bpf: standalone BTF support for modules #3955
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Not all kernel modules can be built in-tree when the core kernel is built. This presents a problem for split BTF, because split module BTF refers to type ids in the base kernel BTF, and if that base kernel BTF changes (even in minor ways) those references become invalid. Such modules then cannot take advantage of BTF (or at least they only can until the kernel changes enough to invalidate their vmlinux type id references). This problem has been discussed before, and the initial approach was to allow BTF mismatch but fail to load BTF. See [1] for more discussion. Generating standalone BTF for modules helps solve this problem because the BTF generated is self-referential only. However, tooling is geared towards split BTF - for example bpftool assumes a module's BTF is defined relative to vmlinux BTF. To handle this, dynamic remapping of standalone BTF is done on module load to make it appear like split BTF - type ids and string offsets are remapped such that they appear as they would in split BTF. It just so happens that the BTF is self-referential. With this approach, existing tooling works with standalone module BTF from /sys/kernel/btf in the same way as before; no knowledge of split versus standalone BTF is required. Currently, the approach taken is to assume that the BTF associated with a module is split BTF. If however the checking of types fails, we fall back to interpreting it as standalone BTF and carrying out remapping. As discussed in [1] there are some heuristics we could use to identify standalone versus split module BTF, but for now the simplistic fallback method is used. Signed-off-by: Alan Maguire <alan.maguire@oracle.com> [1] https://lore.kernel.org/bpf/YfK18x%2FXrYL4Vw8o@syu-laptop/
By having a BTF_BASE variable defaulting to using vmlinux as base BTF, we allow module builders to build standalone BTF such that it is generated independently and not de-duplicated with core vmlinux BTF. This allows such modules to be more resilient to changes in vmlinux BTF if they occur, as would happen if a change resulted in a different vmlinux BTF id mapping. Opt-out of split BTF is done via make BTF_BASE= M=path/2/module Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
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Upstream branch: f71b2f6 |
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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=692173 expired. Closing PR. |
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Pull request for series with
subject: bpf: standalone BTF support for modules
version: 1
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=692173