debug/elf: return FormatError when reading short files

NewFile returns the raw error from ReadAt when failing to read the
ELF identifier, typically io.EOF for empty or short files. This breaks
the API contract that all parsing failures should return *FormatError.

Wrap the error in FormatError for consistency with other error paths.

Fixes #76338

Change-Id: Ic4ed77316fcc459ce8cbe9e9506d7cf8e9286623
Reviewed-on: https://go-review.googlesource.com/c/go/+/736600
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

src/debug/elf/file.go

@@ -290,7 +290,7 @@ func NewFile(r io.ReaderAt) (*File, error) {
 	// Read and decode ELF identifier
 	var ident [16]uint8
 	if _, err := r.ReadAt(ident[0:], 0); err != nil {
-		return nil, err
+		return nil, &FormatError{0, "cannot read ELF identifier", err}
 	}
 	if ident[0] != '\x7f' || ident[1] != 'E' || ident[2] != 'L' || ident[3] != 'F' {
 		return nil, &FormatError{0, "bad magic number", ident[0:4]}

src/debug/elf/file_test.go

@@ -17,6 +17,7 @@ import (
 	"net"
 	"os"
 	"path"
+	"path/filepath"
 	"reflect"
 	"runtime"
 	"slices"
@@ -1622,3 +1623,46 @@ func BenchmarkSymbols32(b *testing.B) {
 		}
 	}
 }
+
+func TestOpenEmptyFile(t *testing.T) {
+	name := filepath.Join(t.TempDir(), "empty")
+	if err := os.WriteFile(name, nil, 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := Open(name)
+	if err == nil {
+		t.Fatal("Open on empty file: got nil error, want non-nil")
+	}
+
+	var formatErr *FormatError
+	if !errors.As(err, &formatErr) {
+		t.Errorf("Open on empty file: got %T (%v), want *FormatError", err, err)
+	}
+}
+
+func TestNewFileShortReader(t *testing.T) {
+	tests := []struct {
+		name string
+		data []byte
+	}{
+		{"empty", []byte{}},
+		{"one byte", []byte{0x7f}},
+		{"four bytes", []byte{0x7f, 'E', 'L', 'F'}},
+		{"fifteen bytes", make([]byte, 15)},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := NewFile(bytes.NewReader(tt.data))
+			if err == nil {
+				t.Fatal("NewFile with short data: got nil error, want non-nil")
+			}
+
+			var formatErr *FormatError
+			if !errors.As(err, &formatErr) {
+				t.Errorf("NewFile with short data: got %T (%v), want *FormatError", err, err)
+			}
+		})
+	}
+}