asm: use correct bitfield order on big-endian arches

Strictly speaking, bitfield order is defined by a respective arch ABI,
however, in practice it always the same as byte order, and LLVM already
makes use of this correlation, therefore do the same in here.

Signed-off-by: Ilya Leoshkevich <[email protected]>

asm/instruction.go

@@ -3,6 +3,7 @@ package asm
 import (
 	"encoding/binary"
 	"fmt"
+	"github.com/cilium/ebpf/internal"
 	"io"
 	"math"
 	"strings"
@@ -433,15 +434,27 @@ type bpfInstruction struct {
 type bpfRegisters uint8
 
 func newBPFRegisters(dst, src Register) bpfRegisters {
-	return bpfRegisters((src << 4) | (dst & 0xF))
+	if internal.NativeEndian == binary.LittleEndian {
+		return bpfRegisters((src << 4) | (dst & 0xF))
+	} else {
+		return bpfRegisters((dst << 4) | (src & 0xF))
+	}
 }
 
 func (r bpfRegisters) Dst() Register {
-	return Register(r & 0xF)
+	if internal.NativeEndian == binary.LittleEndian {
+		return Register(r & 0xF)
+	}else {
+		return Register(r >> 4)
+	}
 }
 
 func (r bpfRegisters) Src() Register {
-	return Register(r >> 4)
+	if internal.NativeEndian == binary.LittleEndian {
+		return Register(r >> 4)
+	} else {
+		return Register(r & 0xf)
+	}
 }
 
 type unreferencedSymbolError struct {

asm/instruction_test.go

@@ -8,6 +8,8 @@ import (
 	"io/ioutil"
 	"math"
 	"testing"
+
+	"github.com/cilium/ebpf/internal"
 )
 
 var test64bitImmProg = []byte{
@@ -179,3 +181,32 @@ func ExampleInstructions_Format() {
 	// 	1: LdImmDW dst: r0 imm: 42
 	// 	3: Exit
 }
+
+var testSrcDstProg = []byte{
+	// on little-endian: r0 = r1
+	// on big-endian: be: r1 = r0
+	0xbf, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+}
+
+func TestReadSrcDst(t *testing.T) {
+	var ins Instruction
+	n, err := ins.Unmarshal(bytes.NewReader(testSrcDstProg), binary.BigEndian)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := uint64(InstructionSize); n != want {
+		t.Errorf("Expected %d bytes to be read, got %d", want, n)
+	}
+	var expDst, expSrc Register
+	if internal.NativeEndian == binary.LittleEndian {
+		expDst, expSrc = 0, 1
+	} else {
+		expDst, expSrc = 1, 0
+	}
+	if ins.Dst != expDst {
+		t.Errorf("Expected destination to be %v, got %v", expDst, ins.Dst)
+	}
+	if ins.Src != expSrc {
+		t.Errorf("Expected source to be %v, got %v", expSrc, ins.Src)
+	}
+}