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

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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: f8a10bed32 ("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>
This commit is contained in:
Breno Leitao 2026-05-01 02:58:41 -07:00 committed by Jakub Kicinski
parent d82ba05263
commit 76b93a8107
1 changed files with 13 additions and 10 deletions
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23
net/core/netpoll.c
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@ -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) {