Fix jump calculation

[jschwinger233]

Currently, an error is triggered when calling ToEBPF() on cBPF instructions generated by the filter "ip6 protochain 58":

$ tcpdump -dd ip6 protochain 58 | cat -n
Warning: assuming Ethernet
     1	{ 0x28, 0, 0, 0x0000000c },
     2	{ 0x15, 0, 33, 0x000086dd },
     3	{ 0x30, 0, 0, 0x00000014 },
     4	{ 0x1, 0, 0, 0x00000028 },
     5	{ 0x15, 27, 0, 0x0000003a },
     6	{ 0x15, 26, 0, 0x0000003b },
     7	{ 0x15, 3, 0, 0x00000000 },
     8	{ 0x15, 2, 0, 0x0000003c },
     9	{ 0x15, 1, 0, 0x0000002b },
    10	{ 0x15, 0, 9, 0x0000002c },
    11	{ 0x50, 0, 0, 0x0000000e },
    12	{ 0x2, 0, 0, 0x00000000 },
    13	{ 0x50, 0, 0, 0x0000000f },
    14	{ 0x4, 0, 0, 0x00000001 },
    15	{ 0x24, 0, 0, 0x00000008 },
    16	{ 0xc, 0, 0, 0x00000000 },
    17	{ 0x7, 0, 0, 0x00000000 },
    18	{ 0x60, 0, 0, 0x00000000 },
    19	{ 0x5, 0, 0, 0xfffffff1 },
    20	{ 0x15, 0, 12, 0x00000033 },
    21	{ 0x87, 0, 0, 0x00000000 },
    22	{ 0x50, 0, 0, 0x0000000e },
    23	{ 0x2, 0, 0, 0x00000000 },
    24	{ 0x87, 0, 0, 0x00000000 },
    25	{ 0x4, 0, 0, 0x00000001 },
    26	{ 0x7, 0, 0, 0x00000000 },
    27	{ 0x50, 0, 0, 0x0000000e },
    28	{ 0x4, 0, 0, 0x00000002 },
    29	{ 0x24, 0, 0, 0x00000004 },
    30	{ 0x7, 0, 0, 0x00000000 },
    31	{ 0x60, 0, 0, 0x00000000 },
    32	{ 0x5, 0, 0, 0xffffffe4 },
    33	{ 0x4, 0, 0, 0x00000000 },
    34	{ 0x15, 0, 1, 0x0000003a },
    35	{ 0x6, 0, 0, 0x00040000 },
    36	{ 0x6, 0, 0, 0x00000000 },

Line 19 {0x5, 0, 0, 0xfffffff1} is a "JA 0xfffffff1" instruction. When cbpfc.ToEBPF() attempts to translate this jump, it miscalculates the destination due to the incorrect type of the pos variable.

The bug arises because pos is currently of type uint (which is uint64 on 64-bit systems), whereas it should be uint32 to correctly emulate cBPF's 32-bit overflow semantics. Instead of computing the correct jump destination as:

uint32(18 + 1 + 0xfffffff1) = 4

It evaluates to:

18 + 1 + 0xfffffff1 = 0x100000004 (due to 64-bit addition)

As a result, the jump target becomes invalid, and ToEBPF() errors out with:

instruction 18: ja 4294967281 flows past last instruction

This patch changes the type of pos to uint32, ensuring correct overflow behavior and jump target computation consistent with cBPF expectations.

[arthurfabre]

Great catch, thank you for the PR! Do you mind adding a test? I think we can add one in insn_test.go. No worries if not, I can add one otherwise.

[jschwinger233]

Hey Arthur, could you help me with the test please, I am afraid I am off keyboard for a few days 🙏

[jschwinger233]

One more thing that I’m sure you’re already aware of: this PR does not fix cloudflare/cbpfc#37, because our cbpf.CompileEBPF() still gets stuck in an infinite loop due to the backwards jump.