Cranelift JIT

Relevant source files

The cranelift feature adds a second JIT, built on the Cranelift code generator (crates cranelift-codegen, -frontend, -jit, -native and -module, version 0.127). It targets whatever host cranelift_native::builder() supports. Unlike the handwritten backend, it inserts bounds checks on memory accesses.

API

Every VM wrapper gets two methods when feature = "cranelift":


pub fn cranelift_compile(&mut self) -> Result<(), Error>;
 
// EbpfVmMbuff
pub fn execute_program_cranelift(&self, mem: &mut [u8], mbuff: &'a mut [u8]) -> Result<u64, Error>;
// EbpfVmFixedMbuff
pub fn execute_program_cranelift(&mut self, mem: &'a mut [u8]) -> Result<u64, Error>;
// EbpfVmRaw
pub fn execute_program_cranelift(&self, mem: &'a mut [u8]) -> Result<u64, Error>;
// EbpfVmNoData
pub fn execute_program_cranelift(&self) -> Result<u64, Error>;

cranelift_compile clones the VM’s helper map into a CraneliftCompiler. That compiler registers each helper as a JIT symbol named helper_<id>, translates the program into a single function, and stores the resulting CraneliftProgram in the VM. Unlike execute_program_jit, execute_program_cranelift is not marked unsafe.

Internal ABI


pub type JittedFunction = extern "C" fn(
    *mut u8, // mem_ptr
    usize,   // mem_len
    *mut u8, // mbuff_ptr
    usize,   // mbuff_len
) -> u64;

The function prelude sets up:

a 512-byte (STACK_SIZE) explicit stack slot, with r10 at its end,
r1 = mbuff_ptr if mbuff_len != 0, otherwise mem_ptr,
r2 = the matching length. The comment in build_function_prelude notes the conformance tests expect this; for that reason the Cranelift conformance run does not exclude the mem-len tests,
variables holding the start and end of stack, mem and mbuff, for bounds checks.

Translation

Behaviour notes:

Bounds checks: every load and store checks that [addr, addr+size) does not overflow and fits inside the stack, mem (if non-null) or mbuff (if non-null). A failed check executes trapz(..., TrapCode::HEAP_OUT_OF_BOUNDS), which is a hardware trap, not a Rust Err. Ranges added with register_allowed_memory are not consulted.
Calls: CALL is always lowered as a helper call through the imported helper_<imm> symbol. An unregistered ID fails compilation with “[CRANELIFT] Error: unknown helper function”. eBPF-to-eBPF local calls are not translated.
Division by zero: division gives 0, and modulo leaves dst unchanged (implemented with select, not a trap).
Unsupported: TAIL_CALL and any opcode without a translation hit unimplemented!().
Memory release: dropping CraneliftProgram calls JITModule::free_memory.

Tests

tests/cranelift.rs is compiled only with #![cfg(feature = "cranelift")] and holds 24 tests. CI also runs the bpf_conformance suite through rbpf_plugin --cranelift, excluding only the lock (atomics) tests. That step is blocking.