Re: [PATCH net v2 1/4] ipv4: add FIB_LOOKUP_REPORT_MISS

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Nov 10, 2026 at 06:15:33AM +0000, liamsmith827 wrote:
> From: liamsmith827 <liamsmith827@proton.me>
> 
> IPv4 FIB lookup maps a routing miss to -ENETUNREACH, which is also
> returned when an unreachable rule is encountered. This flag prevents the
> mapping, returning -ESRCH instead.

Please include the motivation in the commit message. Something like:

"ip_route_output_key_hash_rcu() will need to distinguish between a route
miss and an unreachable rule. Currently, both result in fib_lookup()
returning the same error code (-ENETUNREACH).

Add a new FIB lookup flag that will instruct fib_lookup() to return a
different error code (-ESRCH) when it encounters a route miss."

> 
> Cc: stable@vger.kernel.org
> Signed-off-by: liamsmith827 <liamsmith827@proton.me>
> ---
>  include/net/fib_rules.h | 5 +++++
>  include/net/ip_fib.h    | 6 +++---
>  net/ipv4/fib_rules.c    | 2 +-
>  3 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/fib_rules.h b/include/net/fib_rules.h
> index c6b94790fa81..bcb6809dbd10 100644
> --- a/include/net/fib_rules.h
> +++ b/include/net/fib_rules.h
> @@ -59,6 +59,11 @@ struct fib_lookup_arg {
>  	int			flags;
>  #define FIB_LOOKUP_NOREF		1
>  #define FIB_LOOKUP_IGNORE_LINKSTATE	2
> +/* Return -ESRCH instead of mapping it to -ENETUNREACH. Only applies to IPv4
> + * lookups. When used with FIB_LOOKUP_NOREF, -ESRCH indicates the lookup
> + * exhausted all rules and tables without finding a usable route.
> + */

The comment seems redundant and IPv4 always sets FIB_LOOKUP_NOREF. I
suggest removing the comment.

> +#define FIB_LOOKUP_REPORT_MISS		4
>  };
>  
>  struct fib_rules_ops {
> diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
> index 92f4eb8b86d7..d0b4f261585a 100644
> --- a/include/net/ip_fib.h
> +++ b/include/net/ip_fib.h
> @@ -318,7 +318,7 @@ static inline int fib_lookup(struct net *net, const struct flowi4 *flp,
>  			     struct fib_result *res, unsigned int flags)
>  {
>  	struct fib_table *tb;
> -	int err = -ENETUNREACH;
> +	int err = -EAGAIN;
>  
>  	rcu_read_lock();
>  
> @@ -327,7 +327,7 @@ static inline int fib_lookup(struct net *net, const struct flowi4 *flp,
>  		err = fib_table_lookup(tb, flp, res, flags | FIB_LOOKUP_NOREF);
>  
>  	if (err == -EAGAIN)
> -		err = -ENETUNREACH;
> +		err = (flags & FIB_LOOKUP_REPORT_MISS) ? -ESRCH : -ENETUNREACH;
>  
>  	rcu_read_unlock();
>  
> @@ -398,7 +398,7 @@ static inline int fib_lookup(struct net *net, struct flowi4 *flp,
>  		err = fib_table_lookup(tb, flp, res, flags);
>  
>  	if (err == -EAGAIN)
> -		err = -ENETUNREACH;
> +		err = (flags & FIB_LOOKUP_REPORT_MISS) ? -ESRCH : -ENETUNREACH;
>  out:
>  	rcu_read_unlock();
>  
> diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
> index 060501b376a8..3d7dcb8d0865 100644
> --- a/net/ipv4/fib_rules.c
> +++ b/net/ipv4/fib_rules.c
> @@ -101,7 +101,7 @@ int __fib_lookup(struct net *net, struct flowi4 *flp,
>  		res->tclassid = 0;
>  #endif
>  
> -	if (err == -ESRCH)
> +	if (err == -ESRCH && !(flags & FIB_LOOKUP_REPORT_MISS))
>  		err = -ENETUNREACH;
>  
>  	return err;
> -- 
> 2.47.3
>