netdev - [PATCH net v3] selftests: net: fix "buffer overflow detected" for tap.c

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Message-ID: <20251216170641.250494-1-alice.munduruca@canonical.com>
Date: Tue, 16 Dec 2025 12:06:41 -0500
From: "Alice C. Munduruca" <alice.munduruca@...onical.com>
To: netdev@...r.kernel.org
Cc: linux-kselftest@...r.kernel.org,
	"Alice C. Munduruca" <alice.munduruca@...onical.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Shuah Khan <shuah@...nel.org>,
	Willem de Bruijn <willemb@...gle.com>,
	Cengiz Can <cengiz.can@...onical.com>
Subject: [PATCH net v3] selftests: net: fix "buffer overflow detected" for tap.c

When the selftest 'tap.c' is compiled with '-D_FORTIFY_SOURCE=3',
the strcpy() in rtattr_add_strsz() is replaced with a checked
version which causes the test to consistently fail when compiled
with toolchains for which this option is enabled by default.

 TAP version 13
 1..3
 # Starting 3 tests from 1 test cases.
 #  RUN           tap.test_packet_valid_udp_gso ...
 *** buffer overflow detected ***: terminated
 # test_packet_valid_udp_gso: Test terminated by assertion
 #          FAIL  tap.test_packet_valid_udp_gso
 not ok 1 tap.test_packet_valid_udp_gso
 #  RUN           tap.test_packet_valid_udp_csum ...
 *** buffer overflow detected ***: terminated
 # test_packet_valid_udp_csum: Test terminated by assertion
 #          FAIL  tap.test_packet_valid_udp_csum
 not ok 2 tap.test_packet_valid_udp_csum
 #  RUN           tap.test_packet_crash_tap_invalid_eth_proto ...
 *** buffer overflow detected ***: terminated
 # test_packet_crash_tap_invalid_eth_proto: Test terminated by assertion
 #          FAIL  tap.test_packet_crash_tap_invalid_eth_proto
 not ok 3 tap.test_packet_crash_tap_invalid_eth_proto
 # FAILED: 0 / 3 tests passed.
 # Totals: pass:0 fail:3 xfail:0 xpass:0 skip:0 error:0

A buffer overflow is detected by the fortified glibc __strcpy_chk()
since the __builtin_object_size() of `RTA_DATA(rta)` is incorrectly
reported as 1, even though there is ample space in its bounding
buffer `req`.

Additionally, given that IFLA_IFNAME also expects a null-terminated
string, callers of rtaddr_add_str{,sz}() could simply use the
rtaddr_add_strsz() variant. (which has been renamed to remove the
trailing `sz`) memset() has been used for this function since it
is unchecked and thus circumvents the issue discussed in the
previous paragraph.

Fixes: 2e64fe4624d1 ("selftests: add few test cases for tap driver")
Signed-off-by: Alice C. Munduruca <alice.munduruca@...onical.com>
Reviewed-by: Cengiz Can <cengiz.can@...onical.com>
---
 tools/testing/selftests/net/tap.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/tools/testing/selftests/net/tap.c b/tools/testing/selftests/net/tap.c
index 247c3b3ac1c9..51a209014f1c 100644
--- a/tools/testing/selftests/net/tap.c
+++ b/tools/testing/selftests/net/tap.c
@@ -56,18 +56,12 @@ static void rtattr_end(struct nlmsghdr *nh, struct rtattr *attr)
 static struct rtattr *rtattr_add_str(struct nlmsghdr *nh, unsigned short type,
 				     const char *s)
 {
-	struct rtattr *rta = rtattr_add(nh, type, strlen(s));
+	unsigned int strsz = strlen(s) + 1;
+	struct rtattr *rta;
 
-	memcpy(RTA_DATA(rta), s, strlen(s));
-	return rta;
-}
-
-static struct rtattr *rtattr_add_strsz(struct nlmsghdr *nh, unsigned short type,
-				       const char *s)
-{
-	struct rtattr *rta = rtattr_add(nh, type, strlen(s) + 1);
+	rta = rtattr_add(nh, type, strsz);
 
-	strcpy(RTA_DATA(rta), s);
+	memcpy(RTA_DATA(rta), s, strsz);
 	return rta;
 }
 
@@ -119,7 +113,7 @@ static int dev_create(const char *dev, const char *link_type,
 
 	link_info = rtattr_begin(&req.nh, IFLA_LINKINFO);
 
-	rtattr_add_strsz(&req.nh, IFLA_INFO_KIND, link_type);
+	rtattr_add_str(&req.nh, IFLA_INFO_KIND, link_type);
 
 	if (fill_info_data) {
 		info_data = rtattr_begin(&req.nh, IFLA_INFO_DATA);
-- 
2.48.1