Re: [PATCH net-next 2/2] KEYS: annotate struct user_key_payload with __counted_by

[Thorsten Blum <thorsten.blum@linux.dev>]

On Wed, Apr 08, 2026 at 12:02:25PM +0300, Jarkko Sakkinen wrote:
> On Mon, Apr 06, 2026 at 07:58:10PM +0200, Thorsten Blum wrote:
> > Add the __counted_by() compiler attribute to the flexible array member
> > 'data' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> > CONFIG_FORTIFY_SOURCE.
> > 
> > Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
> > ---
> >  include/keys/user-type.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/keys/user-type.h b/include/keys/user-type.h
> > index 386c31432789..2305991f4fcd 100644
> > --- a/include/keys/user-type.h
> > +++ b/include/keys/user-type.h
> > @@ -27,7 +27,8 @@
> >  struct user_key_payload {
> >  	struct rcu_head	rcu;		/* RCU destructor */
> >  	unsigned short	datalen;	/* length of this data */
> > -	char		data[] __aligned(__alignof__(u64)); /* actual data */
> > +	char		data[]		/* actual data */
> > +			__aligned(__alignof__(u64)) __counted_by(datalen);
> >  };
> >  
> >  extern struct key_type key_type_user;
> 
> You don't provide any evidence of any improvement.

It's a proactive hardening change to help avoid future mistakes.

The __counted_by() annotation makes the bounds visible to the compiler
and at runtime so that future ->data accesses can be checked against
->datalen.

The current code is correct regarding ->data accesses and doesn't
require any changes.