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Diffstat (limited to 'src/archive/tar/format.go')
| -rw-r--r-- | src/archive/tar/format.go | 307 |
1 files changed, 307 insertions, 0 deletions
diff --git a/src/archive/tar/format.go b/src/archive/tar/format.go new file mode 100644 index 0000000..e50124d --- /dev/null +++ b/src/archive/tar/format.go @@ -0,0 +1,307 @@ +// 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 "<unknown>" + 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] } |