Add EBPFOpts.PacketStartOffset for non-zero-based PacketStart#45
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ToEBPF's indirect packet-load guard assumed PacketStart points at offset 0 of the packet. It bounds the data-dependent index against maxStartOffset() = maxPacketOffset - length + 1, relying on the verifier's rule that a packet pointer only gets a range when off + umax_value <= MAX_PACKET_OFF (0xFFFF). When the caller sets PacketStart to a pointer that already carries a fixed offset (e.g. it points at the L3 header rather than the start of the frame), that fixed offset is added on top of the guarded index. An indirect load with a data-dependent index (such as walking TCP options) can then push off + umax_value past MAX_PACKET_OFF, and the verifier rejects the program with "invalid access to packet" even though the access is safe. Add EBPFOpts.PacketStartOffset so callers can declare PacketStart's fixed offset. maxStartOffsetWithBase() reserves that many bytes of the packet-offset budget in the indirect guard so the bounded range stays within MAX_PACKET_OFF. It fails closed (returns 0, i.e. noMatch) when no room remains, rather than emitting an unbounded, unverifiable load. maxStartOffsetWithBase also rejects a negative base offset (PacketStart cannot precede the packet) and fails closed, so bogus input never widens the guard's budget. PacketStartOffset defaults to 0, preserving existing behaviour.
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ToEBPF's indirect packet-load guard assumed PacketStart points at offset 0 of the packet. It bounds the data-dependent index against maxStartOffset() = maxPacketOffset - length + 1, relying on the verifier's rule that a packet pointer only gets a range when off + umax_value <= MAX_PACKET_OFF (0xFFFF).
When the caller sets PacketStart to a pointer that already carries a fixed offset (e.g. it points at the L3 header rather than the start of the frame), that fixed offset is added on top of the guarded index. An indirect load with a data-dependent index (such as walking TCP options) can then push off + umax_value past MAX_PACKET_OFF, and the verifier rejects the program with "invalid access to packet" even though the access is safe.
Add EBPFOpts.PacketStartOffset so callers can declare PacketStart's fixed offset. maxStartOffsetWithBase() reserves that many bytes of the packet-offset budget in the indirect guard so the bounded range stays within MAX_PACKET_OFF. It fails closed (returns 0, i.e. noMatch) when no room remains, rather than emitting an unbounded, unverifiable load. maxStartOffsetWithBase also rejects a negative base offset (PacketStart cannot precede the packet) and fails closed, so bogus input never widens the guard's budget.
PacketStartOffset defaults to 0, preserving existing behaviour.