Account for PacketStart offsets in packet guards - #47
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The verifier rejects any programs that accesses packets with offsets greater than 0xFFFF. For absolute loads we can detect this statically, for indirect loads we insert a runtime check. But our checks didn't account for any pre-existing offset to the packet pointer the caller passes in! (eg because the caller has parsed / skipped the ethernet header). This must be accounted for in the maximum offset check. Add an option to both EBPFOpts and COpts to allow users to specify the maximum offset the packet pointer may have, so it can be included in the static analysis for absolute loads, and the runtime checks for indirect loads. Thank you to @Dhiver for figuring this out in #45.
Dhiver
approved these changes
Jul 28, 2026
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The verifier rejects any programs that accesses packets with offsets greater than 0xFFFF.
For absolute loads we can detect this statically, for indirect loads we insert a runtime check.
But our checks didn't account for any pre-existing offset to the packet pointer the caller passes in! (eg because the caller has parsed / skipped the ethernet header).
This must be accounted for in the maximum offset check.
Add an option to both EBPFOpts and COpts to allow users to specify the maximum offset the packet pointer may have, so it can be included in the static analysis for absolute loads, and the runtime checks for indirect loads.
Thank you to @Dhiver for figuring this out in #45.