tcp: Clean up tcpi_snd_wnd probing

When available, we want to retrieve our socket peer's advertised window and
forward that to the guest.  That information has been available from the
kernel via the TCP_INFO getsockopt() since kernel commit 8f7baad7f035.

Currently our probing for this is a bit odd.  The HAS_SND_WND define
determines if our headers include the tcp_snd_wnd field, but that doesn't
necessarily mean the running kernel supports it.  Currently we start by
assuming it's _not_ available, but mark it as available if we ever see
a non-zero value in the field.  This is a bit hit and miss in two ways:
 * Zero is perfectly possible window the peer could report, so we can
   get false negatives
 * We're reading TCP_INFO into a local variable, which might not be zero
   initialised, so if the kernel _doesn't_ write it it could have non-zero
   garbage, giving us false positives.

We can use a more direct way of probing for this: getsockopt() reports the
length of the information retreived.  So, check whether that's long enough
to include the field.  This lets us probe the availability of the field
once and for all during initialisation.  That in turn allows ctx to become
a const pointer to tcp_prepare_flags() which cascades through many other
functions.

We also move the flag for the probe result from the ctx structure to a
global, to match peek_offset_cap.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 5 insertions, 8 deletions

diff --git a/tcp.h b/tcp.h
index e9ff019..5585924 100644
--- a/tcp.h
+++ b/tcp.h
@@ -10,11 +10,12 @@
 
 struct ctx;
 
-void tcp_timer_handler(struct ctx *c, union epoll_ref ref);
-void tcp_listen_handler(struct ctx *c, union epoll_ref ref,
+void tcp_timer_handler(const struct ctx *c, union epoll_ref ref);
+void tcp_listen_handler(const struct ctx *c, union epoll_ref ref,
 			const struct timespec *now);
-void tcp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t events);
-int tcp_tap_handler(struct ctx *c, uint8_t pif, sa_family_t af,
+void tcp_sock_handler(const struct ctx *c, union epoll_ref ref,
+		      uint32_t events);
+int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 		    const void *saddr, const void *daddr,
 		    const struct pool *p, int idx, const struct timespec *now);
 int tcp_sock_init(const struct ctx *c, sa_family_t af, const void *addr,
@@ -58,16 +59,12 @@ union tcp_listen_epoll_ref {
  * @fwd_in:		Port forwarding configuration for inbound packets
  * @fwd_out:		Port forwarding configuration for outbound packets
  * @timer_run:		Timestamp of most recent timer run
- * @kernel_snd_wnd:	Kernel reports sending window (with commit 8f7baad7f035)
  * @pipe_size:		Size of pipes for spliced connections
  */
 struct tcp_ctx {
 	struct fwd_ports fwd_in;
 	struct fwd_ports fwd_out;
 	struct timespec timer_run;
-#ifdef HAS_SND_WND
-	int kernel_snd_wnd;
-#endif
 	size_t pipe_size;
 };