212 lines · cpp
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-exception6//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 this12// class because we don't need a full-fledged archive file generator in LLVM13// at the moment.14//15// The filename field in the Unix V7 tar header is 100 bytes. Longer filenames16// are stored using the PAX extension. The PAX header is standardized in17// POSIX.1-2001.18//19// The struct definition of UstarHeader is copied from20// https://www.freebsd.org/cgi/man.cgi?query=tar&sektion=521//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 magic59 memcpy(Hdr.Version, "00", 2); // Ustar version60 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 including65// the length field itself. An example string is this.66//67// 25 ctime=1084839148.1212\n68//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 appending74 // 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 be90 // 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 followed103 // 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 magic110 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 if119//120// - Path is less than 100 characters long, or121// - Path is in the form of "<prefix>/<name>" where <prefix> is less122// than or equal to 155 characters long and <name> is less than 100123// 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 interpreted135 // as an 'oldgnu_header', which has an 'isextended' byte at offset 482 in the136 // header, corresponding to offset 137 in the prefix. That's the version of137 // tar in gnuwin, so only use 137 of the 155 bytes in the prefix. This means138 // 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 just140 // 137 bytes of directory + 100 bytes of basename previously.141 // (tar-1.13 also doesn't support pax headers, but in practice all paths in142 // 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 needs156// 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 that206 // 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