Subject: Re: [PATCH net v2] selftests: net: fix "buffer overflow detected" for tap.c

From: Willem de Bruijn <[email-masked]>
Date: Fri, 12 Dec 2025 14:18:25 -0500

Alice C. Munduruca wrote:
> 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`.
> 
> Using the unchecked function memcpy() here instead allows us to match
> the way rtattr_add_str() is written while avoiding the spurious test
> failure.
> 
> Fixes: 2e64fe4624d1 ("selftests: add few test cases for tap driver")
> Signed-off-by: Alice C. Munduruca <[email-masked]>
> ---
>  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);

Could call strlen(s) only once in the function.

Why does rtattr_add_str do the same without the terminating '\0'?
It is only used with IFNAME so assumes max size of IFNAMSIZ perhaps?
>  	return rta;
>  }
> -- 
> 2.48.1