linux-kernel - Re: [RFC PATCH v1 07/16] unwind_user: Enable archs that do not necessarily save RA

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Message-ID: <v6gwx63cd5divimoadofeuz2vn72uw7zrlcjacufaedeuxbvjc@qmobatrxo66u>
Date: Fri, 18 Jul 2025 09:59:59 -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 07/16] unwind_user: Enable archs that do not
 necessarily save RA

On Fri, Jul 18, 2025 at 10:28:32AM +0200, Jens Remus wrote:
> On 17.07.2025 13:09, Jens Remus wrote:
> > On 17.07.2025 01:01, Josh Poimboeuf wrote:
> >> On Thu, Jul 10, 2025 at 06:35:13PM +0200, Jens Remus wrote:
> >>> +++ b/arch/Kconfig
> >>> @@ -450,6 +450,11 @@ config HAVE_UNWIND_USER_SFRAME
> >>>  	bool
> >>>  	select UNWIND_USER
> >>>  
> >>> +config HAVE_USER_RA_REG
> >>> +	bool
> >>> +	help
> >>> +	  The arch passes the return address (RA) in user space in a register.
> >>
> >> How about "HAVE_UNWIND_USER_RA_REG" so it matches the existing
> >> namespace?
> > 
> > Ok.  I am open to any improvements.
> 
> Thinking about this again I realized that the config option actually
> serves two purposes:
> 
> 1. Enable code (e.g. unwind user) to determine the presence of the new
>    user_return_address().  That is where I derived the name from.
> 2. Enable unwind user (sframe) to behave differently, if an architecture
>    has/uses a RA register (unlike x86, which solely uses the stack).

The sframe CONFIG_HAVE_USER_RA_REG check is redundant with the
unwind_user one, no?  I'm thinking it's better for sframe to just decode
the entry as it is, and then let unwind_user validate things.

> I think the primary notion is that an architecture has/uses a register
> for the return address and thus provides user_return_address().  What
> consumers such as unwind user do with that info is secondary.
> 
> Thoughts?

user_return_address() only has the single user, and is not all that
generically useful anyway (e.g., it warns on x86), so let's keep it
encapsulated in include/linux/unwind_user.h and give it the
"unwind_user" prefix.

Also, "RA_REG" is a bit ambiguous, it sounds almost like that other
option which spills RA to another register.  Conceptually, it's a link
register, so can we rename that to CONFIG_HAVE_UNWIND_USER_LINK_REG and
unwind_user_get_link_reg() or so?

Similarly, CONFIG_HAVE_UNWIND_USER_LOC_REG isn't that descriptive, how
about CONFIG_HAVE_UNWIND_USER_LINK_REG_SPILL?

Also we can get rid of the '#define func_name func_name' things and just
guard those functions with their corresponding CONFIG options in
inclide/linux/unwind_user.h.

Also those two functions should have similar naming and prototypes.

For example, in include/linux/unwind_user.h:

#ifndef CONFIG_HAVE_UNWIND_USER_LINK_REG
int unwind_user_get_link_reg(unsigned long *val)
{
	WARN_ON_ONCE(1);
	return -EINVAL;
}
#endif

#ifndef CONFIG_HAVE_UNWIND_USER_LINK_REG_SPILL
int unwind_user_get_reg(unsigned long *val, unsigned int regnum)
{
	WARN_ON_ONCE(1);
	return -EINVAL;
}
#endif

Then the code can be simplified (assuming no topmost checks):

	/* Get the Return Address (RA) */
	switch (frame->ra.loc) {
	case UNWIND_USER_LOC_NONE:
		if (unwind_user_get_link_reg(&ra))
			goto done;
		break;
	...
	case UNWIND_USER_LOC_REG:
		if (unwind_user_get_reg(&ra, frame->ra.regnum))
			goto done;
		break;
	...

-- 
Josh