elf_reader, linker: add kfunc support

[rgo3]

This commit adds support for handling the following extern 
kfunc declaration in bpf programs:

extern void bpf_kfunc_foo(void *mem) __ksym;

If during relocation a call to a kfunc is found we set the corresponding 
btf.Func as Metadata on the instruction and change Instruction.Src to 
asm.PseudoKfuncCall.

fixupKfunc() has been added to the linker, to find the type id of the 
btf.Func in the running kernel and set Instruction.Constant to the found 
btf id.

[lmb]

Very nice, things slot into place really well and it has a small scope. Thanks!

[lmb]

Yes, I think we'll have to export that function. How about btf.CheckCompatibility(Type, Type) error?

The big comment at the top of the function should move into the function body, it's too much implementation detail. We'll also have to replace errImpossibleRelocation with a more meaningful error, maybe even an exported one.

[rgo3]

How is #896 different from exporting coreAreTypesCompatible(Type, Type)? cc @ti-mo

[rgo3]

Hitting my btf knowledge limit again with the CI failure. Looks like either Return or Parameters of the FuncProto will be a Typedef and we don't support that in CheckTypeCompatibility. Wondering if adding *Typdef to the case *Pointer, *Array statement would solve this?

[lmb]

Ah, the reason that we don't handle Typedef in coreAreTypesCompatible is that the function assumes that all qualifiers and Typedefs have been stripped out using UnderlyingType.

Wondering if adding *Typdef to the case *Pointer, *Array statement would solve this?

Unfortunately not, since we want to treat the following as equal:

local: Typedef{Int}
target: Int{}

If we just add to the switch case, we'd still hit

ebpf/btf/core.go

Lines 878 to 880 in 5ea5368

	if reflect.TypeOf(localType) != reflect.TypeOf(targetType) {
	return fmt.Errorf("type mismatch: %w", errImpossibleRelocation)
	}

which would fail since Typedef != Int.

The best fix is to create a wrapper function I think. Leave coreAreTypesCompatible unexported, and instead export the wrapper. The wrapper calls Copy(local, UnderlyingType), etc. and then invokes coreAreTypesCompatible. CO-RE relocation would still call coreAreTypesCompatible but external callers get the more sensible behaviour.

[rgo3]

All the LibBPFCompat Tests that are failing because of STB_WEAK need to be skipped right?

[lmb]

All the LibBPFCompat Tests that are failing because of STB_WEAK need to be skipped right?

Yes. Does it make sense to return ErrNotSupported for STB_WEAK as well?

[lmb]

Ah, the reason that we don't handle Typedef in coreAreTypesCompatible is that the function assumes that all qualifiers and Typedefs have been stripped out using UnderlyingType.

Wondering if adding *Typdef to the case *Pointer, *Array statement would solve this?

Unfortunately not, since we want to treat the following as equal:

local: Typedef{Int}
target: Int{}

If we just add to the switch case, we'd still hit

ebpf/btf/core.go

Lines 878 to 880 in 5ea5368

	if reflect.TypeOf(localType) != reflect.TypeOf(targetType) {
	return fmt.Errorf("type mismatch: %w", errImpossibleRelocation)
	}

which would fail since Typedef != Int.

The best fix is to create a wrapper function I think. Leave coreAreTypesCompatible unexported, and instead export the wrapper. The wrapper calls Copy(local, UnderlyingType), etc. and then invokes coreAreTypesCompatible. CO-RE relocation would still call coreAreTypesCompatible but external callers get the more sensible behaviour.