link: run TestKprobeOffset on arm64

TestKprobeOffset used to hardcode specific offsets to test against,
which meant we had to make it amd64 specific. It turns out that even
on a single arch the offsets aren't stable due to changes in the
compiler. Hence the test was changed to brute force a valid offset,
but we forgot to run it on all arches.

Trying to execute the test on Linux 5.13 on arm64 triggers EINVAL
instead of the expected EILSEQ however. This is because arm64 returns
EINVAL when trying to use an offset that is not a multiple of four.

Relying on EILSEQ is therefore not portable, and probably shouldn't
be encouraged. Drop the check for EILSEQ from TestKprobeOffset and
execute it on all arches.

internal/unix/types_linux.go

@@ -24,6 +24,7 @@ const (
 	E2BIG   = linux.E2BIG
 	EFAULT  = linux.EFAULT
 	EACCES  = linux.EACCES
+	EILSEQ  = linux.EILSEQ
 
 	BPF_F_NO_PREALLOC        = linux.BPF_F_NO_PREALLOC
 	BPF_F_NUMA_NODE          = linux.BPF_F_NUMA_NODE

internal/unix/types_other.go

@@ -25,6 +25,7 @@ const (
 	E2BIG  = syscall.Errno(0)
 	EFAULT = syscall.EFAULT
 	EACCES = syscall.Errno(0)
+	EILSEQ = syscall.Errno(0)
 
 	BPF_F_NO_PREALLOC        = 0
 	BPF_F_NUMA_NODE          = 0

link/kprobe.go

@@ -307,9 +307,10 @@ func pmuProbe(typ probeType, args probeArgs) (*perfEvent, error) {
 	if errors.Is(err, os.ErrNotExist) {
 		return nil, fmt.Errorf("symbol '%s+%#x' not found: %w", args.symbol, args.offset, os.ErrNotExist)
 	}
-	// Since commit ab105a4fb894, -EILSEQ is returned when a kprobe sym+offset is resolved
-	// to an invalid insn boundary.
-	if errors.Is(err, syscall.EILSEQ) {
+	// Since commit ab105a4fb894, EILSEQ is returned when a kprobe sym+offset is resolved
+	// to an invalid insn boundary. The exact conditions that trigger this error are
+	// arch specific however.
+	if errors.Is(err, unix.EILSEQ) {
 		return nil, fmt.Errorf("symbol '%s+%#x' not found (bad insn boundary): %w", args.symbol, args.offset, os.ErrNotExist)
 	}
 	// Since at least commit cb9a19fe4aa51, ENOTSUPP is returned

link/kprobe_test.go

@@ -53,6 +53,23 @@ func TestKprobe(t *testing.T) {
 	testLink(t, k, prog)
 }
 
+func TestKprobeOffset(t *testing.T) {
+	prog := mustLoadProgram(t, ebpf.Kprobe, 0, "")
+
+	// The layout of a function is compiler and arch dependent, so we try to
+	// find a valid attach target in the first few bytes of the function.
+	for i := uint64(1); i < 16; i++ {
+		k, err := Kprobe("inet6_release", prog, &KprobeOptions{Offset: i})
+		if err != nil {
+			continue
+		}
+		k.Close()
+		return
+	}
+
+	t.Fatal("Can't attach with non-zero offset")
+}
+
 func TestKretprobe(t *testing.T) {
 	prog := mustLoadProgram(t, ebpf.Kprobe, 0, "")