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WIP/RFC: add support for manual link layer offset changes #1477

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WIP/RFC: add support for manual link layer offset changes #1477

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ec3f702
WIP/RFC: add support for manual link layer offset changes
Feb 26, 2025
26233df
WIP/RFC: allow manual computed offset adjusment
Feb 28, 2025
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8 changes: 8 additions & 0 deletions gencode.c
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Original file line number Diff line number Diff line change
Expand Up @@ -10712,3 +10712,11 @@ 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);
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@stefan-baranoff stefan-baranoff Feb 26, 2025

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I couldn't think of a better way of generating a "noop" block. I tried BPF_ALU|BPF_ADD|BPF_K with a value of 0, but the optimizer turned it into an unimplemented operator with an operand of 0xffff.

This is an odd ball in pcap-filter syntax, since it doesn't "check" for anything; it's an "always true" term that controls parsing rather than checking anything. But without returning a block something like offset+6 and host 192.0.2.68 would never work, since and assumes a block on both sides (as best I can tell). Returning NULL or nothing doesn't really work.

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;
}
2 changes: 2 additions & 0 deletions gencode.h
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Expand Up @@ -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_adjustment(compiler_state_t*, int);

/*
* 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.
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5 changes: 5 additions & 0 deletions grammar.y.in
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Expand Up @@ -401,12 +401,14 @@ 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 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 '!'
Expand Down Expand Up @@ -689,6 +691,8 @@ 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))); }
| pfvar { $$ = $1; }
| pqual p80211 { $$ = $2; }
| pllc { $$ = $1; }
Expand Down Expand Up @@ -942,4 +946,5 @@ mtp3fieldvalue: NUM {
mtp3listvalue: mtp3fieldvalue
| mtp3listvalue or mtp3fieldvalue { gen_or($1.b, $3.b); $$ = $3; }
;

%%
5 changes: 5 additions & 0 deletions scanner.l
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Expand Up @@ -471,6 +471,11 @@ 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; }
\+[0-9]+ { stou(yytext+1, yylval, yyextra); return OFFSET_PLUS; }
-[0-9]+ { stou(yytext+1, yylval, yyextra); return OFFSET_MINUS; }

[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; }
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