Commit 472c5dd6 authored Dec 16, 2025 by Alice C. Munduruca Committed by Paolo Abeni Dec 23, 2025

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: 2e64fe46 ("selftests: add few test cases for tap driver")
Signed-off-by: Alice C. Munduruca <alice.munduruca@canonical.com>
Reviewed-by: Cengiz Can <cengiz.can@canonical.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20251216170641.250494-1-alice.munduruca@canonical.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>

parent 12cab119

tools/testing/selftests/net/tap.c

+5 −11

Original line number	Diff line number	Diff line
		@@ -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);