Pflua does not allow negative numbers; tcpdump does

[kbara]

Expected behavior: the behavior of tcpdump and pflua should match.

The fix: change the pflua parser.

Details:
% tcpdump -d "len < 2 * -5"
(000) ld #pktlen
(001) jge #0xfffffff6 jt 2 jf 3
(002) ret #0
(003) ret #65535

luajit: deps/pflua/src/pf/parse.lua:283:
Error: In expression "len < 2 * -5"
^
bad token while parsing arithmetic expression -

stack traceback:
[C]: in function 'primitive_error'
deps/pflua/src/pf/parse.lua:283: in function 'error'
deps/pflua/src/pf/parse.lua:660: in function 'parse_primary_arithmetic'
deps/pflua/src/pf/parse.lua:674: in function 'parse_arithmetic'
deps/pflua/src/pf/parse.lua:681: in function 'parse_arithmetic'
deps/pflua/src/pf/parse.lua:822: in function 'parse_logical_or_arithmetic'
deps/pflua/src/pf/parse.lua:846: in function 'parse_logical'
deps/pflua/src/pf/parse.lua:854: in function 'parse'
deps/pflua/src/pf.lua:30: in function 'compile_filter'
./pflangmatheval.lua:25: in function 'pflang_eval'
./pflua-quickcheck:148: in function 'property_pflua_and_tcpdump_arithmetic_match'
./pflua-quickcheck:191: in function 'main'
./pflua-quickcheck:196: in main chunk
[C]: at 0x00404bc0

[kbara]

Horrifyingly enough, for full compatibility we need to support an arbitrary number of '-'s.
% tcpdump -d "len < 1 ---1"
(000) ld #pktlen
(001) jge #0x0 jt 2 jf 3

% tcpdump -d "len < 1 ----1"
(000) ld #pktlen
(001) jge #0x2 jt 2 jf 3

[kbara]

It gets sillier:
% tcpdump -d "len < 1 - - -- 1"
(000) ld #pktlen
(001) jge #0x2 jt 2 jf 3
Arbitrary spaces seem to be allowed.

[mpeterv]

It seems that tcpdump does not allow negative number literals (e.g. tcpdump -d 'port -1' gives syntax error), but it has unary minus operator (e.g. tcpdump -d '1 = -(-(1))' is valid).

[kbara]

Yep, you're entirely right. The relevant bits of libpcap source define gen_neg and UMINUS, and it's a unary operator.