selftests: net: fix "buffer overflow detected" for tap.c

selftests: net: fix "buffer overflow detected" for tap.c

[ Upstream commit 472c5dd6b95c02b3e5d7395acf542150e91165e7 ]

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@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>
Signed-off-by: Sasha Levin <sashal@kernel.org>