linux-kernel - Re: [RFC PATCH 55/56] x86/debug: Show return thunk in debugfs

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Message-ID: <20251027142419.1f04c7c4@pumpkin>
Date: Mon, 27 Oct 2025 14:24:19 +0000
From: David Laight <david.laight.linux@...il.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: David Kaplan <david.kaplan@....com>, Thomas Gleixner
 <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Peter Zijlstra
 <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta
 <pawan.kumar.gupta@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, Dave
 Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H . Peter Anvin"
 <hpa@...or.com>, Alexander Graf <graf@...zon.com>, Boris Ostrovsky
 <boris.ostrovsky@...cle.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 55/56] x86/debug: Show return thunk in debugfs

On Mon, 27 Oct 2025 14:29:59 +0200
Nikolay Borisov <nik.borisov@...e.com> wrote:

> On 10/13/25 17:34, David Kaplan wrote:
> > Make the value of x86_return_thunk visible in debugfs to support user-space
> > testing.
> > 
> > Signed-off-by: David Kaplan <david.kaplan@....com>  
> 
> Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>
> 
> > ---
> >   arch/x86/kernel/cpu/bugs.c | 44 ++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 44 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 26ceb42e0cfb..8365448b3aef 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -16,6 +16,7 @@
> >   #include <linux/sched/smt.h>
> >   #include <linux/pgtable.h>
> >   #include <linux/bpf.h>
> > +#include <linux/debugfs.h>
> >   
> >   #include <asm/spec-ctrl.h>
> >   #include <asm/cmdline.h>
> > @@ -4065,6 +4066,49 @@ void arch_cpu_reset_mitigations(void)
> >   	tsa_reset_mitigation();
> >   	vmscape_reset_mitigation();
> >   }
> > +
> > +static int rethunk_debug_show(struct seq_file *m, void *p)
> > +{
> > +	if (x86_return_thunk == __x86_return_thunk)
> > +		seq_puts(m, "__x86_return_thunk\n");
> > +	else if (x86_return_thunk == retbleed_return_thunk)
> > +		seq_puts(m, "retbleed_return_thunk\n");
> > +	else if (x86_return_thunk == call_depth_return_thunk)
> > +		seq_puts(m, "call_depth_return_thunk\n");
> > +	else if (x86_return_thunk == its_return_thunk)
> > +		seq_puts(m, "its_return_thunk\n");
> > +	else if (x86_return_thunk == srso_alias_return_thunk)
> > +		seq_puts(m, "srso_alias_return_thunk\n");
> > +	else if (x86_return_thunk == srso_return_thunk)
> > +		seq_puts(m, "srso_return_thunk\n");
> > +	else
> > +		seq_puts(m, "unknown\n");  
> 
> nit: This might be better suited for a switch construct but it's fine 
> either way.

That won't work - they are not integers.
eg:
#ifdef CONFIG_MITIGATION_UNRET_ENTRY
extern void retbleed_return_thunk(void);
#else
static inline void retbleed_return_thunk(void) {}
#endif

I'm not even sure you want to be testing anything against a static inline.

As coded the compiler might generate all 7 copies of seq_puts().
So better to use temporary for the result.

	David

> 
> <snip>
> 
>