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

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, Apr 08, 2026 at 02:21:19PM +0200, Thorsten Blum wrote:
> 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.

OK I'll buy that but send +1 version:

~/work/kernel.org/jarkko/linux-tpmdd next
❯ git am -3 20260406_thorsten_blum_keys_dns_drop_unused_upayload_data_nul_terminator.mbx
Applying: keys, dns: drop unused upayload->data NUL terminator
error: sha1 information is lacking or useless (net/dns_resolver/dns_key.c).
error: could not build fake ancestor
Patch failed at 0001 keys, dns: drop unused upayload->data NUL terminator
hint: Use 'git am --show-current-patch=diff' to see the failed patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

BR, Jarkko