bpf: fib: stream-line fib_do_redirect() #39490
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We expect all supported kernels to offer HAVE_FIB_INDEX, and there's no code that would still use the `ifindex` field. Reclaim these two bytes for the future. Note that in particular there's no downgrade concerns here - even if we go back to v1.17, the underlying kernel is still the same and provides HAVE_FIB_INDEX. And thus Cilium v1.17 doesn't expect us to provide CT entries with a valid `ifindex`. Signed-off-by: Julian Wiedmann <jwi@isovalent.com>
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Move the selection of the egress interface into fib_do_redirect()'s callers, and provide it through a single `oif` parameter (which now becomes read-only). The callers know much better what information is available (FIB lookup result, DIRECT_ROUTING_DEV_IFINDEX, ...) and what outcome they want. And some of them already look at `fib_params->l.ifindex` anyway. Note that is just a cleanup, there shouldn't be any change in behavior. While at it also update the big code comment to reflect current reality. Signed-off-by: Julian Wiedmann <jwi@isovalent.com>
XDP programs have no business using redirect_neigh(). This turns a dormant runtime error into an immediate build-time bug. Signed-off-by: Julian Wiedmann <jwi@isovalent.com>
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ldelossa
approved these changes
May 12, 2025
YutaroHayakawa
approved these changes
May 12, 2025
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#38308 helped to simplify the BPF-redirect logic, as we can now assume support for
HAVE_FIB_IFINDEX. Therefore let's lift the remaining processing of the FIB result from the low-level redirect helper, and make it the caller's responsibility to select the egress interface.While at it also free up some related space in the CT entry thanks to #38308, and clarify the use of
redirect_neigh()in XDP context.