cmd/internal/objfile,cmd/objdump: add disassembly support for riscv64

Add support to `go tool objdump` for disassembling riscv64 binaries.

Revendor to bring in cmd/vendor/golang.org/x/arch/riscv64/riscv64asm,
which provides the actual disassembly implementation.

Fixes #36738

Change-Id: I0f29968509041c0c5698fc2d6910a6a0bea9d3c0
Cq-Include-Trybots: luci.golang.try:gotip-linux-riscv64
Reviewed-on: https://go-review.googlesource.com/c/go/+/622257
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/cmd/internal/objfile/disasm.go

@@ -25,6 +25,7 @@ import (
 	"golang.org/x/arch/arm64/arm64asm"
 	"golang.org/x/arch/loong64/loong64asm"
 	"golang.org/x/arch/ppc64/ppc64asm"
+	"golang.org/x/arch/riscv64/riscv64asm"
 	"golang.org/x/arch/s390x/s390xasm"
 	"golang.org/x/arch/x86/x86asm"
 )
@@ -62,7 +63,7 @@ func (e *Entry) Disasm() (*Disasm, error) {
 	disasm := disasms[goarch]
 	byteOrder := byteOrders[goarch]
 	if disasm == nil || byteOrder == nil {
-		return nil, fmt.Errorf("unsupported architecture")
+		return nil, fmt.Errorf("unsupported architecture %q", goarch)
 	}
 
 	// Filter out section symbols, overwriting syms in place.
@@ -398,6 +399,19 @@ func disasm_ppc64(code []byte, pc uint64, lookup lookupFunc, byteOrder binary.By
 	return text, size
 }
 
+func disasm_riscv64(code []byte, pc uint64, lookup lookupFunc, byteOrder binary.ByteOrder, gnuAsm bool) (string, int) {
+	inst, err := riscv64asm.Decode(code)
+	var text string
+	if err != nil || inst.Op == 0 {
+		text = "?"
+	} else if gnuAsm {
+		text = fmt.Sprintf("%-36s // %s", riscv64asm.GoSyntax(inst, pc, lookup, textReader{code, pc}), riscv64asm.GNUSyntax(inst))
+	} else {
+		text = riscv64asm.GoSyntax(inst, pc, lookup, textReader{code, pc})
+	}
+	return text, 4
+}
+
 func disasm_s390x(code []byte, pc uint64, lookup lookupFunc, _ binary.ByteOrder, gnuAsm bool) (string, int) {
 	inst, err := s390xasm.Decode(code)
 	var text string
@@ -423,6 +437,7 @@ var disasms = map[string]disasmFunc{
 	"loong64": disasm_loong64,
 	"ppc64":   disasm_ppc64,
 	"ppc64le": disasm_ppc64,
+	"riscv64": disasm_riscv64,
 	"s390x":   disasm_s390x,
 }
 
@@ -434,6 +449,7 @@ var byteOrders = map[string]binary.ByteOrder{
 	"loong64": binary.LittleEndian,
 	"ppc64":   binary.BigEndian,
 	"ppc64le": binary.LittleEndian,
+	"riscv64": binary.LittleEndian,
 	"s390x":   binary.BigEndian,
 }
 

src/cmd/internal/objfile/elf.go

@@ -127,6 +127,10 @@ func (f *elfFile) goarch() string {
 			return "ppc64le"
 		}
 		return "ppc64"
+	case elf.EM_RISCV:
+		if f.elf.Class == elf.ELFCLASS64 {
+			return "riscv64"
+		}
 	case elf.EM_S390:
 		return "s390x"
 	}

src/cmd/objdump/objdump_test.go

@@ -105,8 +105,6 @@ func mustHaveDisasm(t *testing.T) {
 	switch runtime.GOARCH {
 	case "mips", "mipsle", "mips64", "mips64le":
 		t.Skipf("skipping on %s, issue 12559", runtime.GOARCH)
-	case "riscv64":
-		t.Skipf("skipping on %s, issue 36738", runtime.GOARCH)
 	}
 }