netlink: add support for formatted extack messages

Include an 80-byte buffer in struct netlink_ext_ack that can be used
 for scnprintf()ed messages.  This does mean that the resulting string
 can't be enumerated, translated etc. in the way NL_SET_ERR_MSG() was
 designed to allow.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index d51e041d2242681c67cc2499682bc8bfb16dce82..d81bde5a5844a9aba852490d7be68cdb6261a044 100644 (file)
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -64,6 +64,7 @@ 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 NETLINK_MAX_FMTMSG_LEN 80
 
 /**
  * struct netlink_ext_ack - netlink extended ACK report struct
@@ -75,6 +76,8 @@ netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
  * @miss_nest: nest missing an attribute (%NULL if missing top level attr)
  * @cookie: cookie data to return to userspace (for success)
  * @cookie_len: actual cookie data length
+ * @_msg_buf: output buffer for formatted message strings - don't access
+ *     directly, use %NL_SET_ERR_MSG_FMT
  */
 struct netlink_ext_ack {
        const char *_msg;
@@ -84,13 +87,13 @@ struct netlink_ext_ack {
        u16 miss_type;
        u8 cookie[NETLINK_MAX_COOKIE_LEN];
        u8 cookie_len;
+       char _msg_buf[NETLINK_MAX_FMTMSG_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.)
+ * If string formatting is needed use NL_SET_ERR_MSG_FMT.
  */
 #define NL_SET_ERR_MSG(extack, msg) do {               \
        static const char __msg[] = msg;                \
@@ -102,9 +105,31 @@ struct netlink_ext_ack {
                __extack->_msg = __msg;                 \
 } while (0)
 
+/* We splice fmt with %s at each end even in the snprintf so that both calls
+ * can use the same string constant, avoiding its duplication in .ro
+ */
+#define NL_SET_ERR_MSG_FMT(extack, fmt, args...) do {                         \
+       struct netlink_ext_ack *__extack = (extack);                           \
+                                                                              \
+       if (!__extack)                                                         \
+               break;                                                         \
+       if (snprintf(__extack->_msg_buf, NETLINK_MAX_FMTMSG_LEN,               \
+                    "%s" fmt "%s", "", ##args, "") >=                         \
+           NETLINK_MAX_FMTMSG_LEN)                                            \
+               net_warn_ratelimited("%s" fmt "%s", "truncated extack: ",      \
+                                    ##args, "\n");                            \
+                                                                              \
+       do_trace_netlink_extack(__extack->_msg_buf);                           \
+                                                                              \
+       __extack->_msg = __extack->_msg_buf;                                   \
+} while (0)
+
 #define NL_SET_ERR_MSG_MOD(extack, msg)                        \
        NL_SET_ERR_MSG((extack), KBUILD_MODNAME ": " msg)
 
+#define NL_SET_ERR_MSG_FMT_MOD(extack, fmt, args...)   \
+       NL_SET_ERR_MSG_FMT((extack), KBUILD_MODNAME ": " fmt, ##args)
+
 #define NL_SET_BAD_ATTR_POLICY(extack, attr, pol) do { \
        if ((extack)) {                                 \
                (extack)->bad_attr = (attr);            \