netdev - Re: [PATCH net-next] udp_tunnel: Use flex array to simplify code

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Date: Sun, 24 Sep 2023 12:00:58 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>, 
 "David S. Miller" <davem@...emloft.net>, 
 David Ahern <dsahern@...nel.org>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: linux-kernel@...r.kernel.org, 
 kernel-janitors@...r.kernel.org, 
 Christophe JAILLET <christophe.jaillet@...adoo.fr>, 
 netdev@...r.kernel.org
Subject: Re: [PATCH net-next] udp_tunnel: Use flex array to simplify code

Christophe JAILLET wrote:
> 'n_tables' is small, UDP_TUNNEL_NIC_MAX_TABLES	= 4 as a maximum. So there
> is no real point to allocate the 'entries' pointers array with a dedicate
> memory allocation.
> 
> Using a flexible array for struct udp_tunnel_nic->entries avoids the
> overhead of an additional memory allocation.
> 
> This also saves an indirection when the array is accessed.
> 
> Finally, __counted_by() can be used for run-time bounds checking if
> configured and supported by the compiler.
> 
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
>  net/ipv4/udp_tunnel_nic.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/net/ipv4/udp_tunnel_nic.c b/net/ipv4/udp_tunnel_nic.c
> index 029219749785..b6d2d16189c0 100644
> --- a/net/ipv4/udp_tunnel_nic.c
> +++ b/net/ipv4/udp_tunnel_nic.c
> @@ -47,7 +47,7 @@ struct udp_tunnel_nic {
>  
>  	unsigned int n_tables;
>  	unsigned long missed;
> -	struct udp_tunnel_nic_table_entry **entries;
> +	struct udp_tunnel_nic_table_entry *entries[] __counted_by(n_tables);
>  };
>  
>  /* We ensure all work structs are done using driver state, but not the code.
> @@ -725,16 +725,12 @@ udp_tunnel_nic_alloc(const struct udp_tunnel_nic_info *info,
>  	struct udp_tunnel_nic *utn;
>  	unsigned int i;
>  
> -	utn = kzalloc(sizeof(*utn), GFP_KERNEL);
> +	utn = kzalloc(struct_size(utn, entries, n_tables), GFP_KERNEL);
>  	if (!utn)
>  		return NULL;
>  	utn->n_tables = n_tables;

Should utn->n_tables be initialized before first use of
struct_size(utn, entries, n_tables)?