WIP/RFC: allow manual computed offset adjusment

Some keywords, like VLAN and MPLS, implicitly adjust offsets. New protocols not
yet handled by pcap-filter may also require offset jumping, but are not part of
the existing known protocol code. Allowing arbitrary offset adjustments by users
supports overcoming long standing "vlan X or vlan Y" issues, albeit with awkward
syntax, and for jumping over new protocols not yet handled explicitly, like
VNTag.

This second attempt switches from adjusting compile time static offsets to using
the variable arguments already baked into the offset structs to allow use with
nvarg to generate either fixed or dynamically computed jumps. There is no
support for "set to X"; all actions are increment or decrement. There is also no
support at this time for selecting which offsets to adjust. That can be added
once the basic concept is flushed out and there's some agreement on if this is a
desired feature, syntax, and implementation approach.

Initially my thoughts for a next commit to support selecting offset are
something like `off-linkhdr` or `off-nl` or `off-linkpl` for each type of offset
that might be reasonable for an end user to want to adjust themseleves.

Author: Stefan Baranoff

Diff for: gencode.c

@@ -10713,10 +10713,49 @@ gen_atmmulti_abbrev(compiler_state_t *cstate, int type)
 	return b1;
 }
 
-struct block *gen_offset_adjustment(compiler_state_t *cstate, int n) {
-    struct block *b = new_block(cstate, BPF_JMP|BPF_JA);
-    b->s.k = 0;  // Jump by 0 bytes, effectively a no-op
-    cstate->off_linkpl.constant_part += n;
-    cstate->off_linktype.constant_part += n;
-    return b;
+struct block*
+gen_offset(compiler_state_t *cstate, struct arth *a_arg)
+{
+	struct arth *a = a_arg;
+	struct block *b;
+	struct slist *s, *s1;
+	bpf_abs_offset *offsets[2] = {&(cstate->off_linkpl), &(cstate->off_linktype)};
+	int i;
+
+	/*
+	 * Catch errors reported by us and routines below us, and return NULL
+	 * on an error.
+	 */
+	if (setjmp(cstate->top_ctx))
+		return (NULL);
+
+	s = NULL;
+	b = gen_true(cstate);
+	for (i = 0; i < 2; ++i) {
+		bpf_abs_offset *off = offsets[i];
+		// Make sure the offset being adjusted is variable
+		if (!off->is_variable)
+			off->is_variable = 1;
+		if (off->reg == -1)
+			off->reg = alloc_reg(cstate);
+
+		// Put the current offset into the accumulator
+		s = new_stmt(cstate, BPF_LD|BPF_MEM);
+		s->s.k = off->reg;
+
+		// Put the computed offset into the extra register
+		s1 = xfer_to_x(cstate, a);
+		sappend(s, s1);
+
+		// Add to the accumulator from the computed offset in the extra register
+		s1 = new_stmt(cstate, BPF_ALU|BPF_ADD|BPF_X);
+		sappend(s, s1);
+
+		s1 = new_stmt(cstate, BPF_ST);
+		s1->s.k = off->reg;
+		sappend(s, s1);
+	}
+	sappend(a->s, s);
+	b->stmts = a->s;
+	return b;
 }

Diff for: gencode.h

@@ -378,7 +378,7 @@ struct block *gen_pf_action(compiler_state_t *, int);
 struct block *gen_p80211_type(compiler_state_t *, bpf_u_int32, bpf_u_int32);
 struct block *gen_p80211_fcdir(compiler_state_t *, bpf_u_int32);
 
-struct block *gen_offset_adjustment(compiler_state_t*, int);
+struct block *gen_offset(compiler_state_t *, struct arth *);
 
 /*
  * Representation of a program as a tree of blocks, plus current mark.

Diff for: grammar.y.in

@@ -401,14 +401,13 @@ DIAG_OFF_BISON_BYACC
 %token	RADIO
 %token	FISU LSSU MSU HFISU HLSSU HMSU
 %token	SIO OPC DPC SLS HSIO HOPC HDPC HSLS
-%token  OFFSET OFFSET_PLUS OFFSET_MINUS
+%token	OFFSET
 %token	LEX_ERROR
 
 %type	<s> ID EID AID
 %type	<s> HID HID6
 %type	<h> NUM
 %type	<i> action reason type subtype type_subtype dir
-%type   <h> OFFSET_PLUS OFFSET_MINUS
 
 %left OR AND
 %nonassoc  '!'
@@ -691,8 +690,7 @@ other:	  pqual TK_BROADCAST	{ CHECK_PTR_VAL(($$ = gen_broadcast(cstate, $1))); }
 	| GENEVE		{ CHECK_PTR_VAL(($$ = gen_geneve(cstate, 0, 0))); }
 	| VXLAN pnum		{ CHECK_PTR_VAL(($$ = gen_vxlan(cstate, $2, 1))); }
 	| VXLAN			{ CHECK_PTR_VAL(($$ = gen_vxlan(cstate, 0, 0))); }
-	| OFFSET OFFSET_PLUS    { CHECK_PTR_VAL(($$ = gen_offset_adjustment(cstate, $2))); }
-    	| OFFSET OFFSET_MINUS   { CHECK_PTR_VAL(($$ = gen_offset_adjustment(cstate, -$2))); }
+	| OFFSET arth		{ CHECK_PTR_VAL(($$ = gen_offset(cstate, $2))); }
 	| pfvar			{ $$ = $1; }
 	| pqual p80211		{ $$ = $2; }
 	| pllc			{ $$ = $1; }
@@ -946,5 +944,4 @@ mtp3fieldvalue: NUM {
 mtp3listvalue: mtp3fieldvalue
 	| mtp3listvalue or mtp3fieldvalue { gen_or($1.b, $3.b); $$ = $3; }
 	;
-
 %%

Diff for: scanner.l

@@ -472,9 +472,7 @@ tcp-urg			{ yylval->h = 0x20; return NUM; }
 tcp-ece			{ yylval->h = 0x40; return NUM; }
 tcp-cwr			{ yylval->h = 0x80; return NUM; }
 
-offset		{ return OFFSET; }
-\+[0-9]+        { stou(yytext+1, yylval, yyextra); return OFFSET_PLUS; }
--[0-9]+         { stou(yytext+1, yylval, yyextra); return OFFSET_MINUS; }
+offset			return OFFSET;
 
 [A-Za-z0-9]([-_.A-Za-z0-9]*[.A-Za-z0-9])? {
 			 yylval->s = sdup(yyextra, (char *)yytext); return ID; }