linux-kernel - Re: [RFC PATCH v1 08/16] unwind_user: Enable archs that save RA/FP in other registers

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Message-ID: <oasyyga72yuiad7y2nzh7wcd7t7wmxnsbo2kuvsn5xsnuypewd@ukxxgdjbvegz>
Date: Wed, 16 Jul 2025 19:01:06 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, x86@...nel.org, Steven Rostedt <rostedt@...nel.org>, 
	Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, 
	Ilya Leoshkevich <iii@...ux.ibm.com>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, 
	Indu Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, 
	Beau Belgrave <beaub@...ux.microsoft.com>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Jens Axboe <axboe@...nel.dk>, 
	Florian Weimer <fweimer@...hat.com>, Sam James <sam@...too.org>
Subject: Re: [RFC PATCH v1 08/16] unwind_user: Enable archs that save RA/FP
 in other registers

On Thu, Jul 10, 2025 at 06:35:14PM +0200, Jens Remus wrote:
> +#ifndef unwind_user_get_reg
> +
> +/**
> + * generic_unwind_user_get_reg - Get register value.
> + * @val: Register value.
> + * @regnum: DWARF register number to obtain the value from.
> + *
> + * Returns zero if successful. Otherwise -EINVAL.
> + */
> +static inline int generic_unwind_user_get_reg(unsigned long *val, int regnum)
> +{
> +	return -EINVAL;
> +}
> +
> +#define unwind_user_get_reg generic_unwind_user_get_reg
> +
> +#endif /* !unwind_user_get_reg */

I believe the traditional way to do this is to give the function the
same name as the define:

#ifndef unwind_user_get_reg
static inline int unwind_user_get_reg(unsigned long *val, int regnum)
{
	return -EINVAL;
}
#define unwind_user_get_reg unwind_user_get_reg
#endif

> +/**
> + * generic_sframe_set_frame_reginfo - Populate info to unwind FP/RA register
> + * from SFrame offset.
> + * @reginfo: Unwind info for FP/RA register.
> + * @offset: SFrame offset value.
> + *
> + * A non-zero offset value denotes a stack offset from CFA and indicates
> + * that the register is saved on the stack. A zero offset value indicates
> + * that the register is not saved.
> + */
> +static inline void generic_sframe_set_frame_reginfo(
> +	struct unwind_user_reginfo *reginfo,
> +	s32 offset)
> +{
> +	if (offset) {
> +		reginfo->loc = UNWIND_USER_LOC_STACK;
> +		reginfo->frame_off = offset;
> +	} else {
> +		reginfo->loc = UNWIND_USER_LOC_NONE;
> +	}
> +}

This just inits the reginfo struct, can we call it sframe_init_reginfo()?

Also the function comment seems completely superfluous as the function
is completely obvious.

Also the signature should match kernel style, something like:

static inline void
sframe_init_reginfo(struct unwind_user_reginfo *reginfo, s32 offset)

> @@ -98,26 +98,57 @@ static int unwind_user_next(struct unwind_user_state *state)
>  
>  
>  	/* Get the Return Address (RA) */
> -	if (frame->ra_off) {
> +	switch (frame->ra.loc) {
> +	case UNWIND_USER_LOC_NONE:
> +		if (!IS_ENABLED(CONFIG_HAVE_USER_RA_REG) || !topmost)
> +			goto done;
> +		ra = user_return_address(task_pt_regs(current));
> +		break;
> +	case UNWIND_USER_LOC_STACK:
> +		if (!frame->ra.frame_off)
> +			goto done;
>  		/* Make sure that the address is word aligned */
>  		shift = sizeof(long) == 4 || compat_fp_state(state) ? 2 : 3;
> -		if ((cfa + frame->ra_off) & ((1 << shift) - 1))
> +		if ((cfa + frame->ra.frame_off) & ((1 << shift) - 1))
>  			goto done;
> -		if (unwind_get_user_long(ra, cfa + frame->ra_off, state))
> +		if (unwind_get_user_long(ra, cfa + frame->ra.frame_off, state))
>  			goto done;
> -	} else {
> -		if (!IS_ENABLED(CONFIG_HAVE_USER_RA_REG) || !topmost)
> +		break;
> +	case UNWIND_USER_LOC_REG:
> +		if (!IS_ENABLED(CONFIG_HAVE_UNWIND_USER_LOC_REG) || !topmost)
>  			goto done;
> -		ra = user_return_address(task_pt_regs(current));
> +		if (unwind_user_get_reg(&ra, frame->ra.regnum))
> +			goto done;
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
> +		goto done;

The default case will never happen, can we make it a BUG()?

>  	}
>  
>  	/* Get the Frame Pointer (FP) */
> -	if (frame->fp_off && unwind_get_user_long(fp, cfa + frame->fp_off, state))
> +	switch (frame->fp.loc) {
> +	case UNWIND_USER_LOC_NONE:
> +		break;

The UNWIND_USER_LOC_NONE behavior is different here compared to above.
Do we also need UNWIND_USER_LOC_PT_REGS?

> +	case UNWIND_USER_LOC_STACK:
> +		if (!frame->fp.frame_off)
> +			goto done;
> +		if (unwind_get_user_long(fp, cfa + frame->fp.frame_off, state))
> +			goto done;
> +		break;
> +	case UNWIND_USER_LOC_REG:
> +		if (!IS_ENABLED(CONFIG_HAVE_UNWIND_USER_LOC_REG) || !topmost)
> +			goto done;

The topmost checking is *really* getting cumbersome, I do hope we can
get rid of that.

> +		if (unwind_user_get_reg(&fp, frame->fp.regnum))
> +			goto done;
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
>  		goto done;
> +	}

BUG(1) here as well.

>  	state->ip = ra;
>  	state->sp = sp;
> -	if (frame->fp_off)
> +	if (frame->fp.loc != UNWIND_USER_LOC_NONE)
>  		state->fp = fp;

Instead of the extra conditional here, can fp be initialized to zero?
Either at the top of the function or in the RA switch statement?

-- 
Josh