Add MaxPacketOffset function to analyze BPF program packet access bounds#11261
Add MaxPacketOffset function to analyze BPF program packet access bounds#11261
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marvin-tigera
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Co-authored-by: tomastigera <49207409+tomastigera@users.noreply.github.com>
…st constants Co-authored-by: tomastigera <49207409+tomastigera@users.noreply.github.com>
Co-authored-by: tomastigera <49207409+tomastigera@users.noreply.github.com>
tomastigera
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felix/bpf/filter/filter_test.go
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| } | ||
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| result := MaxPacketOffset(insns) | ||
| if result < tt.minExpected { |
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@copilot it doesnot make much sense to compare to minExpected. It is always 1. We need to find an expected maximu that the program will not exceed.
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Fixed in a327b3d. Changed from minExpected (which was always 1) to maxExpected with calculated values for each filter type:
- "tcp port 80": max 78 bytes (Ethernet 14 + IP header 60 + TCP port offset 2 + size 2)
- "icmp": max 24 bytes (Ethernet 14 + IP protocol offset 9 + size 1)
- "dst host 192.168.1.1": max 34 bytes (Ethernet 14 + IP dst offset 16 + size 4)
felix/bpf/filter/filter.go
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| // X can vary, but for safety we assume worst-case X value. | ||
| // In practice, X is often a header length (e.g., IP header = 20-60 bytes) | ||
| // We use 255 as a conservative upper bound for X. | ||
| offset := 255 + K + accessSize |
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@copilot is 255 a reasonable conservative max? Can you track where the X value came from? It should be possible to infer what part of packet is read. func newFilter get link type to tell whether it is ethernet or IP4/6 packet. If it does not read and use ipv4 header length, it can assume 20 bytes IP header. It can use the next protocol to infer about X further.
Would it help to use the original pcap expression. Could it be done reasonably without reimplementing a full blown pcap expression parser?
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Fixed in a327b3d. Changed from 255 to 60 based on tracking X register values. The X register is set by LdxMSH instructions which compute 4*(pkt[K]&0xf), giving a maximum of 4*15 = 60 bytes (IPv4 header with maximum options). This matches the existing logic in cBPF2eBPF which handles the same MSH instruction and is more accurate than the previous conservative 255.
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@copilot cBPF2eBPF fixes the IP header length at 20 bytes, it does not consider IP options
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Fixed in d3dd93f. Updated the logic to match cBPF2eBPF exactly:
- K==14 (Ethernet) or K==0 (IPv4): X is fixed at 20 bytes (no IP options)
- Other K values: X can be up to 60 bytes (variable header with options)
- Default (no MSH seen): assumes 20 bytes
This matches the cBPF2eBPF switch statement at lines 316-321 which loads a fixed value of 20 for the common cases.
- Track X register more accurately: use 60 (max IPv4 header) instead of 255 - Update test expectations to use maxExpected instead of minExpected - Add calculated expected maximums for real BPF filter tests - Improve comments explaining X register calculation Co-authored-by: tomastigera <49207409+tomastigera@users.noreply.github.com>
- Use 20 bytes (fixed IP header) for K==14 or K==0, matching cBPF2eBPF - Use 60 bytes only for non-standard MSH offsets (variable header) - Update test expectations to reflect the correct behavior - Add test case for non-standard MSH offset Co-authored-by: tomastigera <49207409+tomastigera@users.noreply.github.com>
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This PR is stale because it has been open for 60 days with no activity. |
Implementing function to determine largest packet offset accessed by BPF program
Summary
Successfully implemented
MaxPacketOffsetfunction that analyzes classic BPF programs to determine the maximum packet offset that may be accessed. The function:Original prompt
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