WIP/RFC: add support for manual link layer offset changes

gencode.c

@@ -10712,3 +10712,52 @@ gen_atmmulti_abbrev(compiler_state_t *cstate, int type)
 	}
 	return b1;
 }
+
+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];
+	int i;
+	offsets[0] = &(cstate->off_linkpl);
+	offsets[1] = &(cstate->off_linktype)};
+
+	/*
+	 * 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;
+}

gencode.h

@@ -378,6 +378,8 @@ 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(compiler_state_t *, struct arth *);
+
 /*
  * Representation of a program as a tree of blocks, plus current mark.
  * A block is marked if only if its mark equals the current mark.

grammar.y.in

@@ -401,6 +401,7 @@ DIAG_OFF_BISON_BYACC
 %token	RADIO
 %token	FISU LSSU MSU HFISU HLSSU HMSU
 %token	SIO OPC DPC SLS HSIO HOPC HDPC HSLS
+%token	OFFSET
 %token	LEX_ERROR
 
 %type	<s> ID EID AID
@@ -689,6 +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 arth		{ CHECK_PTR_VAL(($$ = gen_offset(cstate, $2))); }
 	| pfvar			{ $$ = $1; }
 	| pqual p80211		{ $$ = $2; }
 	| pllc			{ $$ = $1; }

scanner.l

@@ -471,6 +471,8 @@ tcp-ack			{ yylval->h = 0x10; return NUM; }
 tcp-urg			{ yylval->h = 0x20; return NUM; }
 tcp-ece			{ yylval->h = 0x40; return NUM; }
 tcp-cwr			{ yylval->h = 0x80; return NUM; }
+
+offset			return OFFSET;
 [A-Za-z0-9]([-_.A-Za-z0-9]*[.A-Za-z0-9])? {
 			 yylval->s = sdup(yyextra, (char *)yytext); return ID; }
 "\\"[^ !()\n\t]+	{ yylval->s = sdup(yyextra, (char *)yytext + 1); return ID; }