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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package tar
// Constants to identify various tar formats.
const (
// The format is unknown.
formatUnknown = (1 << iota) / 2 // Sequence of 0, 1, 2, 4, 8, etc...
// The format of the original Unix V7 tar tool prior to standardization.
formatV7
// The old and new GNU formats, which are incompatible with USTAR.
// This does cover the old GNU sparse extension.
// This does not cover the GNU sparse extensions using PAX headers,
// versions 0.0, 0.1, and 1.0; these fall under the PAX format.
formatGNU
// Schily's tar format, which is incompatible with USTAR.
// This does not cover STAR extensions to the PAX format; these fall under
// the PAX format.
formatSTAR
// USTAR is the former standardization of tar defined in POSIX.1-1988.
// This is incompatible with the GNU and STAR formats.
formatUSTAR
// PAX is the latest standardization of tar defined in POSIX.1-2001.
// This is an extension of USTAR and is "backwards compatible" with it.
//
// Some newer formats add their own extensions to PAX, such as GNU sparse
// files and SCHILY extended attributes. Since they are backwards compatible
// with PAX, they will be labelled as "PAX".
formatPAX
)
// Magics used to identify various formats.
const (
magicGNU, versionGNU = "ustar ", " \x00"
magicUSTAR, versionUSTAR = "ustar\x00", "00"
trailerSTAR = "tar\x00"
)
// Size constants from various tar specifications.
const (
blockSize = 512 // Size of each block in a tar stream
nameSize = 100 // Max length of the name field in USTAR format
prefixSize = 155 // Max length of the prefix field in USTAR format
)
var zeroBlock block
type block [blockSize]byte
// Convert block to any number of formats.
func (b *block) V7() *headerV7 { return (*headerV7)(b) }
func (b *block) GNU() *headerGNU { return (*headerGNU)(b) }
func (b *block) STAR() *headerSTAR { return (*headerSTAR)(b) }
func (b *block) USTAR() *headerUSTAR { return (*headerUSTAR)(b) }
func (b *block) Sparse() sparseArray { return (sparseArray)(b[:]) }
// GetFormat checks that the block is a valid tar header based on the checksum.
// It then attempts to guess the specific format based on magic values.
// If the checksum fails, then formatUnknown is returned.
func (b *block) GetFormat() (format int) {
// Verify checksum.
var p parser
value := p.parseOctal(b.V7().Chksum())
chksum1, chksum2 := b.ComputeChecksum()
if p.err != nil || (value != chksum1 && value != chksum2) {
return formatUnknown
}
// Guess the magic values.
magic := string(b.USTAR().Magic())
version := string(b.USTAR().Version())
trailer := string(b.STAR().Trailer())
switch {
case magic == magicUSTAR && trailer == trailerSTAR:
return formatSTAR
case magic == magicUSTAR:
return formatUSTAR
case magic == magicGNU && version == versionGNU:
return formatGNU
default:
return formatV7
}
}
// SetFormat writes the magic values necessary for specified format
// and then updates the checksum accordingly.
func (b *block) SetFormat(format int) {
// Set the magic values.
switch format {
case formatV7:
// Do nothing.
case formatGNU:
copy(b.GNU().Magic(), magicGNU)
copy(b.GNU().Version(), versionGNU)
case formatSTAR:
copy(b.STAR().Magic(), magicUSTAR)
copy(b.STAR().Version(), versionUSTAR)
copy(b.STAR().Trailer(), trailerSTAR)
case formatUSTAR, formatPAX:
copy(b.USTAR().Magic(), magicUSTAR)
copy(b.USTAR().Version(), versionUSTAR)
default:
panic("invalid format")
}
// Update checksum.
// This field is special in that it is terminated by a NULL then space.
var f formatter
field := b.V7().Chksum()
chksum, _ := b.ComputeChecksum() // Possible values are 256..128776
f.formatOctal(field[:7], chksum) // Never fails since 128776 < 262143
field[7] = ' '
}
// ComputeChecksum computes the checksum for the header block.
// POSIX specifies a sum of the unsigned byte values, but the Sun tar used
// signed byte values.
// We compute and return both.
func (b *block) ComputeChecksum() (unsigned, signed int64) {
for i, c := range b {
if 148 <= i && i < 156 {
c = ' ' // Treat the checksum field itself as all spaces.
}
unsigned += int64(uint8(c))
signed += int64(int8(c))
}
return unsigned, signed
}
type headerV7 [blockSize]byte
func (h *headerV7) Name() []byte { return h[000:][:100] }
func (h *headerV7) Mode() []byte { return h[100:][:8] }
func (h *headerV7) UID() []byte { return h[108:][:8] }
func (h *headerV7) GID() []byte { return h[116:][:8] }
func (h *headerV7) Size() []byte { return h[124:][:12] }
func (h *headerV7) ModTime() []byte { return h[136:][:12] }
func (h *headerV7) Chksum() []byte { return h[148:][:8] }
func (h *headerV7) TypeFlag() []byte { return h[156:][:1] }
func (h *headerV7) LinkName() []byte { return h[157:][:100] }
type headerGNU [blockSize]byte
func (h *headerGNU) V7() *headerV7 { return (*headerV7)(h) }
func (h *headerGNU) Magic() []byte { return h[257:][:6] }
func (h *headerGNU) Version() []byte { return h[263:][:2] }
func (h *headerGNU) UserName() []byte { return h[265:][:32] }
func (h *headerGNU) GroupName() []byte { return h[297:][:32] }
func (h *headerGNU) DevMajor() []byte { return h[329:][:8] }
func (h *headerGNU) DevMinor() []byte { return h[337:][:8] }
func (h *headerGNU) AccessTime() []byte { return h[345:][:12] }
func (h *headerGNU) ChangeTime() []byte { return h[357:][:12] }
func (h *headerGNU) Sparse() sparseArray { return (sparseArray)(h[386:][:24*4+1]) }
func (h *headerGNU) RealSize() []byte { return h[483:][:12] }
type headerSTAR [blockSize]byte
func (h *headerSTAR) V7() *headerV7 { return (*headerV7)(h) }
func (h *headerSTAR) Magic() []byte { return h[257:][:6] }
func (h *headerSTAR) Version() []byte { return h[263:][:2] }
func (h *headerSTAR) UserName() []byte { return h[265:][:32] }
func (h *headerSTAR) GroupName() []byte { return h[297:][:32] }
func (h *headerSTAR) DevMajor() []byte { return h[329:][:8] }
func (h *headerSTAR) DevMinor() []byte { return h[337:][:8] }
func (h *headerSTAR) Prefix() []byte { return h[345:][:131] }
func (h *headerSTAR) AccessTime() []byte { return h[476:][:12] }
func (h *headerSTAR) ChangeTime() []byte { return h[488:][:12] }
func (h *headerSTAR) Trailer() []byte { return h[508:][:4] }
type headerUSTAR [blockSize]byte
func (h *headerUSTAR) V7() *headerV7 { return (*headerV7)(h) }
func (h *headerUSTAR) Magic() []byte { return h[257:][:6] }
func (h *headerUSTAR) Version() []byte { return h[263:][:2] }
func (h *headerUSTAR) UserName() []byte { return h[265:][:32] }
func (h *headerUSTAR) GroupName() []byte { return h[297:][:32] }
func (h *headerUSTAR) DevMajor() []byte { return h[329:][:8] }
func (h *headerUSTAR) DevMinor() []byte { return h[337:][:8] }
func (h *headerUSTAR) Prefix() []byte { return h[345:][:155] }
type sparseArray []byte
func (s sparseArray) Entry(i int) sparseNode { return (sparseNode)(s[i*24:]) }
func (s sparseArray) IsExtended() []byte { return s[24*s.MaxEntries():][:1] }
func (s sparseArray) MaxEntries() int { return len(s) / 24 }
type sparseNode []byte
func (s sparseNode) Offset() []byte { return s[00:][:12] }
func (s sparseNode) NumBytes() []byte { return s[12:][:12] }