netfilter: x_tables: replace strlcat() with snprintf()

PATCH In review Ian Bridges 3 participants 4 messages Jun 26 – Jul 1

Subsystems (3)

Networking Network drivers netfilter

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Ian Bridges

author

2026-06-26 22:25 UTC #0 lore raw

PATCH 1/1 netfilter: x_tables: replace strlcat() with snprintf()

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 <[email protected]>

---
 net/netfilter/x_tables.c | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)

net/netfilter/x_tables.c +8 ~~-16~~

index 4e6708c23922..56f4546be336 100644
@@ -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);
 	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*/
 }

Signature · 1 line

2.47.3

David Laight

2026-06-27 21:16 UTC #1 lore raw

Quoted 22 lines · On Fri, 26 Jun 2026 17:25:35 -0500 Ian Bridges <[email protected]> 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 <[email protected]>
---
 net/netfilter/x_tables.c | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)
net/netfilter/x_tables.c +0 -0
index 4e6708c23922..56f4546be336 100644

net/netfilter/x_tables.c int xt_proto_init(struct net *net, u_int8_t af) · line 2036

Quoted 6 lines

net/netfilter/x_tables.c +1 -2

@@ -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

Quoted 59 lines

net/netfilter/x_tables.c +7 ~~-14~~

 	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*/
 }

Ian Bridges

author

2026-07-01 06:25 UTC #2 lore raw

Quoted 28 lines · David Laight

Quoted 25 lines · On Fri, 26 Jun 2026 17:25:35 -0500 Ian Bridges <[email protected]> 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 <[email protected]>
---
 net/netfilter/x_tables.c | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)
net/netfilter/x_tables.c +0 -0
index 4e6708c23922..56f4546be336 100644
@@ -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);
 

Quoted 9 lines

net/netfilter/x_tables.c int xt_proto_init(struct net *net, u_int8_t af) · line 2036
Quoted 3 lines
net/netfilter/x_tables.c +1 -2
-	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]);

I learned something new today, thanks. I'll use the first form in v2.

Quoted 1 line

FORMAT_TABLES should also be FORMAT_NAMES.

The macro is already named FORMAT_TABLES today, so that rename would be a cleanup of pre-existing code rather than part of the strlcat conversion. I'm happy to fold it into v2 if a maintainer is fine including the tidy-up in this patch.

Thanks for the review,
Ian

Quoted 63 lines

-- David

Quoted 59 lines

net/netfilter/x_tables.c +7 ~~-14~~

 	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*/
 }

Florian Westphal

maintainer

2026-07-01 06:31 UTC #3 lore raw

Quoted 3 lines · Ian Bridges

I learned something new today, thanks. I'll use the first form in v2.
Quoted 1 line
FORMAT_TABLES should also be FORMAT_NAMES.

No. The name is fine.

Quoted 4 lines

The macro is already named FORMAT_TABLES today, so that rename would be a cleanup of pre-existing code rather than part of the strlcat conversion. I'm happy to fold it into v2 if a maintainer is fine including the tidy-up in this patch.

No need for a v2, I mangled this patch locally already to use "%s_FOO" in FORMAT_FOO.