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# `bpf()` syscall compatibility with Linux ABI | ||
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There is a desire to have first-class eBPF support for Windows for the Go ecosystem. | ||
The Go community prefers for libraries that can be easily cross-compiled and distributed. | ||
However, calling into libbpf via Go's foreign function interface (CGo) goes against this spirit, as it requires distributing C headers, a C toolchain, and other dependencies. | ||
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To address this, one of the most popular ways to interact with eBPF on Linux is through the [ebpf-go library], which intentionally avoids CGo. | ||
Given these considerations, this proposal explores what is necessary to port the ebpf-go library to the eBPF for Windows runtime. | ||
Such a port would provide a solution that aligns with Go's cross-platform philosophy while offering robust eBPF support for Windows. | ||
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## ebpf-go architecture | ||
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ebpf-go uses BTF from a Linux kernel image to generate syscall bindings. | ||
This happens at development time, and the bindings are checked in. | ||
These are used from the rest of the library to provide an "idiomatic" public API. | ||
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```mermaid | ||
flowchart LR | ||
public[public API] --> sys[syscall bindings] | ||
sys --> bpf["syscall(SYS_BPF)"] | ||
subgraph auto[auto-generated from BTF] | ||
sys | ||
end | ||
``` | ||
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The library follows the same ELF conventions as libbpf so you are able to load the same ELF files. | ||
The API however is [completely distinct][architecture] and doesn't follow libbpf semantics. | ||
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## ebpf-go on Windows | ||
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The goal of porting ebpf-go is to end up with a library that facilitates porting programs written for Linux to Windows. | ||
In the best case the public API stays the same regardless of operating system. | ||
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A natural point to start a port is to replace the auto-generated syscall bindings with a Windows-specific wrapper. | ||
[An attempt][stable ioctl] was made to directly issue `DeviceIoControl` from ebpf-go. | ||
This approach was discarded because the exact format of the request and response buffers is currently an implementation detail, which is a good thing considering that the current approach [has performance implications][ioctl allocs]. | ||
It also forces us to reimplement the scheme used to load signed native images, which might also still change. | ||
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Instead of replacing the auto-generated syscall bindings we can replace the `bpf()` syscall on Windows with an existing [wrapper][bpf wrapper]. | ||
This has a number of benefits: | ||
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- The necessary changes are much smaller since we don't need to write new code to generate bindings for Windows. | ||
- We don't need to add a new stable API to the Windows runtime and instead can reuse the existing `bpf()` emulation. | ||
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For this to work we need the `bpf()` emulation to be binary compatible with the Linux syscall. | ||
Otherwise the auto generated bindings in ebpf-go won't work, negating one of the key benefits of this approach. | ||
This is problematic since so far eBPF for Windows has only targeted source-level compatibility, not binary compatibility. | ||
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### Differing `bpf_cmd_id` values | ||
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The values of `bpf_cmd_id` do not match Linux, which leads to ebpf-go performing the wrong syscalls. | ||
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### Differing field size | ||
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On Windows the length of an object's name is restricted to 64 characters, while Linux only allows 16. | ||
Unfortunately this constant is used to size an array embedded in key structures like `bpf_map_info` and `bpf_prog_info`: | ||
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```C | ||
struct bpf_map_info | ||
{ | ||
// ... | ||
char name[BPF_OBJ_NAME_LEN]; ///< Null-terminated map name. | ||
// ... | ||
``` | ||
The result is that fields after `name` have a different offset. | ||
### Differing field order / presence | ||
Sometimes the field order doesn't match. For example, `id` and `type` are swapped in `bpf_map_info`. | ||
```C | ||
struct bpf_map_info | ||
{ | ||
// Cross-platform fields. | ||
ebpf_id_t id; ///< Map ID. | ||
ebpf_map_type_t type; ///< Type of map. | ||
// ... | ||
``` | ||
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Some unsupported fields are missing completely: | ||
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```C | ||
struct bpf_prog_info | ||
{ | ||
ebpf_id_t id; ///< Program ID. | ||
enum bpf_prog_type type; ///< Program type, if a cross-platform type. | ||
// 8 missing fields | ||
uint32_t nr_map_ids; ///< Number of maps associated with this program. | ||
``` | ||
### Additional fields | ||
Some structures contain Windows-specific fields: | ||
```C | ||
struct bpf_map_info | ||
{ | ||
// Cross-platform fields. | ||
ebpf_id_t id; ///< Map ID. | ||
// ... | ||
// Windows-specific fields. | ||
ebpf_id_t inner_map_id; ///< ID of inner map template. | ||
uint32_t pinned_path_count; ///< Number of pinned paths. | ||
}; | ||
``` | ||
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This is problematic because Linux exposes many more fields in `bpf_map_info`, aliasing with the Windows-specific fields. | ||
Adding a new cross-platform field before the Windows-specific fields will break compatibility in several ways. | ||
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## Resolve ABI incompatibilities in the `bpf()` syscall emulation | ||
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Changing the problematic types globally is possible but undesirable, since it will cause problems for existing users of the C API. | ||
Instead we will restrict the necessary changes to the `bpf()` syscall emulation. | ||
A separate header will contain type definitions compatible with the Linux ABI. | ||
The syscall emulation layer is then responsible for translating between the Linux | ||
and native types. | ||
This localises breakage to users of `bpf()` and allows the C API to evolve on its own. | ||
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[ebpf-go library]: https://ebpf-go.dev | ||
[architecture]: https://ebpf-go.dev/contributing/architecture/ | ||
[stable ioctl]: https://github.com/microsoft/ebpf-for-windows/issues/3700 | ||
[ioctl allocs]: https://github.com/microsoft/ebpf-for-windows/issues/3726 | ||
[bpf wrapper]: https://github.com/microsoft/ebpf-for-windows/blob/main/libs/api/bpf_syscall.cpp | ||
[api tests]: https://github.com/microsoft/ebpf-for-windows/issues/3729#issuecomment-2259330472 |
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Submodule ebpf-verifier
updated
57 files
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