Re: [PATCH] netfilter: x_tables: replace strlcat() with snprintf()

[David Laight <david.laight.linux@gmail.com>]

On Fri, 26 Jun 2026 17:25:35 -0500
Ian Bridges <icb@fastmail.org> wrote:

> In preparation for removing the deprecated strlcat() API[1], replace the
> strscpy()/strlcat() pairs in xt_proto_init() and xt_proto_fini() with
> snprintf(), which builds each /proc file name in a single call.
> 
> Each name is "<prefix><suffix>", where <prefix> is the address-family
> string xt_prefix[af] and <suffix> is one of the FORMAT_TABLES,
> FORMAT_MATCHES or FORMAT_TARGETS literals. snprintf() with a "%s%s"
> format produces the same NUL-terminated, length-bounded string as the
> strscpy()/strlcat() chain it replaces, so the proc entry names are
> unchanged.
> 
> Link: https://github.com/KSPP/linux/issues/370 [1]
> Signed-off-by: Ian Bridges <icb@fastmail.org>
> ---
>  net/netfilter/x_tables.c | 24 ++++++++----------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
> 
> diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
> index 4e6708c23922..56f4546be336 100644
> --- a/net/netfilter/x_tables.c
> +++ b/net/netfilter/x_tables.c
> @@ -2033,8 +2033,7 @@ int xt_proto_init(struct net *net, u_int8_t af)
>  	root_uid = make_kuid(net->user_ns, 0);
>  	root_gid = make_kgid(net->user_ns, 0);
>  
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_TABLES, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_TABLES);

If you are going to use snprintf either paste the strings together:
	snprintf(buf, sizeof(buf), "%s" FORMAT_TABLES, xt_prefix[af]);
or prepend the "%s" onto the #define of FORMAT_TABLES itself:
	snprintf(buf, sizeof(buf), FORMAT_TABLES, xt_prefix[af]);

FORMAT_TABLES should also be FORMAT_NAMES.

-- David

>  	proc = proc_create_net_data(buf, 0440, net->proc_net, &xt_table_seq_ops,
>  			sizeof(struct seq_net_private),
>  			(void *)(unsigned long)af);
> @@ -2043,8 +2042,7 @@ int xt_proto_init(struct net *net, u_int8_t af)
>  	if (uid_valid(root_uid) && gid_valid(root_gid))
>  		proc_set_user(proc, root_uid, root_gid);
>  
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_MATCHES, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_MATCHES);
>  	proc = proc_create_seq_private(buf, 0440, net->proc_net,
>  			&xt_match_seq_ops, sizeof(struct nf_mttg_trav),
>  			(void *)(unsigned long)af);
> @@ -2053,8 +2051,7 @@ int xt_proto_init(struct net *net, u_int8_t af)
>  	if (uid_valid(root_uid) && gid_valid(root_gid))
>  		proc_set_user(proc, root_uid, root_gid);
>  
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_TARGETS, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_TARGETS);
>  	proc = proc_create_seq_private(buf, 0440, net->proc_net,
>  			 &xt_target_seq_ops, sizeof(struct nf_mttg_trav),
>  			 (void *)(unsigned long)af);
> @@ -2068,13 +2065,11 @@ int xt_proto_init(struct net *net, u_int8_t af)
>  
>  #ifdef CONFIG_PROC_FS
>  out_remove_matches:
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_MATCHES, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_MATCHES);
>  	remove_proc_entry(buf, net->proc_net);
>  
>  out_remove_tables:
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_TABLES, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_TABLES);
>  	remove_proc_entry(buf, net->proc_net);
>  out:
>  	return -1;
> @@ -2087,16 +2082,13 @@ void xt_proto_fini(struct net *net, u_int8_t af)
>  #ifdef CONFIG_PROC_FS
>  	char buf[XT_FUNCTION_MAXNAMELEN];
>  
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_TABLES, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_TABLES);
>  	remove_proc_entry(buf, net->proc_net);
>  
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_TARGETS, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_TARGETS);
>  	remove_proc_entry(buf, net->proc_net);
>  
> -	strscpy(buf, xt_prefix[af], sizeof(buf));
> -	strlcat(buf, FORMAT_MATCHES, sizeof(buf));
> +	snprintf(buf, sizeof(buf), "%s%s", xt_prefix[af], FORMAT_MATCHES);
>  	remove_proc_entry(buf, net->proc_net);
>  #endif /*CONFIG_PROC_FS*/
>  }