bpf: Fix csum logic in snat_v4_rewrite_headers #28768
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The old_port != new_port flow in snat_v4_rewrite_headers has a few bugs in checksum calculation: 1. l4_sum is overwritten, so the change in the IP addresses is not taken into account when later calling csum_l4_replace. 2. The change in ports is accounted with BPF_F_PSEUDO_HDR, which leads to incorrect calculations because ports are not part of the pseudo-header. 3. The ICMPv4 branch is correct as is, because it takes into account only the ports and clears the BPF_F_PSEUDO_HDR flag for them, but it has to be tweaked as well after fixing (1) and (2). Fix the above bugs by separating patching the L4 checksum due to changes in the ports and the addresses, pass BPF_F_PSEUDO_HDR only for the addresses, add the explanatory comments, and ensure that ICMPv4 is still handled correctly after the fixes. Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
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julianwiedmann
approved these changes
Oct 25, 2023
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The old_port != new_port flow in snat_v4_rewrite_headers has a few bugs in checksum calculation:
Fix the above bugs by separating patching the L4 checksum due to changes in the ports and the addresses, pass BPF_F_PSEUDO_HDR only for the addresses, add the explanatory comments, and ensure that ICMPv4 is still handled correctly after the fixes.