linux-kernel - Re: [PATCH] unwind deferred: Annotate struct unwind_cache with __counted

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Message-ID: <20251114143104.0d1aead3@pumpkin>
Date: Fri, 14 Nov 2025 14:31:04 +0000
From: David Laight <david.laight.linux@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Thorsten Blum <thorsten.blum@...ux.dev>, Josh Poimboeuf
 <jpoimboe@...nel.org>, Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva"
 <gustavoars@...nel.org>, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] unwind deferred: Annotate struct unwind_cache with
 __counted_by

On Fri, 14 Nov 2025 08:43:46 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 14 Nov 2025 13:27:47 +0100
> Thorsten Blum <thorsten.blum@...ux.dev> wrote:
> 
> > Add the __counted_by() compiler attribute to the flexible array member
> > 'entries' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> > CONFIG_FORTIFY_SOURCE.
> > 
> > Signed-off-by: Thorsten Blum <thorsten.blum@...ux.dev>
> > ---
> >  include/linux/unwind_deferred_types.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> > index 33b62ac25c86..d4b67f0116f3 100644
> > --- a/include/linux/unwind_deferred_types.h
> > +++ b/include/linux/unwind_deferred_types.h
> > @@ -5,7 +5,7 @@
> >  struct unwind_cache {
> >  	unsigned long		unwind_completed;
> >  	unsigned int		nr_entries;
> > -	unsigned long		entries[];
> > +	unsigned long		entries[] __counted_by(nr_entries);
> >  };
> >  
> >  /*  
> 
> Wrong!
> 
> I need to add a comment here that entries is not bound by nr_entries.
> 
>    https://lore.kernel.org/all/20250730093249.4833be14@gandalf.local.home/
> 
> Maybe this?:
> 
> diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> index 33b62ac25c86..d05409bb14fa 100644
> --- a/include/linux/unwind_deferred_types.h
> +++ b/include/linux/unwind_deferred_types.h
> @@ -5,7 +5,7 @@
>  struct unwind_cache {
>  	unsigned long		unwind_completed;

Does that need to be 'long' - 'int' would fit in the padding on 64bit.

>  	unsigned int		nr_entries;
> -	unsigned long		entries[];
> +	unsigned long		entries[]; /* Fixed size, not bound by nr_entries */
>  };

Perhaps it should be:
	unsigned long entries[ /* MAX_UNWIND_ENTRIES */ ];

    David

>  
>  /*
> 
> 
> 
> -- Steve
>