add zip-byte-offset support to zip utils

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

internal/file/zip_read_closer.go

@@ -0,0 +1,149 @@
+package file
+
+import (
+	"archive/zip"
+	"encoding/binary"
+	"fmt"
+	"io"
+	"os"
+)
+
+// directoryEndLen, readByf, directoryEnd, and findSignatureInBlock were copied from the golang stdlib, specifically:
+// - https://github.com/golang/go/blob/go1.16.4/src/archive/zip/struct.go
+// - https://github.com/golang/go/blob/go1.16.4/src/archive/zip/reader.go
+// findArchiveStartOffset is derived from the same stdlib utils, specifically the readDirectoryEnd function.
+
+const directoryEndLen = 22
+
+// ZipReadCloser is a drop-in replacement for zip.ReadCloser (from zip.OpenReader) that additionally considers zips
+// that have bytes prefixed to the front of the archive (common with self-extracting jars).
+type ZipReadCloser struct {
+	*zip.Reader
+	io.Closer
+}
+
+// OpenZip provides a ZipReadCloser for the given filepath.
+func OpenZip(filepath string) (*ZipReadCloser, error) {
+	f, err := os.Open(filepath)
+	if err != nil {
+		return nil, err
+	}
+	fi, err := f.Stat()
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+
+	// some archives may have bytes prepended to the front of the archive, such as with self executing JARs. We first
+	// need to find the start of the archive and keep track of this offset.
+	offset, err := findArchiveStartOffset(f, fi.Size())
+	if err != nil {
+		return nil, fmt.Errorf("cannot find beginning of zip archive=%q : %w", filepath, err)
+	}
+
+	if _, err := f.Seek(0, io.SeekStart); err != nil {
+		return nil, fmt.Errorf("unable to seek to beginning of archive: %w", err)
+	}
+
+	size := fi.Size() - int64(offset)
+
+	r, err := zip.NewReader(io.NewSectionReader(f, int64(offset), size), size)
+	if err != nil {
+		return nil, fmt.Errorf("unable to open ZipReadCloser @ %q: %w", filepath, err)
+	}
+
+	return &ZipReadCloser{
+		Reader: r,
+		Closer: f,
+	}, nil
+}
+
+type readBuf []byte
+
+func (b *readBuf) uint16() uint16 {
+	v := binary.LittleEndian.Uint16(*b)
+	*b = (*b)[2:]
+	return v
+}
+
+func (b *readBuf) uint32() uint32 {
+	v := binary.LittleEndian.Uint32(*b)
+	*b = (*b)[4:]
+	return v
+}
+
+type directoryEnd struct {
+	diskNbr            uint32 // unused
+	dirDiskNbr         uint32 // unused
+	dirRecordsThisDisk uint64 // unused
+	directoryRecords   uint64
+	directorySize      uint64
+	directoryOffset    uint64 // relative to file
+}
+
+// note: this is derived from readDirectoryEnd within the archive/zip package
+func findArchiveStartOffset(r io.ReaderAt, size int64) (startOfArchive uint64, err error) {
+	// look for directoryEndSignature in the last 1k, then in the last 65k
+	var buf []byte
+	var directoryEndOffset int64
+	for i, bLen := range []int64{1024, 65 * 1024} {
+		if bLen > size {
+			bLen = size
+		}
+		buf = make([]byte, int(bLen))
+		if _, err := r.ReadAt(buf, size-bLen); err != nil && err != io.EOF {
+			return 0, err
+		}
+		if p := findSignatureInBlock(buf); p >= 0 {
+			buf = buf[p:]
+			directoryEndOffset = size - bLen + int64(p)
+			break
+		}
+		if i == 1 || bLen == size {
+			return 0, zip.ErrFormat
+		}
+	}
+
+	if buf == nil {
+		// we were unable to find the directoryEndSignature block
+		return 0, zip.ErrFormat
+	}
+
+	// read header into struct
+	b := readBuf(buf[4:]) // skip signature
+	d := &directoryEnd{
+		diskNbr:            uint32(b.uint16()),
+		dirDiskNbr:         uint32(b.uint16()),
+		dirRecordsThisDisk: uint64(b.uint16()),
+		directoryRecords:   uint64(b.uint16()),
+		directorySize:      uint64(b.uint32()),
+		directoryOffset:    uint64(b.uint32()),
+	}
+	// Calculate where the zip data actually begins
+	startOfArchive = uint64(directoryEndOffset) - d.directorySize - d.directoryOffset
+
+	// These values mean that the file can be a zip64 file
+	if d.directoryRecords == 0xffff || d.directorySize == 0xffff || d.directoryOffset == 0xffffffff {
+		startOfArchive = 0 // Prefixed data not supported
+	}
+
+	// Make sure directoryOffset points to somewhere in our file.
+	if o := int64(d.directoryOffset); o < 0 || o >= size {
+		return 0, zip.ErrFormat
+	}
+	return startOfArchive, nil
+}
+
+func findSignatureInBlock(b []byte) int {
+	for i := len(b) - directoryEndLen; i >= 0; i-- {
+		// defined from directoryEndSignature
+		if b[i] == 'P' && b[i+1] == 'K' && b[i+2] == 0x05 && b[i+3] == 0x06 {
+			// n is length of comment
+			n := int(b[i+directoryEndLen-2]) | int(b[i+directoryEndLen-1])<<8
+			if n+directoryEndLen+i <= len(b) {
+				return i
+			}
+		}
+	}
+	return -1
+}