1//===-- TarWriter.cpp - Tar archive file creator --------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// TarWriter class provides a feature to create a tar archive file.
10//
11// I put emphasis on simplicity over comprehensiveness when implementing this
12// class because we don't need a full-fledged archive file generator in LLVM
13// at the moment.
14//
15// The filename field in the Unix V7 tar header is 100 bytes. Longer filenames
16// are stored using the PAX extension. The PAX header is standardized in
17// POSIX.1-2001.
18//
19// The struct definition of UstarHeader is copied from
20// https://www.freebsd.org/cgi/man.cgi?query=tar&sektion=5
21//
22//===----------------------------------------------------------------------===//
23
24#include "llvm/Support/TarWriter.h"
25#include "llvm/ADT/StringRef.h"
26#include "llvm/Support/FileSystem.h"
27#include "llvm/Support/MathExtras.h"
28#include "llvm/Support/Path.h"
29
30using namespace llvm;
31
32// Each file in an archive must be aligned to this block size.
33static const int BlockSize = 512;
34
35struct UstarHeader {
36 char Name[100];
37 char Mode[8];
38 char Uid[8];
39 char Gid[8];
40 char Size[12];
41 char Mtime[12];
42 char Checksum[8];
43 char TypeFlag;
44 char Linkname[100];
45 char Magic[6];
46 char Version[2];
47 char Uname[32];
48 char Gname[32];
49 char DevMajor[8];
50 char DevMinor[8];
51 char Prefix[155];
52 char Pad[12];
53};
54static_assert(sizeof(UstarHeader) == BlockSize, "invalid Ustar header");
55
56static UstarHeader makeUstarHeader() {
57 UstarHeader Hdr = {};
58 memcpy(dest: Hdr.Magic, src: "ustar", n: 5); // Ustar magic
59 memcpy(dest: Hdr.Version, src: "00", n: 2); // Ustar version
60 return Hdr;
61}
62
63// A PAX attribute is in the form of "<length> <key>=<value>\n"
64// where <length> is the length of the entire string including
65// the length field itself. An example string is this.
66//
67// 25 ctime=1084839148.1212\n
68//
69// This function create such string.
70static std::string formatPax(StringRef Key, StringRef Val) {
71 int Len = Key.size() + Val.size() + 3; // +3 for " ", "=" and "\n"
72
73 // We need to compute total size twice because appending
74 // a length field could change total size by one.
75 int Total = Len + Twine(Len).str().size();
76 Total = Len + Twine(Total).str().size();
77 return (Twine(Total) + " " + Key + "=" + Val + "\n").str();
78}
79
80// Headers in tar files must be aligned to 512 byte boundaries.
81// This function forwards the current file position to the next boundary.
82static void pad(raw_fd_ostream &OS) {
83 uint64_t Pos = OS.tell();
84 OS.seek(off: alignTo(Value: Pos, Align: BlockSize));
85}
86
87// Computes a checksum for a tar header.
88static void computeChecksum(UstarHeader &Hdr) {
89 // Before computing a checksum, checksum field must be
90 // filled with space characters.
91 memset(s: Hdr.Checksum, c: ' ', n: sizeof(Hdr.Checksum));
92
93 // Compute a checksum and set it to the checksum field.
94 unsigned Chksum = 0;
95 for (size_t I = 0; I < sizeof(Hdr); ++I)
96 Chksum += reinterpret_cast<uint8_t *>(&Hdr)[I];
97 snprintf(s: Hdr.Checksum, maxlen: sizeof(Hdr.Checksum), format: "%06o", Chksum);
98}
99
100// Create a tar header and write it to a given output stream.
101static void writePaxHeader(raw_fd_ostream &OS, StringRef Path) {
102 // A PAX header consists of a 512-byte header followed
103 // by key-value strings. First, create key-value strings.
104 std::string PaxAttr = formatPax(Key: "path", Val: Path);
105
106 // Create a 512-byte header.
107 UstarHeader Hdr = makeUstarHeader();
108 snprintf(s: Hdr.Size, maxlen: sizeof(Hdr.Size), format: "%011zo", PaxAttr.size());
109 Hdr.TypeFlag = 'x'; // PAX magic
110 computeChecksum(Hdr);
111
112 // Write them down.
113 OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
114 OS << PaxAttr;
115 pad(OS);
116}
117
118// Path fits in a Ustar header if
119//
120// - Path is less than 100 characters long, or
121// - Path is in the form of "<prefix>/<name>" where <prefix> is less
122// than or equal to 155 characters long and <name> is less than 100
123// characters long. Both <prefix> and <name> can contain extra '/'.
124//
125// If Path fits in a Ustar header, updates Prefix and Name and returns true.
126// Otherwise, returns false.
127static bool splitUstar(StringRef Path, StringRef &Prefix, StringRef &Name) {
128 if (Path.size() < sizeof(UstarHeader::Name)) {
129 Prefix = "";
130 Name = Path;
131 return true;
132 }
133
134 // tar 1.13 and earlier unconditionally look at the tar header interpreted
135 // as an 'oldgnu_header', which has an 'isextended' byte at offset 482 in the
136 // header, corresponding to offset 137 in the prefix. That's the version of
137 // tar in gnuwin, so only use 137 of the 155 bytes in the prefix. This means
138 // we'll need a pax header after 237 bytes of path instead of after 255,
139 // but in return paths up to 237 bytes work with gnuwin, instead of just
140 // 137 bytes of directory + 100 bytes of basename previously.
141 // (tar-1.13 also doesn't support pax headers, but in practice all paths in
142 // llvm's test suite are short enough for that to not matter.)
143 const int MaxPrefix = 137;
144 size_t Sep = Path.rfind(C: '/', From: MaxPrefix + 1);
145 if (Sep == StringRef::npos)
146 return false;
147 if (Path.size() - Sep - 1 >= sizeof(UstarHeader::Name))
148 return false;
149
150 Prefix = Path.substr(Start: 0, N: Sep);
151 Name = Path.substr(Start: Sep + 1);
152 return true;
153}
154
155// The PAX header is an extended format, so a PAX header needs
156// to be followed by a "real" header.
157static void writeUstarHeader(raw_fd_ostream &OS, StringRef Prefix,
158 StringRef Name, size_t Size) {
159 UstarHeader Hdr = makeUstarHeader();
160 memcpy(dest: Hdr.Name, src: Name.data(), n: Name.size());
161 memcpy(dest: Hdr.Mode, src: "0000664", n: 8);
162 snprintf(s: Hdr.Size, maxlen: sizeof(Hdr.Size), format: "%011zo", Size);
163 memcpy(dest: Hdr.Prefix, src: Prefix.data(), n: Prefix.size());
164 computeChecksum(Hdr);
165 OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
166}
167
168// Creates a TarWriter instance and returns it.
169Expected<std::unique_ptr<TarWriter>> TarWriter::create(StringRef OutputPath,
170 StringRef BaseDir) {
171 using namespace sys::fs;
172 int FD;
173 if (std::error_code EC =
174 openFileForWrite(Name: OutputPath, ResultFD&: FD, Disp: CD_CreateAlways, Flags: OF_None))
175 return make_error<StringError>(Args: "cannot open " + OutputPath, Args&: EC);
176 return std::unique_ptr<TarWriter>(new TarWriter(FD, BaseDir));
177}
178
179TarWriter::TarWriter(int FD, StringRef BaseDir)
180 : OS(FD, /*shouldClose=*/true, /*unbuffered=*/false),
181 BaseDir(std::string(BaseDir)) {}
182
183// Append a given file to an archive.
184void TarWriter::append(StringRef Path, StringRef Data) {
185 // Write Path and Data.
186 std::string Fullpath = BaseDir + "/" + sys::path::convert_to_slash(path: Path);
187
188 // We do not want to include the same file more than once.
189 if (!Files.insert(key: Fullpath).second)
190 return;
191
192 StringRef Prefix;
193 StringRef Name;
194 if (splitUstar(Path: Fullpath, Prefix, Name)) {
195 writeUstarHeader(OS, Prefix, Name, Size: Data.size());
196 } else {
197 writePaxHeader(OS, Path: Fullpath);
198 writeUstarHeader(OS, Prefix: "", Name: "", Size: Data.size());
199 }
200
201 OS << Data;
202 pad(OS);
203
204 // POSIX requires tar archives end with two null blocks.
205 // Here, we write the terminator and then seek back, so that
206 // the file being output is terminated correctly at any moment.
207 uint64_t Pos = OS.tell();
208 OS << std::string(BlockSize * 2, '\0');
209 OS.seek(off: Pos);
210 OS.flush();
211}
212