[PATCH net-next] devlink: Replace strlcat() with seq_buf

[Ian Bridges <icb@fastmail.org>]

In preparation for removing the strlcat() API[1], replace its uses in
__devlink_compat_running_version().

The function accumulates a variable number of version strings into a
fixed buffer, which is what seq_buf is for. The seq_buf is anchored at
the end of any existing string in the buffer and each version string
is appended with a single seq_buf_printf(). The output is unchanged,
including under truncation.

Link: https://github.com/KSPP/linux/issues/370 [1]
Signed-off-by: Ian Bridges <icb@fastmail.org>
---
The patch was tested as follows, on top of net-next:
 - x86_64 allmodconfig and allyesconfig builds of net/devlink/dev.o at
   W=1 produce no warnings.
 - A userspace comparison of the old and new construction ran with
   randomized version lists and buffer contents across all buffer
   fill levels. The outputs are byte-identical in every case,
   including on overflow.
 - The changed path was exercised in a QEMU guest through the ethtool
   GDRVINFO ioctl against a netdevsim device, before and after the
   change, with identical fw_version output.

 net/devlink/dev.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/devlink/dev.c b/net/devlink/dev.c
index 57b2b8f03543..0d4301267171 100644
--- a/net/devlink/dev.c
+++ b/net/devlink/dev.c
@@ -5,6 +5,7 @@
  */
 
 #include <linux/device.h>
+#include <linux/seq_buf.h>
 #include <net/genetlink.h>
 #include <net/sock.h>
 #include "devl_internal.h"
@@ -1188,8 +1189,10 @@ static void __devlink_compat_running_version(struct devlink *devlink,
 					     char *buf, size_t len)
 {
 	struct devlink_info_req req = {};
+	size_t used = strnlen(buf, len);
 	const struct nlattr *nlattr;
 	struct sk_buff *msg;
+	struct seq_buf sb;
 	int rem, err;
 
 	msg = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_KERNEL);
@@ -1201,6 +1204,8 @@ static void __devlink_compat_running_version(struct devlink *devlink,
 	if (err)
 		goto free_msg;
 
+	seq_buf_init(&sb, buf + used, len - used);
+
 	nla_for_each_attr_type(nlattr, DEVLINK_ATTR_INFO_VERSION_RUNNING,
 			       (void *)msg->data, msg->len, rem) {
 		const struct nlattr *kv;
@@ -1208,8 +1213,8 @@ static void __devlink_compat_running_version(struct devlink *devlink,
 
 		nla_for_each_nested_type(kv, DEVLINK_ATTR_INFO_VERSION_VALUE,
 					 nlattr, rem_kv) {
-			strlcat(buf, nla_data(kv), len);
-			strlcat(buf, " ", len);
+			seq_buf_printf(&sb, "%s ",
+				       (const char *)nla_data(kv));
 		}
 	}
 free_msg:
-- 
2.47.3