BTF relocations#3966
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BTF, the BPF Type Format, encodes type information for both the running
Linux kernel and compiled eBPF programs. An eBPF object can carry
relocation records that describe field and aggregate accesses in terms
of BTF types instead of fixed offsets; at load time, the loader compares
the program's BTF with the kernel's BTF and rewrites those accesses to
the correct offsets for the target kernel. This mechanism is often
referred to as "CO-RE relocations" or "BTF relocations".
`offset_of` always folds to a plain layout constant and does not
preserve enough information for BTF CO-RE relocation emission. As a
result, it is not suitable for relocatable field queries on BPF targets.
Add three explicit intrinsics for BTF field metadata queries:
* `btf_field_byte_offset`
* `btf_field_byte_size`
* `btf_field_exists`
The user-facing BTF relocatable type support remains behind the
`btf_relocations` feature gate, so use of this experimental CO-RE
surface requires an explicit nightly opt-in.
These intrinsics provide a frontend surface for CO-RE relocations.
Unlike `offset_of`, they remain visible to backend codegen and can lower
to relocatable field-info queries instead of immediate layout constants.
For LLVM, lower these intrinsics through `@llvm.bpf.preserve.field.info`
with the corresponding query kind. The necessary
`@llvm.preserve.{struct,array,union}.access.index` chain is constructed
internally during lowering, but it is not exposed as part of the
user-facing API.
On targets or backends without BTF relocation support, fall back to the
ordinary layout-computed result: the field offset for
`btf_field_byte_offset`, the field size for `btf_field_byte_size`, and
`true` for `btf_field_exists`.
The language-level design for this feature is proposed in
rust-lang/rfcs#3966.
vadorovsky
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BTF, the BPF Type Format, encodes type information for both the running
Linux kernel and compiled eBPF programs. An eBPF object can carry
relocation records that describe field and aggregate accesses in terms
of BTF types instead of fixed offsets; at load time, the loader compares
the program's BTF with the kernel's BTF and rewrites those accesses to
the correct offsets for the target kernel. This mechanism is often
referred to as "CO-RE relocations" or "BTF relocations".
`offset_of` always folds to a plain layout constant and does not
preserve enough information for BTF CO-RE relocation emission. As a
result, it is not suitable for relocatable field queries on BPF targets.
Add three intrinsics for BTF field metadata queries:
* `btf_field_byte_offset`
* `btf_field_byte_size`
* `btf_field_exists`
Their availability is hidden behind the `btf_relocations` feature gate.
Unlike `offset_of`, they remain visible to backend codegen and can lower
to relocatable field-info queries instead of immediate layout constants.
For LLVM, lower these intrinsics through `@llvm.bpf.preserve.field.info`
with the corresponding query kind. The necessary
`@llvm.preserve.{struct,array,union}.access.index` chain is constructed
internally during lowering, but it is not exposed as part of the
user-facing API.
On targets or backends without BTF relocation support, fall back to:
* The field offset for `btf_field_byte_offset`.
* The field size for `btf_field_byte_size`.
* `true` for `btf_field_exists`.
The language-level design for this feature is proposed in
rust-lang/rfcs#3966.
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The RFC proposes the `btf_relocations` feature gate, which provides a `#[repr(Btf)]` representation and `core::btf` field-info macros that emit BTF (BPF Type Format)[0] CO-RE (Compile Once, Run Everywhere)[1] relocations. It also documents the relationship to `offset_of!`, ordinary field projection, and LLVM BPF lowering. This feature was originally proposed as a pre-RFC[2]. [0] https://docs.kernel.org/bpf/btf.html [1] https://nakryiko.com/posts/bpf-portability-and-co-re/ [2] https://internals.rust-lang.org/t/pre-rfc-btf-relocations/24161/25
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The RFC proposes the
btf_relocatable_typesfeature gate, that provides a#[repr(Btf)]representation and low-level field-info intrinsics that emit BTF (BPF TypeFormat) CO-RE (Compile Once, Run Everywhere) relocations.It also documents the relationship to
offset_of!, ordinary field projection and LLVM BPF lowering.This feature was originally proposed as pre-RFC.
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