netpoll: pass buffer size to egress_dev() to avoid MAC truncation

egress_dev() formats np->dev_mac via snprintf() but receives buf as
a bare char *, so it cannot derive the buffer size from the pointer. The
size argument was hardcoded to MAC_ADDR_STR_LEN (3 * ETH_ALEN - 1 = 17),
which is silly wrong in two ways:

 1) misleading kernel log output on the MAC-selected target path
    (np->dev_name[0] == '\0'); for example "aa:bb:cc:dd:ee:ff doesn't
    exist, aborting" was logged as "aa:bb:cc:dd:ee:f doesn't exist,
    aborting".

 2) the second argument of snprintf is the size of the buffer, not the
    size of what you want to write.

Add a bufsz parameter to egress_dev() and pass sizeof(buf) from each
caller, matching the standard snprintf() idiom and removing the
hardcoded size from the helper.

Every caller already declares "char buf[MAC_ADDR_STR_LEN + 1]" so the
formatted MAC continues to fit.

Tested by booting with
  netconsole=6665@/aa:bb:cc:dd:ee:ff,6666@10.0.0.1/00:11:22:33:44:55
on a kernel without a matching device. Pre-fix dmesg shows
"aa:bb:cc:dd:ee:f doesn't exist, aborting"; post-fix shows the full
"aa:bb:cc:dd:ee:ff doesn't exist, aborting".

Fixes: f8a10bed32f5 ("netconsole: allow selection of egress interface via MAC address")
Cc: stable@vger.kernel.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260501-netpoll_snprintf_fix-v1-1-84b0566e6597@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 4381e0fc25bf474c1c16a16ea15a418dc36134e4..84faace50ac281d6ca638819b09ceb09f2ed5c68 100644 (file)
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -608,14 +608,16 @@ EXPORT_SYMBOL_GPL(__netpoll_setup);
 /*
  * Returns a pointer to a string representation of the identifier used
  * to select the egress interface for the given netpoll instance. buf
- * must be a buffer of length at least MAC_ADDR_STR_LEN + 1.
+ * is used to format np->dev_mac when np->dev_name is empty; bufsz must
+ * be at least MAC_ADDR_STR_LEN + 1 to fit the formatted MAC address
+ * and its NUL terminator.
  */
-static char *egress_dev(struct netpoll *np, char *buf)
+static char *egress_dev(struct netpoll *np, char *buf, size_t bufsz)
 {
        if (np->dev_name[0])
                return np->dev_name;
 
-       snprintf(buf, MAC_ADDR_STR_LEN, "%pM", np->dev_mac);
+       snprintf(buf, bufsz, "%pM", np->dev_mac);
        return buf;
 }
 
@@ -645,7 +647,7 @@ static int netpoll_take_ipv6(struct netpoll *np, struct net_device *ndev)
 
        if (!IS_ENABLED(CONFIG_IPV6)) {
                np_err(np, "IPv6 is not supported %s, aborting\n",
-                      egress_dev(np, buf));
+                      egress_dev(np, buf, sizeof(buf)));
                return -EINVAL;
        }
 
@@ -667,7 +669,7 @@ static int netpoll_take_ipv6(struct netpoll *np, struct net_device *ndev)
        }
        if (err) {
                np_err(np, "no IPv6 address for %s, aborting\n",
-                      egress_dev(np, buf));
+                      egress_dev(np, buf, sizeof(buf)));
                return err;
        }
 
@@ -687,14 +689,14 @@ static int netpoll_take_ipv4(struct netpoll *np, struct net_device *ndev)
        in_dev = __in_dev_get_rtnl(ndev);
        if (!in_dev) {
                np_err(np, "no IP address for %s, aborting\n",
-                      egress_dev(np, buf));
+                      egress_dev(np, buf, sizeof(buf)));
                return -EDESTADDRREQ;
        }
 
        ifa = rtnl_dereference(in_dev->ifa_list);
        if (!ifa) {
                np_err(np, "no IP address for %s, aborting\n",
-                      egress_dev(np, buf));
+                      egress_dev(np, buf, sizeof(buf)));
                return -EDESTADDRREQ;
        }
 
@@ -736,7 +738,8 @@ int netpoll_setup(struct netpoll *np)
                ndev = dev_getbyhwaddr(net, ARPHRD_ETHER, np->dev_mac);
 
        if (!ndev) {
-               np_err(np, "%s doesn't exist, aborting\n", egress_dev(np, buf));
+               np_err(np, "%s doesn't exist, aborting\n",
+                      egress_dev(np, buf, sizeof(buf)));
                err = -ENODEV;
                goto unlock;
        }
@@ -744,14 +747,14 @@ int netpoll_setup(struct netpoll *np)
 
        if (netdev_master_upper_dev_get(ndev)) {
                np_err(np, "%s is a slave device, aborting\n",
-                      egress_dev(np, buf));
+                      egress_dev(np, buf, sizeof(buf)));
                err = -EBUSY;
                goto put;
        }
 
        if (!netif_running(ndev)) {
                np_info(np, "device %s not up yet, forcing it\n",
-                       egress_dev(np, buf));
+                       egress_dev(np, buf, sizeof(buf)));
 
                err = dev_open(ndev, NULL);
                if (err) {