linux-kernel - Re: [PATCH 11/12] unwind: Implement compat fp unwind

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Message-ID: <20251020103859.GO3419281@noisy.programming.kicks-ass.net>
Date: Mon, 20 Oct 2025 12:38:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: jpoimboe@...nel.org, rostedt@...nel.org,
	Indu Bhagat <indu.bhagat@...cle.com>, linux-kernel@...r.kernel.org,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH 11/12] unwind: Implement compat fp unwind

On Fri, Oct 17, 2025 at 05:47:26PM +0200, Jens Remus wrote:
> On 9/24/2025 9:59 AM, Peter Zijlstra wrote:
> 
> > --- a/include/linux/unwind_user_types.h
> > +++ b/include/linux/unwind_user_types.h
> > @@ -36,6 +36,7 @@ struct unwind_user_state {
> >  	unsigned long				ip;
> >  	unsigned long				sp;
> >  	unsigned long				fp;
> > +	unsigned int				ws;
> 
> Factoring out the word size (ws) from the CFA, FP, and RA offsets is
> clever.  Wondering though whether that would be an issue for unwind user
> sframe.  Do all architectures guarantee that those offsets are aligned
> to the native word size?

I would hope so, but this is all opt-in, I'm sure the first architecture
with an unaligned stack trying to support this will let us know ;-)

> >  	/* Make sure that the address is word aligned */
> > -	if (cfa & (sizeof(long) - 1))
> > +	if (cfa & (state->ws - 1))
> >  		return -EINVAL;
> 
> Alternatively using a state->ws of 1 in uwind user sframe would defeat
> this alignment check.

Indeed. Or rather, with a words size of 1, everything is aligned :-)