[PATCH v4] pasta: make it possible to disable socket splicing - dev

8 Dec 2024

During testing it is sometimes useful to force traffic which would
normally be forwared by socket splicing through the tap interface.

In this commit, we add a command switch enabling such funtionality
for inbound local traffic.

For outbound local traffic this is much trickier, if even possible,
so leave that for a later commit.

Suggested-by: David Gibson &lt;david(a)gibson.dropbear.id.au&gt;
Signed-off-by: Jon Maloy &lt;jmaloy(a)redhat.com&gt;

---
v2: Some minor changes based on feedback from PASST team
v3: More changes based on feedback from D. Gibson and S. Brivio
    -Moved new option to pasta-only section
    -Added description to man-page
v4: -Changed test on (mode == PASST) to (mode != PASTA) as
     suggested by Stefano Brivio.
---
 conf.c  | 7 ++++++-
 fwd.c   | 2 +-
 passt.1 | 4 ++++
 passt.h | 2 ++
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/conf.c b/conf.c
index eaa7d99..97d8beb 100644
--- a/conf.c
+++ b/conf.c
@@ -977,7 +977,8 @@ pasta_opts:
 		"			Don't copy all routes to namespace\n"
 		"  --no-copy-addrs	DEPRECATED:\n"
 		"			Don't copy all addresses to namespace\n"
-		"  --ns-mac-addr ADDR	Set MAC address on tap interface\n");
+		"  --ns-mac-addr ADDR	Set MAC address on tap interface\n"
+		"  --no-splice		Disable inbound socket splicing\n");
 
 	exit(status);
 }
@@ -1319,6 +1320,7 @@ void conf(struct ctx *c, int argc, char **argv)
 		{"no-dhcpv6",	no_argument,		&c->no_dhcpv6,	1 },
 		{"no-ndp",	no_argument,		&c->no_ndp,	1 },
 		{"no-ra",	no_argument,		&c->no_ra,	1 },
+		{"no-splice",	no_argument,		&c->no_splice,	1 },
 		{"freebind",	no_argument,		&c->freebind,	1 },
 		{"no-map-gw",	no_argument,		&no_map_gw,	1 },
 		{"ipv4-only",	no_argument,		NULL,		'4' },
@@ -1756,6 +1758,9 @@ void conf(struct ctx *c, int argc, char **argv)
 		}
 	} while (name != -1);
 
+	if (c->mode != MODE_PASTA)
+		c->no_splice = 1;
+
 	if (c->mode == MODE_PASTA && !c->pasta_conf_ns) {
 		if (copy_routes_opt)
 			die("--no-copy-routes needs --config-net");
diff --git a/fwd.c b/fwd.c
index 0b7f8b1..2829cd2 100644
--- a/fwd.c
+++ b/fwd.c
@@ -443,7 +443,7 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto,
 	else if (proto == IPPROTO_UDP)
 		tgt->eport += c->udp.fwd_in.delta[tgt->eport];
 
-	if (c->mode == MODE_PASTA && inany_is_loopback(&ini->eaddr)
&&
+	if (!c->no_splice && inany_is_loopback(&ini->eaddr) &&
 	    (proto == IPPROTO_TCP || proto == IPPROTO_UDP)) {
 		/* spliceable */
 
diff --git a/passt.1 b/passt.1
index b2896a2..c8a5783 100644
--- a/passt.1
+++ b/passt.1
@@ -695,6 +695,10 @@ Configure MAC address \fIaddr\fR on the tap interface in the
namespace.
 
 Default is to let the tap driver build a pseudorandom hardware address.
 
+.TP
+.BR \-\-no-splice
+Disable socket splicing for host to NS traffic.
+
 .SH EXAMPLES
 
 .SS \fBpasta
diff --git a/passt.h b/passt.h
index c038630..0dd4efa 100644
--- a/passt.h
+++ b/passt.h
@@ -229,6 +229,7 @@ struct ip6_ctx {
  * @no_dhcpv6:		Disable DHCPv6 server
  * @no_ndp:		Disable NDP handler altogether
  * @no_ra:		Disable router advertisements
+ * @no_splice:		Disable socket splicing for inbound traffic
  * @host_lo_to_ns_lo:	Map host loopback addresses to ns loopback addresses
  * @freebind:		Allow binding of non-local addresses for forwarding
  * @low_wmem:		Low probed net.core.wmem_max
@@ -291,6 +292,7 @@ struct ctx {
 	int no_dhcpv6;
 	int no_ndp;
 	int no_ra;
+	int no_splice;
 	int host_lo_to_ns_lo;
 	int freebind;
 
-- 
2.45.2


    