #endif
> +	struct llist_node *kr_cur;
>  };

Please update the comment above this structure to describe the new member
you're adding. If it's only relevant for kprobes, then let's define it
conditionally too (based on CONFIG_KRETPROBES ?)

>  extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
> diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> index 8982a2b78acf..f1eef5745542 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -129,6 +129,8 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>  		frame->pc = ret_stack->ret;
>  	}
>  #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> +	if (is_kretprobe_trampoline(frame->pc))
> +		frame->pc = kretprobe_find_ret_addr(tsk, (void *)frame->fp, &frame->kr_cur);
>  
>  	frame->pc = ptrauth_strip_insn_pac(frame->pc);
>  
> @@ -224,6 +226,7 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
>  {
>  	struct stackframe frame;
>  
> +	memset(&frame, 0, sizeof(frame));

Why do we need this?

Will