[PATCH V2 net-next 1/6] netlink: Convert extack msg to a formattable buffer

[Saeed Mahameed <saeedm@mellanox.com>]

Before this patch the extack msg had to be a const char and didn't leave
much room for developers to report formattable and flexible messages.

All current usages are oneliner messages, hence, a buffer of size 128B
should be sufficient to replace the const char pointer.

This will allow future usages to provide formattable messages and more
flexible error reporting, without any impact on current users.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
---
 include/linux/netlink.h | 27 ++++++++++++---------------
 lib/nlattr.c            |  2 +-
 2 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 205fa7b1f07a..de9004da0288 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -62,6 +62,7 @@ netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
 
 /* this can be increased when necessary - don't expose to userland */
 #define NETLINK_MAX_COOKIE_LEN	20
+#define NL_EXTACK_MAX_MSG_SZ	128
 
 /**
  * struct netlink_ext_ack - netlink extended ACK report struct
@@ -72,40 +73,36 @@ netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
  * @cookie_len: actual cookie data length
  */
 struct netlink_ext_ack {
-	const char *_msg;
+	char _msg[NL_EXTACK_MAX_MSG_SZ];
 	const struct nlattr *bad_attr;
 	u8 cookie[NETLINK_MAX_COOKIE_LEN];
 	u8 cookie_len;
 };
 
-/* Always use this macro, this allows later putting the
- * message into a separate section or such for things
- * like translation or listing all possible messages.
- * Currently string formatting is not supported (due
- * to the lack of an output buffer.)
- */
-#define NL_SET_ERR_MSG(extack, msg) do {		\
-	static const char __msg[] = msg;		\
+#define NL_MSG_FMT(extack, fmt, ...) \
+	WARN_ON(snprintf(extack->_msg, NL_EXTACK_MAX_MSG_SZ, fmt, ## __VA_ARGS__) \
+		>= NL_EXTACK_MAX_MSG_SZ)
+
+#define NL_SET_ERR_MSG(extack, fmt, ...) do {		\
 	struct netlink_ext_ack *__extack = (extack);	\
 							\
 	if (__extack)					\
-		__extack->_msg = __msg;			\
+		NL_MSG_FMT(__extack, fmt, ## __VA_ARGS__); \
 } while (0)
 
-#define NL_SET_ERR_MSG_MOD(extack, msg)			\
-	NL_SET_ERR_MSG((extack), KBUILD_MODNAME ": " msg)
+#define NL_SET_ERR_MSG_MOD(extack, fmt, ...)			\
+	NL_SET_ERR_MSG((extack), KBUILD_MODNAME ": " fmt, ## __VA_ARGS__)
 
 #define NL_SET_BAD_ATTR(extack, attr) do {		\
 	if ((extack))					\
 		(extack)->bad_attr = (attr);		\
 } while (0)
 
-#define NL_SET_ERR_MSG_ATTR(extack, attr, msg) do {	\
-	static const char __msg[] = msg;		\
+#define NL_SET_ERR_MSG_ATTR(extack, attr, fmt, ...) do {	\
 	struct netlink_ext_ack *__extack = (extack);	\
 							\
 	if (__extack) {					\
-		__extack->_msg = __msg;			\
+		NL_MSG_FMT(__extack, fmt, ## __VA_ARGS__); \
 		__extack->bad_attr = (attr);		\
 	}						\
 } while (0)
diff --git a/lib/nlattr.c b/lib/nlattr.c
index cace9b307781..2ce1d6b68ce8 100644
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -208,7 +208,7 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 	case NLA_REJECT:
 		if (extack && pt->validation_data) {
 			NL_SET_BAD_ATTR(extack, nla);
-			extack->_msg = pt->validation_data;
+			NL_MSG_FMT(extack, pt->validation_data);
 			return -EINVAL;
 		}
 		err = -EINVAL;
-- 
2.21.0