debug/macho: support reading imported symbols without LC_DYSYMTAB

Currently, the ImportedSymbols method requires an LC_DYSYMTAB load
command to exist. However, a Mach-O object file may not have an
LC_DYSYMTAB load command, e.g. the one produced by "ld -r".
Support this case by just reading the symbol table and gathers
undefined symbols.

Updates #61229.

Change-Id: I8b4761ac7d99e1f1f378e883e9be75ee4049ffbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/692995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/debug/macho/file.go

@@ -719,15 +719,28 @@ func (f *File) DWARF() (*dwarf.Data, error) {
 // referred to by the binary f that are expected to be
 // satisfied by other libraries at dynamic load time.
 func (f *File) ImportedSymbols() ([]string, error) {
-	if f.Dysymtab == nil || f.Symtab == nil {
+	if f.Symtab == nil {
 		return nil, &FormatError{0, "missing symbol table", nil}
 	}
 
 	st := f.Symtab
 	dt := f.Dysymtab
 	var all []string
-	for _, s := range st.Syms[dt.Iundefsym : dt.Iundefsym+dt.Nundefsym] {
-		all = append(all, s.Name)
+	if dt != nil {
+		for _, s := range st.Syms[dt.Iundefsym : dt.Iundefsym+dt.Nundefsym] {
+			all = append(all, s.Name)
+		}
+	} else {
+		// From Darwin's include/mach-o/nlist.h
+		const (
+			N_TYPE = 0x0e
+			N_UNDF = 0x0
+		)
+		for _, s := range st.Syms {
+			if s.Type&N_TYPE == N_UNDF && s.Sect == 0 {
+				all = append(all, s.Name)
+			}
+		}
 	}
 	return all, nil
 }

src/debug/macho/file_test.go

@@ -9,15 +9,17 @@ import (
 	"internal/obscuretestdata"
 	"io"
 	"reflect"
+	"slices"
 	"testing"
 )
 
 type fileTest struct {
-	file        string
-	hdr         FileHeader
-	loads       []any
-	sections    []*SectionHeader
-	relocations map[string][]Reloc
+	file         string
+	hdr          FileHeader
+	loads        []any
+	sections     []*SectionHeader
+	relocations  map[string][]Reloc
+	importedSyms []string
 }
 
 var fileTests = []fileTest{
@@ -46,6 +48,7 @@ var fileTests = []fileTest{
 			{"__jump_table", "__IMPORT", 0x3000, 0xa, 0x2000, 0x6, 0x0, 0x0, 0x4000008},
 		},
 		nil,
+		nil,
 	},
 	{
 		"testdata/gcc-amd64-darwin-exec.base64",
@@ -74,6 +77,7 @@ var fileTests = []fileTest{
 			{"__la_symbol_ptr", "__DATA", 0x100001058, 0x10, 0x1058, 0x2, 0x0, 0x0, 0x7},
 		},
 		nil,
+		nil,
 	},
 	{
 		"testdata/gcc-amd64-darwin-exec-debug.base64",
@@ -102,6 +106,7 @@ var fileTests = []fileTest{
 			{"__debug_str", "__DWARF", 0x10000215c, 0x60, 0x115c, 0x0, 0x0, 0x0, 0x0},
 		},
 		nil,
+		nil,
 	},
 	{
 		"testdata/clang-386-darwin-exec-with-rpath.base64",
@@ -126,6 +131,7 @@ var fileTests = []fileTest{
 		},
 		nil,
 		nil,
+		nil,
 	},
 	{
 		"testdata/clang-amd64-darwin-exec-with-rpath.base64",
@@ -150,6 +156,7 @@ var fileTests = []fileTest{
 		},
 		nil,
 		nil,
+		nil,
 	},
 	{
 		"testdata/clang-386-darwin.obj.base64",
@@ -185,6 +192,7 @@ var fileTests = []fileTest{
 				},
 			},
 		},
+		nil,
 	},
 	{
 		"testdata/clang-amd64-darwin.obj.base64",
@@ -221,6 +229,15 @@ var fileTests = []fileTest{
 				},
 			},
 		},
+		[]string{"_printf"},
+	},
+	{
+		"testdata/clang-amd64-darwin-ld-r.obj.base64",
+		FileHeader{0xfeedfacf, CpuAmd64, 0x3, 0x1, 0x4, 0x1c0, 0x2000},
+		nil,
+		nil,
+		nil,
+		[]string{"_printf"},
 	},
 }
 
@@ -345,6 +362,17 @@ func TestOpen(t *testing.T) {
 				}
 			}
 		}
+
+		if tt.importedSyms != nil {
+			ss, err := f.ImportedSymbols()
+			if err != nil {
+				t.Errorf("open %s: fail to read imported symbols: %v", tt.file, err)
+			}
+			want := tt.importedSyms
+			if !slices.Equal(ss, want) {
+				t.Errorf("open %s: imported symbols differ:\n\thave %v\n\twant %v", tt.file, ss, want)
+			}
+		}
 	}
 }
 

src/debug/macho/testdata/clang-amd64-darwin-ld-r.obj.base64

@@ -0,0 +1 @@
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