tcp: Remove compile-time dependency on struct tcp_info version

In the Makefile we probe to create several defines based on the presence
of particular fields in struct tcp_info.  These defines are used for two
purposes, neither of which they accomplish well:

1) Determining if the tcp_info fields are available at runtime.  For this
   purpose the defines are Just Plain Wrong, since the runtime kernel may
   not be the same as the compile time kernel. We corrected this for
   tcp_snd_wnd, but not for tcpi_bytes_acked or tcpi_min_rtt

2) Allowing the source to compile against older kernel headers which don't
   have the fields in question.  This works in theory, but it does mean
   we won't be able to use the fields, even if later run against a
   newer kernel.  Furthermore, it's quite fragile: without much more
   thorough tests of builds in different environments that we're currently
   set up for, it's very easy to miss cases where we're accessing a field
   without protection from an #ifdef.  For example we currently access
   tcpi_snd_wnd without #ifdefs in tcp_update_seqack_wnd().

Improve this with a different approach, borrowed from qemu (which has many
instances of similar problems).  Don't compile against linux/tcp.h, using
netinet/tcp.h instead.  Then for when we need an extension field, define
a struct tcp_info_linux, copied from the kernel, with all the fields we're
interested in.  That may need updating from future kernel versions, but
only when we want to use a new extension, so it shouldn't be frequent.

This allows us to remove the HAS_SND_WND define entirely.  We keep
HAS_BYTES_ACKED and HAS_MIN_RTT now, since they're used for purpose (1),
we'll fix that in a later patch.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Trivial grammar fixes in comments]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 0 insertions, 5 deletions

diff --git a/Makefile b/Makefile
index 74a9513..6faa501 100644
--- a/Makefile
+++ b/Makefile
@@ -67,11 +67,6 @@ PASST_HEADERS = arch.h arp.h checksum.h conf.h dhcp.h dhcpv6.h flow.h fwd.h \
 	udp.h udp_flow.h util.h
 HEADERS = $(PASST_HEADERS) seccomp.h
 
-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 $$?),0)
-	FLAGS += -DHAS_SND_WND
-endif
-
 C := \#include <linux/tcp.h>\nstruct tcp_info x = { .tcpi_bytes_acked = 0 };
 ifeq ($(shell printf "$(C)" | $(CC) -S -xc - -o - >/dev/null 2>&1; echo $$?),0)
 	FLAGS += -DHAS_BYTES_ACKED