Add RelativeOffset field to UprobeOptions struct

[mertyildiran]

Alternative to #683

This PR, for a bpftrace program like below:

#!/usr/bin/env bpftrace

uprobe:./main:"main.probeMe"+42
{
    printf("rax: %d\n", reg("ax"));
}

enables probing without the need of recalculating the offset of main.probeMe (which is already calculated and stored inside offsets field), adding 0x2a and setting Offset field to the resulting value:

up, err := ex.Uprobe("main.probeMe", objs.ExampleUprobe, &link.UprobeOptions{RelativeOffset: 0x2a})
if err != nil {
	log.Fatalf("creating uprobe: %s", err)
}
defer up.Close()

The PR also improves some of the comments because it's hard to understand at first glance that with the block below, the Offset field nullifies the offset that's calculated from the given symbol and the Offset is actually an absolute offset instead of a relative one:

ebpf/link/uprobe.go

Lines 255 to 262 in 951bb28

	offset := opts.Offset
	if offset == 0 {
	off, err := ex.offset(symbol)
	if err != nil {
	return nil, err
	}
	offset = off
	}

[mmat11]

this shouldn't be the offset from the start (so maybe absolute is not the best term). Maybe leave this comment as is and specify in the comment below that RelativeOffset is an additional offset relative to the calculated or provided offset?

[mertyildiran]

@mmat11 reverted with mertyildiran@ddbe83e

[mmat11]

nit: what about doing offset: offset+opts.RelativeOffset in probeArgs on line 287? this way the "relative offset" is added both for a specified one and for an internally calculated one

[mertyildiran]

@mmat11 done with mertyildiran@8a7955a

[lmb]

Hi, can you add a test that covers the feature?

[mertyildiran]

@lmb I have added a test case with mertyildiran@89cb6f2 But IMHO, exporting the Offset method was a simpler change. Which I have provided as an alternative in the PR description: #683 I'm not sure why the offset method is preferred as unexported:

ebpf/link/uprobe.go

Line 160 in 0f69484

func (ex *Executable) offset(symbol string) (uint64, error) {

[lmb]

Looks good to me, I've pushed 401e03f with small changes:

• Merge offset and offsetWithOpts, don't allow passing nil opts
• Add ReferenceOffset even when UprobeOptions.Offset is given. This makes more sense to me since Uprobe.Offset represents the start of the function / symbol. It also means that the behaviour of ReferenceOffset is easier to understand: it is always added to the "base", no matter what.

Regarding the API: RelativeOffset is more convenient for the user than Offset() since they don't have to deal with error handling. So if specifying a relative offset is at least somewhat common going for the more convenient API is better.

If you are OK with these changes I'm happy to merge the PR.