// 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 import "strings" // Format represents the tar archive format. // // The original tar format was introduced in Unix V7. // Since then, there have been multiple competing formats attempting to // standardize or extend the V7 format to overcome its limitations. // The most common formats are the USTAR, PAX, and GNU formats, // each with their own advantages and limitations. // // The following table captures the capabilities of each format: // // | USTAR | PAX | GNU // ------------------+--------+-----------+---------- // Name | 256B | unlimited | unlimited // Linkname | 100B | unlimited | unlimited // Size | uint33 | unlimited | uint89 // Mode | uint21 | uint21 | uint57 // Uid/Gid | uint21 | unlimited | uint57 // Uname/Gname | 32B | unlimited | 32B // ModTime | uint33 | unlimited | int89 // AccessTime | n/a | unlimited | int89 // ChangeTime | n/a | unlimited | int89 // Devmajor/Devminor | uint21 | uint21 | uint57 // ------------------+--------+-----------+---------- // string encoding | ASCII | UTF-8 | binary // sub-second times | no | yes | no // sparse files | no | yes | yes // // The table's upper portion shows the [Header] fields, where each format reports // the maximum number of bytes allowed for each string field and // the integer type used to store each numeric field // (where timestamps are stored as the number of seconds since the Unix epoch). // // The table's lower portion shows specialized features of each format, // such as supported string encodings, support for sub-second timestamps, // or support for sparse files. // // The Writer currently provides no support for sparse files. type Format int // Constants to identify various tar formats. const ( // Deliberately hide the meaning of constants from public API. _ Format = (1 << iota) / 4 // Sequence of 0, 0, 1, 2, 4, 8, etc... // FormatUnknown indicates that the format is unknown. FormatUnknown // The format of the original Unix V7 tar tool prior to standardization. formatV7 // FormatUSTAR represents the USTAR header format defined in POSIX.1-1988. // // While this format is compatible with most tar readers, // the format has several limitations making it unsuitable for some usages. // Most notably, it cannot support sparse files, files larger than 8GiB, // filenames larger than 256 characters, and non-ASCII filenames. // // Reference: // http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06 FormatUSTAR // FormatPAX represents the PAX header format defined in POSIX.1-2001. // // PAX extends USTAR by writing a special file with Typeflag TypeXHeader // preceding the original header. This file contains a set of key-value // records, which are used to overcome USTAR's shortcomings, in addition to // providing the ability to have sub-second resolution for timestamps. // // Some newer formats add their own extensions to PAX by defining their // own keys and assigning certain semantic meaning to the associated values. // For example, sparse file support in PAX is implemented using keys // defined by the GNU manual (e.g., "GNU.sparse.map"). // // Reference: // http://pubs.opengroup.org/onlinepubs/009695399/utilities/pax.html FormatPAX // FormatGNU represents the GNU header format. // // The GNU header format is older than the USTAR and PAX standards and // is not compatible with them. The GNU format supports // arbitrary file sizes, filenames of arbitrary encoding and length, // sparse files, and other features. // // It is recommended that PAX be chosen over GNU unless the target // application can only parse GNU formatted archives. // // Reference: // https://www.gnu.org/software/tar/manual/html_node/Standard.html 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 formatMax ) func (f Format) has(f2 Format) bool { return f&f2 != 0 } func (f *Format) mayBe(f2 Format) { *f |= f2 } func (f *Format) mayOnlyBe(f2 Format) { *f &= f2 } func (f *Format) mustNotBe(f2 Format) { *f &^= f2 } var formatNames = map[Format]string{ formatV7: "V7", FormatUSTAR: "USTAR", FormatPAX: "PAX", FormatGNU: "GNU", formatSTAR: "STAR", } func (f Format) String() string { var ss []string for f2 := Format(1); f2 < formatMax; f2 <<= 1 { if f.has(f2) { ss = append(ss, formatNames[f2]) } } switch len(ss) { case 0: return "" case 1: return ss[0] default: return "(" + strings.Join(ss, " | ") + ")" } } // 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 // Max length of a special file (PAX header, GNU long name or link). // This matches the limit used by libarchive. maxSpecialFileSize = 1 << 20 ) // blockPadding computes the number of bytes needed to pad offset up to the // nearest block edge where 0 <= n < blockSize. func blockPadding(offset int64) (n int64) { return -offset & (blockSize - 1) } var zeroBlock block type block [blockSize]byte // Convert block to any number of formats. func (b *block) toV7() *headerV7 { return (*headerV7)(b) } func (b *block) toGNU() *headerGNU { return (*headerGNU)(b) } func (b *block) toSTAR() *headerSTAR { return (*headerSTAR)(b) } func (b *block) toUSTAR() *headerUSTAR { return (*headerUSTAR)(b) } func (b *block) toSparse() 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 { // Verify checksum. var p parser value := p.parseOctal(b.toV7().chksum()) chksum1, chksum2 := b.computeChecksum() if p.err != nil || (value != chksum1 && value != chksum2) { return FormatUnknown } // Guess the magic values. magic := string(b.toUSTAR().magic()) version := string(b.toUSTAR().version()) trailer := string(b.toSTAR().trailer()) switch { case magic == magicUSTAR && trailer == trailerSTAR: return formatSTAR case magic == magicUSTAR: return FormatUSTAR | FormatPAX 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 Format) { // Set the magic values. switch { case format.has(formatV7): // Do nothing. case format.has(FormatGNU): copy(b.toGNU().magic(), magicGNU) copy(b.toGNU().version(), versionGNU) case format.has(formatSTAR): copy(b.toSTAR().magic(), magicUSTAR) copy(b.toSTAR().version(), versionUSTAR) copy(b.toSTAR().trailer(), trailerSTAR) case format.has(FormatUSTAR | FormatPAX): copy(b.toUSTAR().magic(), magicUSTAR) copy(b.toUSTAR().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.toV7().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(c) signed += int64(int8(c)) } return unsigned, signed } // reset clears the block with all zeros. func (b *block) reset() { *b = block{} } 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) sparseElem { return sparseElem(s[i*24:]) } func (s sparseArray) isExtended() []byte { return s[24*s.maxEntries():][:1] } func (s sparseArray) maxEntries() int { return len(s) / 24 } type sparseElem []byte func (s sparseElem) offset() []byte { return s[00:][:12] } func (s sparseElem) length() []byte { return s[12:][:12] }