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

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

[Alice C. Munduruca]

When '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

Using `memcpy`, an unchecked function, avoids this issue and allows
the tests to go forwards as expected.

Fixes: 2e64fe4624d1 ("selftests: add few test cases for tap driver")
Signed-off-by: Alice C. Munduruca <alice.munduruca@canonical.com>
---
 tools/testing/selftests/net/tap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/net/tap.c b/tools/testing/selftests/net/tap.c
index 247c3b3ac1c9..dd961b629295 100644
--- a/tools/testing/selftests/net/tap.c
+++ b/tools/testing/selftests/net/tap.c
@@ -67,7 +67,7 @@ static struct rtattr *rtattr_add_strsz(struct nlmsghdr *nh, unsigned short type,
 {
 	struct rtattr *rta = rtattr_add(nh, type, strlen(s) + 1);
 
-	strcpy(RTA_DATA(rta), s);
+	memcpy(RTA_DATA(rta), s, strlen(s) + 1);
 	return rta;
 }
 
-- 
2.48.1

Re: [PATCH net] selftests: net: fix "buffer overflow detected" for tap.c

[Simon Horman]

On Wed, Dec 10, 2025 at 05:39:32PM -0500, Alice C. Munduruca wrote:
> When '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
> 
> Using `memcpy`, an unchecked function, avoids this issue and allows
> the tests to go forwards as expected.
> 
> Fixes: 2e64fe4624d1 ("selftests: add few test cases for tap driver")
> Signed-off-by: Alice C. Munduruca <alice.munduruca@canonical.com>

Thanks Alice,

I agree that this approach makes sense.
But I wonder if the commit message should also mention:

1. That this is consistent with usage elsewhere in this file
2. Why there is actually no overflow.

...