Re: [PATCH v5 3/8] arm64: entry: add unwind info for various kernel entries

[Mark Rutland <mark.rutland@arm.com>]

Hi Dylan,

On Tue, Apr 28, 2026 at 06:36:38PM +0000, Dylan Hatch wrote:
> From: Weinan Liu <wnliu@google.com>
> 
> DWARF CFI (Call Frame Information) specifies how to recover the return
> address and callee-saved registers at each PC in a given function.
> Compilers are able to generate the CFI annotations when they compile
> the code to assembly language. For handcrafted assembly, we need to
> annotate them by hand.
> 
> Annotate minimal CFI to enable stacktracing using SFrame for kernel
> exception entries through el1*_64_*() paths 

I thought we were only consuming SFrame when unwinding an exeption
boundary?

We shouldn't be taking exceptions _from_ the entry assembly functions
unless something has gone horribly wrong, and so I don't see why we'd
need CFI entries for the entry assembly functions.

Am I missing some reason we need CFI entries for the entry assembly
functions? I strongly suspect it is not necessary to add these, and I'd
prefer to omit them.

> and irq entries through call_on_irq_stack()

Needing some sort of unwind annotations for call_on_irq_stack() makes
sense to me, but don't we need something for other assembly functions
too?

We can interrupt things like memset(); I assume we'll treat those as
unreliable until annotated?

Mark.

> Signed-off-by: Weinan Liu <wnliu@google.com>
> Suggested-by: Jens Remus <jremus@linux.ibm.com>
> Reviewed-by: Jens Remus <jremus@linux.ibm.com>
> Signed-off-by: Dylan Hatch <dylanbhatch@google.com>
> ---
>  arch/arm64/kernel/entry.S | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index f8018b5c1f9a..dc55b0b19cfa 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -30,6 +30,12 @@
>  #include <asm/asm-uaccess.h>
>  #include <asm/unistd.h>
>  
> +/*
> + * Do not generate .eh_frame.  Only generate .debug_frame and optionally
> + * .sframe (via assembler option --gsframe[-N]).
> + */
> +	.cfi_sections .debug_frame
> +
>  	.macro	clear_gp_regs
>  	.irp	n,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29
>  	mov	x\n, xzr
> @@ -575,7 +581,16 @@ SYM_CODE_START_LOCAL(el\el\ht\()_\regsize\()_\label)
>  	.if \el == 0
>  	b	ret_to_user
>  	.else
> +	/*
> +	 * Minimal DWARF CFI for unwinding across the call above.
> +	 * Enable unwinding for el1*_64_*() path only.
> +	 */
> +	.cfi_startproc
> +	.cfi_def_cfa_offset PT_REGS_SIZE
> +	.cfi_offset 29, S_FP - PT_REGS_SIZE
> +	.cfi_offset 30, S_LR - PT_REGS_SIZE
>  	b	ret_to_kernel
> +	.cfi_endproc
>  	.endif
>  SYM_CODE_END(el\el\ht\()_\regsize\()_\label)
>  	.endm
> @@ -872,6 +887,7 @@ NOKPROBE(ret_from_fork)
>   * Calls func(regs) using this CPU's irq stack and shadow irq stack.
>   */
>  SYM_FUNC_START(call_on_irq_stack)
> +	.cfi_startproc
>  	save_and_disable_daif x9
>  #ifdef CONFIG_SHADOW_CALL_STACK
>  	get_current_task x16
> @@ -882,6 +898,9 @@ SYM_FUNC_START(call_on_irq_stack)
>  	/* Create a frame record to save our LR and SP (implicit in FP) */
>  	stp	x29, x30, [sp, #-16]!
>  	mov	x29, sp
> +	.cfi_def_cfa 29, 16
> +	.cfi_offset 29, -16
> +	.cfi_offset 30, -8
>  
>  	ldr_this_cpu x16, irq_stack_ptr, x17
>  
> @@ -897,9 +916,13 @@ SYM_FUNC_START(call_on_irq_stack)
>  	 */
>  	mov	sp, x29
>  	ldp	x29, x30, [sp], #16
> +	.cfi_restore 29
> +	.cfi_restore 30
> +	.cfi_def_cfa 31, 0
>  	scs_load_current
>  	restore_irq x9
>  	ret
> +	.cfi_endproc
>  SYM_FUNC_END(call_on_irq_stack)
>  NOKPROBE(call_on_irq_stack)
>  
> -- 
> 2.54.0.545.g6539524ca2-goog
>