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(Hdr.Magic, "ustar", 5); // Ustar magic
59  memcpy(Hdr.Version, "00", 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(alignTo(Pos, 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(Hdr.Checksum, ' ', 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(Hdr.Checksum, sizeof(Hdr.Checksum), "%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("path", Path);
105
106  // Create a 512-byte header.
107  UstarHeader Hdr = makeUstarHeader();
108  snprintf(Hdr.Size, sizeof(Hdr.Size), "%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('/', 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(0, Sep);
151  Name = Path.substr(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(Hdr.Name, Name.data(), Name.size());
161  memcpy(Hdr.Mode, "0000664", 8);
162  snprintf(Hdr.Size, sizeof(Hdr.Size), "%011zo", Size);
163  memcpy(Hdr.Prefix, Prefix.data(), 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(OutputPath, FD, CD_CreateAlways, OF_None))
175    return make_error<StringError>("cannot open " + OutputPath, 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);
187
188  // We do not want to include the same file more than once.
189  if (!Files.insert(Fullpath).second)
190    return;
191
192  StringRef Prefix;
193  StringRef Name;
194  if (splitUstar(Fullpath, Prefix, Name)) {
195    writeUstarHeader(OS, Prefix, Name, Data.size());
196  } else {
197    writePaxHeader(OS, Fullpath);
198    writeUstarHeader(OS, "", "", 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(Pos);
210  OS.flush();
211}
212