debug/pe, internal/saferio: use saferio to read PE section data

For #47653
Fixes #53189

Change-Id: If35b968fc53e4c96b18964cfb020cdc003b881bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/412014
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/debug/pe/section.go

@@ -7,6 +7,7 @@ package pe
 import (
 	"encoding/binary"
 	"fmt"
+	"internal/saferio"
 	"io"
 	"strconv"
 )
@@ -97,12 +98,7 @@ type Section struct {
 
 // Data reads and returns the contents of the PE section s.
 func (s *Section) Data() ([]byte, error) {
-	dat := make([]byte, s.sr.Size())
-	n, err := s.sr.ReadAt(dat, 0)
-	if n == len(dat) {
-		err = nil
-	}
-	return dat[0:n], err
+	return saferio.ReadDataAt(s.sr, uint64(s.sr.Size()), 0)
 }
 
 // Open returns a new ReadSeeker reading the PE section s.

src/internal/saferio/io.go

@@ -50,3 +50,44 @@ func ReadData(r io.Reader, n uint64) ([]byte, error) {
 	}
 	return buf, nil
 }
+
+// ReadDataAt reads n bytes from the input stream at off, but avoids
+// allocating all n bytes if n is large. This avoids crashing the program
+// by allocating all n bytes in cases where n is incorrect.
+func ReadDataAt(r io.ReaderAt, n uint64, off int64) ([]byte, error) {
+	if int64(n) < 0 || n != uint64(int(n)) {
+		// n is too large to fit in int, so we can't allocate
+		// a buffer large enough. Treat this as a read failure.
+		return nil, io.ErrUnexpectedEOF
+	}
+
+	if n < chunk {
+		buf := make([]byte, n)
+		_, err := r.ReadAt(buf, off)
+		if err != nil {
+			// io.SectionReader can return EOF for n == 0,
+			// but for our purposes that is a success.
+			if err != io.EOF || n > 0 {
+				return nil, err
+			}
+		}
+		return buf, nil
+	}
+
+	var buf []byte
+	buf1 := make([]byte, chunk)
+	for n > 0 {
+		next := n
+		if next > chunk {
+			next = chunk
+		}
+		_, err := r.ReadAt(buf1[:next], off)
+		if err != nil {
+			return nil, err
+		}
+		buf = append(buf, buf1[:next]...)
+		n -= next
+		off += int64(next)
+	}
+	return buf, nil
+}

src/internal/saferio/io_test.go

@@ -6,6 +6,7 @@ package saferio
 
 import (
 	"bytes"
+	"io"
 	"testing"
 )
 
@@ -37,3 +38,46 @@ func TestReadData(t *testing.T) {
 		}
 	})
 }
+
+func TestReadDataAt(t *testing.T) {
+	const count = 100
+	input := bytes.Repeat([]byte{'a'}, count)
+
+	t.Run("small", func(t *testing.T) {
+		got, err := ReadDataAt(bytes.NewReader(input), count, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !bytes.Equal(got, input) {
+			t.Errorf("got %v, want %v", got, input)
+		}
+	})
+
+	t.Run("large", func(t *testing.T) {
+		_, err := ReadDataAt(bytes.NewReader(input), 10<<30, 0)
+		if err == nil {
+			t.Error("large read succeeded unexpectedly")
+		}
+	})
+
+	t.Run("maxint", func(t *testing.T) {
+		_, err := ReadDataAt(bytes.NewReader(input), 1<<62, 0)
+		if err == nil {
+			t.Error("large read succeeded unexpectedly")
+		}
+	})
+
+	t.Run("SectionReader", func(t *testing.T) {
+		// Reading 0 bytes from an io.SectionReader at the end
+		// of the section will return EOF, but ReadDataAt
+		// should succeed and return 0 bytes.
+		sr := io.NewSectionReader(bytes.NewReader(input), 0, 0)
+		got, err := ReadDataAt(sr, 0, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if len(got) > 0 {
+			t.Errorf("got %d bytes, expected 0", len(got))
+		}
+	})
+}