From: Amery Hung ameryhung@gmail.com

[ Upstream commit b99f27e90268b1a814c13f8bd72ea1db448ea257 ]

Fix a race condition between the main test_progs thread and the traffic monitoring thread. The traffic monitor thread tries to print a line using multiple printf and use flockfile() to prevent the line from being torn apart. Meanwhile, the main thread doing io redirection can reassign or close stdout when going through tests. A deadlock as shown below can happen.

main traffic_monitor_thread ==== ====================== show_transport() -> flockfile(stdout)

stdio_hijack_init() -> stdout = open_memstream(log_buf, log_cnt); ... env.subtest_state->stdout_saved = stdout;

... funlockfile(stdout) stdio_restore_cleanup() -> fclose(env.subtest_state->stdout_saved);

After the traffic monitor thread lock stdout, A new memstream can be assigned to stdout by the main thread. Therefore, the traffic monitor thread later will not be able to unlock the original stdout. As the main thread tries to access the old stdout, it will hang indefinitely as it is still locked by the traffic monitor thread.

The deadlock can be reproduced by running test_progs repeatedly with traffic monitor enabled:

for ((i=1;i<=100;i++)); do ./test_progs -a flow_dissector_skb* -m '*' done

Fix this by only calling printf once and remove flockfile()/funlockfile().

Signed-off-by: Amery Hung ameryhung@gmail.com Signed-off-by: Martin KaFai Lau martin.lau@kernel.org Link: https://patch.msgid.link/20250213233217.553258-1-ameryhung@gmail.com Signed-off-by: Sasha Levin sashal@kernel.org --- tools/testing/selftests/bpf/network_helpers.c | 33 ++++++++----------- 1 file changed, 13 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/bpf/network_helpers.c b/tools/testing/selftests/bpf/network_helpers.c index 27784946b01b8..af0ee70a53f9f 100644 --- a/tools/testing/selftests/bpf/network_helpers.c +++ b/tools/testing/selftests/bpf/network_helpers.c @@ -771,12 +771,13 @@ static const char *pkt_type_str(u16 pkt_type) return "Unknown"; }

+#define MAX_FLAGS_STRLEN 21 /* Show the information of the transport layer in the packet */ static void show_transport(const u_char *packet, u16 len, u32 ifindex, const char *src_addr, const char *dst_addr, u16 proto, bool ipv6, u8 pkt_type) { - char *ifname, _ifname[IF_NAMESIZE]; + char *ifname, _ifname[IF_NAMESIZE], flags[MAX_FLAGS_STRLEN] = ""; const char *transport_str; u16 src_port, dst_port; struct udphdr *udp; @@ -817,29 +818,21 @@ static void show_transport(const u_char *packet, u16 len, u32 ifindex,

/* TCP or UDP*/

- flockfile(stdout); + if (proto == IPPROTO_TCP) + snprintf(flags, MAX_FLAGS_STRLEN, "%s%s%s%s", + tcp->fin ? ", FIN" : "", + tcp->syn ? ", SYN" : "", + tcp->rst ? ", RST" : "", + tcp->ack ? ", ACK" : ""); + if (ipv6) - printf("%-7s %-3s IPv6 %s.%d > %s.%d: %s, length %d", + printf("%-7s %-3s IPv6 %s.%d > %s.%d: %s, length %d%s\n", ifname, pkt_type_str(pkt_type), src_addr, src_port, - dst_addr, dst_port, transport_str, len); + dst_addr, dst_port, transport_str, len, flags); else - printf("%-7s %-3s IPv4 %s:%d > %s:%d: %s, length %d", + printf("%-7s %-3s IPv4 %s:%d > %s:%d: %s, length %d%s\n", ifname, pkt_type_str(pkt_type), src_addr, src_port, - dst_addr, dst_port, transport_str, len); - - if (proto == IPPROTO_TCP) { - if (tcp->fin) - printf(", FIN"); - if (tcp->syn) - printf(", SYN"); - if (tcp->rst) - printf(", RST"); - if (tcp->ack) - printf(", ACK"); - } - - printf("\n"); - funlockfile(stdout); + dst_addr, dst_port, transport_str, len, flags); }

static void show_ipv6_packet(const u_char *packet, u32 ifindex, u8 pkt_type)