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btf: provide work around for missing BTF_KIND_ENUM64 support #1116

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lmb opened this issue Aug 11, 2023 · 1 comment · Fixed by #1132
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btf: provide work around for missing BTF_KIND_ENUM64 support #1116

lmb opened this issue Aug 11, 2023 · 1 comment · Fixed by #1132
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lmb commented Aug 11, 2023 •
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pahole from v1.24 generates a new BTF_KIND_ENUM64. The library has support for this as well, and only encodes an enum64 if necessary to represent a type. This still causes a problem when compiling an old kernel with a new pahole, since TestRoundtripVmlinux then tries to load the enum64 into the kernel.

This shows that it would be nice^H^H^H it's necessary to have a general workaround for this. libbpf does the following when enum64 is not supported:

  • It clears the kind flag on BTF_KIND_ENUM and BTF_KIND_ENUM64. This is used to specify signedness and only cosmetic from what I can tell.
  • It replaces BTF_KIND_ENUM64 with a union.

Details are here: https://lwn.net/ml/bpf/20220603015937.1190992-1-yhs@fb.com/
@lmb lmb added the enhancement New feature or request label Aug 11, 2023
@lmb lmb mentioned this issue Aug 14, 2023
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lmb commented Aug 14, 2023

I'm a bit fuzzy why libbpf replaces the enum with a union? It doesn't seem like it can preserve values, etc. that way?

Implementing this also requires at least v6.0 in CI, which is when BTF_KIND_ENUM64 appeared.

@lmb lmb mentioned this issue Aug 15, 2023
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lmb added a commit to lmb/ebpf that referenced this issue Sep 13, 2023
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btf: work around missing ENUM64 support on older kernels
4be94f3 
Replace an ENUM64 with a 64-bit integer on kernels which don't
support the former. This behaviour is similar to libbpf, which replaces
the ENUM64 with a union instead.

Remove the old heuristic which encoded a 64-bit enum as a 32-bit enum
if none of the values exceeded math.MaxUint32. The heuristic has no
tests and doesn't take the signedness of the enum into account.

Fixes cilium#1116

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
@lmb lmb mentioned this issue Sep 13, 2023
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@lmb lmb self-assigned this Sep 13, 2023
@lmb lmb closed this as completed in #1132 Sep 19, 2023
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btf: work around missing ENUM64 support lmb/ebpf
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