Re: [PATCH 2/2] selftests/net: fix uninitialized variables

From: John Hubbard
Date: Mon May 06 2024 - 14:51:14 EST


On 5/6/24 11:00 AM, Willem de Bruijn wrote:
John Hubbard wrote:
..
diff --git a/tools/testing/selftests/net/gro.c b/tools/testing/selftests/net/gro.c
index 353e1e867fbb..0eb61edaad83 100644
--- a/tools/testing/selftests/net/gro.c
+++ b/tools/testing/selftests/net/gro.c
@@ -110,7 +110,8 @@ static void setup_sock_filter(int fd)
const int dport_off = tcp_offset + offsetof(struct tcphdr, dest);
const int ethproto_off = offsetof(struct ethhdr, h_proto);
int optlen = 0;
- int ipproto_off, opt_ipproto_off;
+ int ipproto_off;
+ int opt_ipproto_off = 0;

This is only intended to be used in the case where the IP proto is not TCP:

BPF_STMT(BPF_LD + BPF_B + BPF_ABS, ipproto_off),
+ BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_TCP, 2, 0),
+ BPF_STMT(BPF_LD + BPF_B + BPF_ABS, opt_ipproto_off),
BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_TCP, 0, 5),

In that case the test tries again at a different offset that accounts
for optional IPv6 extension headers.

This is indeed buggy, in that it might accidentally accept packets
that should be dropped.

Initializing to 0 compares against against the first byte of the
Ethernet header. Which is an external argument to the test. So
safest is to initialize opt_ipproto_off to ipproto_off and just
repeat the previous check. Perhaps:

@@ -118,6 +118,7 @@ static void setup_sock_filter(int fd)
else
next_off = offsetof(struct ipv6hdr, nexthdr);
ipproto_off = ETH_HLEN + next_off;
+ opt_ipproto_off = ipproto_off; /* overridden later if may have exthdrs */

OK, thanks for pointing out the right fix, I'll send a v2 that does that.

thanks,
--
John Hubbard
NVIDIA