Makefile, tcp: Don't try to use tcpi_snd_wnd from tcp_info on pre-5.3 kernels

Detect missing tcpi_snd_wnd in struct tcp_info at build time,
otherwise build fails with a pre-5.3 linux/tcp.h header.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

3 files changed, 15 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 85d7400..83140ad 100644
--- a/Makefile
+++ b/Makefile
@@ -29,6 +29,11 @@ endif
 endif
 endif
 
+C := \#include <linux/tcp.h>\nstruct tcp_info x = { .tcpi_snd_wnd = 0 };
+ifeq ($(shell printf "$(C)" | $(CC) -S -xc - -o - >/dev/null 2>&1; echo $$?),0)
+	CFLAGS += -DHAS_SND_WND
+endif
+
 prefix ?= /usr/local
 
 all: passt pasta passt4netns qrap
diff --git a/tcp.c b/tcp.c
index 6b5d29b..813d2ed 100644
--- a/tcp.c
+++ b/tcp.c
@@ -359,6 +359,11 @@
 		 sizeof(struct ipv6hdr) - sizeof(struct tcphdr))
 
 #define WINDOW_DEFAULT			14600		/* RFC 6928 */
+#ifdef HAS_SND_WND
+# define KERNEL_REPORTS_SND_WND(c)	(c->tcp.kernel_snd_wnd)
+#else
+# define KERNEL_REPORTS_SND_WND(c)	(0 && (c))
+#endif
 
 #define SYN_TIMEOUT			240000		/* ms */
 #define ACK_TIMEOUT			2000
@@ -1563,7 +1568,7 @@ static int tcp_update_seqack_wnd(struct ctx *c, struct tcp_tap_conn *conn,
 			conn->seq_ack_to_tap = prev_ack_to_tap;
 	}
 
-	if (!c->tcp.kernel_snd_wnd) {
+	if (!KERNEL_REPORTS_SND_WND(c)) {
 		tcp_get_sndbuf(conn);
 		conn->wnd_to_tap = MIN(conn->snd_buf, MAX_WINDOW);
 		goto out;
@@ -1675,8 +1680,10 @@ static int tcp_send_to_tap(struct ctx *c, struct tcp_tap_conn *conn, int flags,
 		data += OPT_MSS_LEN - 2;
 		th->doff += OPT_MSS_LEN / 4;
 
+#ifdef HAS_SND_WND
 		if (!c->tcp.kernel_snd_wnd && info.tcpi_snd_wnd)
 			c->tcp.kernel_snd_wnd = 1;
+#endif
 
 		conn->ws = MIN(MAX_WS, info.tcpi_snd_wscale);
 
diff --git a/tcp.h b/tcp.h
index bfd6082..dc854b8 100644
--- a/tcp.h
+++ b/tcp.h
@@ -67,7 +67,9 @@ struct tcp_ctx {
 	uint8_t port_to_init	[USHRT_MAX / 8];
 	int ns_detect_ports;
 	struct timespec timer_run;
+#ifdef HAS_SND_WND
 	int kernel_snd_wnd;
+#endif
 	size_t pipe_size;
 	struct timespec refill_ts;
 	struct timespec port_detect_ts;