add haveV4ISA probe for verifying ISA v4 support in the kernel

This commit introduces the haveV4ISA probe and HaveV4ISA API to check
in the running kernel if instructions of the v4 ISA are supported.
The upstream commit used as reference is 1f9a1ea821ff
("bpf: Support new sign-extension load insns").
The probes tests the new asm.LongJump insn given by `BPF_JMP32 | BPF_JA`.
On arm64 with kernel prior to ISA v4 support (<= v6.6), this would return
sys.ENOTSUPP: handle this case by returning our ebpf.ErrNotSupported
instead.

Signed-off-by: Simone Magnani <[email protected]>

features/misc.go

@@ -93,3 +93,25 @@ var haveV3ISA = internal.NewFeatureTest("v3 ISA", func() error {
 		},
 	})
 }, "5.1")
+
+// HaveV4ISA probes the running kernel if instructions of the v4 ISA are supported.
+//
+// Upstream commit 1f9a1ea821ff ("bpf: Support new sign-extension load insns").
+//
+// See the package documentation for the meaning of the error return value.
+func HaveV4ISA() error {
+	return haveV4ISA()
+}
+
+var haveV4ISA = internal.NewFeatureTest("v4 ISA", func() error {
+	return probeProgram(&ebpf.ProgramSpec{
+		Type: ebpf.SocketFilter,
+		Instructions: asm.Instructions{
+			asm.Mov.Imm(asm.R0, 0),
+			asm.JEq.Imm(asm.R0, 1, "error"),
+			asm.LongJump("exit"),
+			asm.Mov.Imm(asm.R0, 1).WithSymbol("error"),
+			asm.Return().WithSymbol("exit"),
+		},
+	})
+}, "6.6")

features/misc_test.go

@@ -21,3 +21,7 @@ func TestHaveV2ISA(t *testing.T) {
 func TestHaveV3ISA(t *testing.T) {
 	testutils.CheckFeatureTest(t, HaveV3ISA)
 }
+
+func TestHaveV4ISA(t *testing.T) {
+	testutils.CheckFeatureTest(t, HaveV4ISA)
+}

features/prog.go

@@ -40,9 +40,9 @@ func probeProgram(spec *ebpf.ProgramSpec) error {
 	}
 
 	switch {
-	// ENOTSUPP with ebpf.StructOps means the program type is at least known to the kernel.
+	// With ebpf.StructOps, ENOTSUPP means the program type is at least known to the kernel.
 	case errors.Is(err, sys.ENOTSUPP) && spec.Type == ebpf.StructOps:
-		return nil
+		err = nil
 	// ENOTSUPP occurs when, for instance, JIT fails.
 	// EINVAL occurs when attempting to create a program with an unknown type.
 	// E2BIG occurs when ProgLoadAttr contains non-zero bytes past the end