debug/buildinfo: correct a typo in calculating next align index

When it calculates the smallest n such that:
  n >= i && n % buildInfoAlign == 0
the expression should be
  (i+buildInfoAlign-1)&^(buildInfoAlign-1)
instead of
  (i+buildInfoAlign-1)&^buildInfoAlign

Fixes #54968.

Change-Id: Ibb7bdf568a521545b2609acc85e2ab4e05da5dae
GitHub-Last-Rev: 479ebc140a
GitHub-Pull-Request: golang/go#54971
Reviewed-on: https://go-review.googlesource.com/c/go/+/429815
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

src/debug/buildinfo/buildinfo.go

@@ -164,7 +164,7 @@ func readRawBuildInfo(r io.ReaderAt) (vers, mod string, err error) {
 			data = data[i:]
 			break
 		}
-		data = data[(i+buildInfoAlign-1)&^buildInfoAlign:]
+		data = data[(i+buildInfoAlign-1)&^(buildInfoAlign-1):]
 	}
 
 	// Decode the blob.

src/debug/buildinfo/buildinfo_test.go

@@ -7,6 +7,8 @@ package buildinfo_test
 import (
 	"bytes"
 	"debug/buildinfo"
+	"debug/pe"
+	"encoding/binary"
 	"flag"
 	"fmt"
 	"internal/testenv"
@@ -245,3 +247,71 @@ func TestReadFile(t *testing.T) {
 		})
 	}
 }
+
+// TestIssue54968 is a regression test for golang.org/issue/54968.
+//
+// The cause of issue 54968 is when the first buildInfoMagic is invalid, it
+// enters an infinite loop.
+func TestIssue54968(t *testing.T) {
+	t.Parallel()
+
+	const (
+		paddingSize    = 200
+		buildInfoAlign = 16
+	)
+	buildInfoMagic := []byte("\xff Go buildinf:")
+
+	// Construct a valid PE header.
+	var buf bytes.Buffer
+
+	buf.Write([]byte{'M', 'Z'})
+	buf.Write(bytes.Repeat([]byte{0}, 0x3c-2))
+	// At location 0x3c, the stub has the file offset to the PE signature.
+	binary.Write(&buf, binary.LittleEndian, int32(0x3c+4))
+
+	buf.Write([]byte{'P', 'E', 0, 0})
+
+	binary.Write(&buf, binary.LittleEndian, pe.FileHeader{NumberOfSections: 1})
+
+	sh := pe.SectionHeader32{
+		Name:             [8]uint8{'t', 0},
+		SizeOfRawData:    uint32(paddingSize + len(buildInfoMagic)),
+		PointerToRawData: uint32(buf.Len()),
+	}
+	sh.PointerToRawData = uint32(buf.Len() + binary.Size(sh))
+
+	binary.Write(&buf, binary.LittleEndian, sh)
+
+	start := buf.Len()
+	buf.Write(bytes.Repeat([]byte{0}, paddingSize+len(buildInfoMagic)))
+	data := buf.Bytes()
+
+	if _, err := pe.NewFile(bytes.NewReader(data)); err != nil {
+		t.Fatalf("need a valid PE header for the misaligned buildInfoMagic test: %s", err)
+	}
+
+	// Place buildInfoMagic after the header.
+	for i := 1; i < paddingSize-len(buildInfoMagic); i++ {
+		// Test only misaligned buildInfoMagic.
+		if i%buildInfoAlign == 0 {
+			continue
+		}
+
+		t.Run(fmt.Sprintf("start_at_%d", i), func(t *testing.T) {
+			d := data[:start]
+			// Construct intentionally-misaligned buildInfoMagic.
+			d = append(d, bytes.Repeat([]byte{0}, i)...)
+			d = append(d, buildInfoMagic...)
+			d = append(d, bytes.Repeat([]byte{0}, paddingSize-i)...)
+
+			_, err := buildinfo.Read(bytes.NewReader(d))
+
+			wantErr := "not a Go executable"
+			if err == nil {
+				t.Errorf("got error nil; want error containing %q", wantErr)
+			} else if errMsg := err.Error(); !strings.Contains(errMsg, wantErr) {
+				t.Errorf("got error %q; want error containing %q", errMsg, wantErr)
+			}
+		})
+	}
+}