tc/skbmod: Remove misinformation about the swap action

Currently man 8 tc-skbmod says that "...the swap action will occur after
any smac/dmac substitutions are executed, if they are present."

This is false.  In fact, trying to "set" and "swap" in a single skbmod
command causes the "set" part to be completely ignored.  As an example:

	$ tc filter add dev eth0 parent 1: protocol ip prio 10 \
		matchall action skbmod \
        	set dmac AA:AA:AA:AA:AA:AA smac BB:BB:BB:BB:BB:BB \
        	swap mac

The above command simply does a "swap", without setting DMAC or SMAC to
AA's or BB's.  The root cause of this is in the kernel, see
net/sched/act_skbmod.c:tcf_skbmod_init():

	parm = nla_data(tb[TCA_SKBMOD_PARMS]);
	index = parm->index;
	if (parm->flags & SKBMOD_F_SWAPMAC)
		lflags = SKBMOD_F_SWAPMAC;
		^^^^^^^^^^^^^^^^^^^^^^^^^^

Doing a "=" instead of "|=" clears all other "set" flags when doing a
"swap".  Discourage using "set" and "swap" in the same command by
documenting it as undefined behavior, and update the "SYNOPSIS" section
as well as tc -help text accordingly.

If one really needs to e.g. "set" DMAC to all AA's then "swap" DMAC and
SMAC, one should do two separate commands and "pipe" them together.

Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

man/man8/tc-skbmod.8

@@ -5,12 +5,12 @@ skbmod - user-friendly packet editor action
 .SH SYNOPSIS
 .in +8
 .ti -8
-.BR tc " ... " "action skbmod " "{ [ " "set "
+.BR tc " ... " "action skbmod " "{ " "set "
-.IR SETTABLE " ] [ "
+.IR SETTABLE " | "
 .BI swap " SWAPPABLE"
-.RI " ] [ " CONTROL " ] [ "
+.RI " } [ " CONTROL " ] [ "
 .BI index " INDEX "
-] }
+]
 
 .ti -8
 .IR SETTABLE " := "
@@ -25,6 +25,7 @@ skbmod - user-friendly packet editor action
 .IR SWAPPABLE " := "
 .B mac
 .ti -8
+
 .IR CONTROL " := {"
 .BR reclassify " | " pipe " | " drop " | " shot " | " continue " | " pass " }"
 .SH DESCRIPTION
@@ -48,10 +49,7 @@ Change the source mac to the specified address.
 Change the ethertype to the specified value.
 .TP
 .BI mac
-Used to swap mac addresses. The
+Used to swap mac addresses.
-.B swap mac
-directive is performed
-after any outstanding D/SMAC changes.
 .TP
 .I CONTROL
 The following keywords allow to control how the tree of qdisc, classes,
@@ -128,9 +126,13 @@ tc filter add dev eth3 parent 1: protocol ip prio 10 \\
 .EE
 .RE
 
-As mentioned above, the swap action will occur after any
+However, trying to
-.B " smac/dmac "
+.B set
-substitutions are executed, if they are present.
+and
+.B swap
+in a single
+.B skbmod
+command will cause undefined behavior.
 
 .SH SEE ALSO
 .BR tc (8),

tc/m_skbmod.c

@@ -28,10 +28,9 @@
 static void skbmod_explain(void)
 {
 	fprintf(stderr,
-		"Usage:... skbmod {[set <SETTABLE>] [swap <SWAPABLE>]} [CONTROL] [index INDEX]\n"
+		"Usage:... skbmod { set <SETTABLE> | swap <SWAPPABLE> } [CONTROL] [index INDEX]\n"
 		"where SETTABLE is: [dmac DMAC] [smac SMAC] [etype ETYPE]\n"
-		"where SWAPABLE is: \"mac\" to swap mac addresses\n"
+		"where SWAPPABLE is: \"mac\" to swap mac addresses\n"
-		"note: \"swap mac\" is done after any outstanding D/SMAC change\n"
 		"\tDMAC := 6 byte Destination MAC address\n"
 		"\tSMAC := optional 6 byte Source MAC address\n"
 		"\tETYPE := optional 16 bit ethertype\n"