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//===-- TarWriter.cpp - Tar archive file creator --------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// TarWriter class provides a feature to create a tar archive file.
//
// I put emphasis on simplicity over comprehensiveness when implementing this
// class because we don't need a full-fledged archive file generator in LLVM
// at the moment.
//
// The filename field in the Unix V7 tar header is 100 bytes. Longer filenames
// are stored using the PAX extension. The PAX header is standardized in
// POSIX.1-2001.
//
// The struct definition of UstarHeader is copied from
// https://www.freebsd.org/cgi/man.cgi?query=tar&sektion=5
//
//===----------------------------------------------------------------------===//
#include "llvm/Support/TarWriter.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/Path.h"
using namespace llvm;
// Each file in an archive must be aligned to this block size.
static const int BlockSize = 512;
struct UstarHeader {
char Name[100];
char Mode[8];
char Uid[8];
char Gid[8];
char Size[12];
char Mtime[12];
char Checksum[8];
char TypeFlag;
char Linkname[100];
char Magic[6];
char Version[2];
char Uname[32];
char Gname[32];
char DevMajor[8];
char DevMinor[8];
char Prefix[155];
char Pad[12];
};
static_assert(sizeof(UstarHeader) == BlockSize, "invalid Ustar header");
static UstarHeader makeUstarHeader() {
UstarHeader Hdr = {};
memcpy(Hdr.Magic, "ustar", 5); // Ustar magic
memcpy(Hdr.Version, "00", 2); // Ustar version
return Hdr;
}
// A PAX attribute is in the form of "<length> <key>=<value>\n"
// where <length> is the length of the entire string including
// the length field itself. An example string is this.
//
// 25 ctime=1084839148.1212\n
//
// This function create such string.
static std::string formatPax(StringRef Key, StringRef Val) {
int Len = Key.size() + Val.size() + 3; // +3 for " ", "=" and "\n"
// We need to compute total size twice because appending
// a length field could change total size by one.
int Total = Len + Twine(Len).str().size();
Total = Len + Twine(Total).str().size();
return (Twine(Total) + " " + Key + "=" + Val + "\n").str();
}
// Headers in tar files must be aligned to 512 byte boundaries.
// This function forwards the current file position to the next boundary.
static void pad(raw_fd_ostream &OS) {
uint64_t Pos = OS.tell();
OS.seek(alignTo(Pos, BlockSize));
}
// Computes a checksum for a tar header.
static void computeChecksum(UstarHeader &Hdr) {
// Before computing a checksum, checksum field must be
// filled with space characters.
memset(Hdr.Checksum, ' ', sizeof(Hdr.Checksum));
// Compute a checksum and set it to the checksum field.
unsigned Chksum = 0;
for (size_t I = 0; I < sizeof(Hdr); ++I)
Chksum += reinterpret_cast<uint8_t *>(&Hdr)[I];
snprintf(Hdr.Checksum, sizeof(Hdr.Checksum), "%06o", Chksum);
}
// Create a tar header and write it to a given output stream.
static void writePaxHeader(raw_fd_ostream &OS, StringRef Path) {
// A PAX header consists of a 512-byte header followed
// by key-value strings. First, create key-value strings.
std::string PaxAttr = formatPax("path", Path);
// Create a 512-byte header.
UstarHeader Hdr = makeUstarHeader();
snprintf(Hdr.Size, sizeof(Hdr.Size), "%011zo", PaxAttr.size());
Hdr.TypeFlag = 'x'; // PAX magic
computeChecksum(Hdr);
// Write them down.
OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
OS << PaxAttr;
pad(OS);
}
// Path fits in a Ustar header if
//
// - Path is less than 100 characters long, or
// - Path is in the form of "<prefix>/<name>" where <prefix> is less
// than or equal to 155 characters long and <name> is less than 100
// characters long. Both <prefix> and <name> can contain extra '/'.
//
// If Path fits in a Ustar header, updates Prefix and Name and returns true.
// Otherwise, returns false.
static bool splitUstar(StringRef Path, StringRef &Prefix, StringRef &Name) {
if (Path.size() < sizeof(UstarHeader::Name)) {
Prefix = "";
Name = Path;
return true;
}
// tar 1.13 and earlier unconditionally look at the tar header interpreted
// as an 'oldgnu_header', which has an 'isextended' byte at offset 482 in the
// header, corresponding to offset 137 in the prefix. That's the version of
// tar in gnuwin, so only use 137 of the 155 bytes in the prefix. This means
// we'll need a pax header after 237 bytes of path instead of after 255,
// but in return paths up to 237 bytes work with gnuwin, instead of just
// 137 bytes of directory + 100 bytes of basename previously.
// (tar-1.13 also doesn't support pax headers, but in practice all paths in
// llvm's test suite are short enough for that to not matter.)
const int MaxPrefix = 137;
size_t Sep = Path.rfind('/', MaxPrefix + 1);
if (Sep == StringRef::npos)
return false;
if (Path.size() - Sep - 1 >= sizeof(UstarHeader::Name))
return false;
Prefix = Path.substr(0, Sep);
Name = Path.substr(Sep + 1);
return true;
}
// The PAX header is an extended format, so a PAX header needs
// to be followed by a "real" header.
static void writeUstarHeader(raw_fd_ostream &OS, StringRef Prefix,
StringRef Name, size_t Size) {
UstarHeader Hdr = makeUstarHeader();
memcpy(Hdr.Name, Name.data(), Name.size());
memcpy(Hdr.Mode, "0000664", 8);
snprintf(Hdr.Size, sizeof(Hdr.Size), "%011zo", Size);
memcpy(Hdr.Prefix, Prefix.data(), Prefix.size());
computeChecksum(Hdr);
OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
}
// Creates a TarWriter instance and returns it.
Expected<std::unique_ptr<TarWriter>> TarWriter::create(StringRef OutputPath,
StringRef BaseDir) {
using namespace sys::fs;
int FD;
if (std::error_code EC =
openFileForWrite(OutputPath, FD, CD_CreateAlways, OF_None))
return make_error<StringError>("cannot open " + OutputPath, EC);
return std::unique_ptr<TarWriter>(new TarWriter(FD, BaseDir));
}
TarWriter::TarWriter(int FD, StringRef BaseDir)
: OS(FD, /*shouldClose=*/true, /*unbuffered=*/false),
BaseDir(std::string(BaseDir)) {}
// Append a given file to an archive.
void TarWriter::append(StringRef Path, StringRef Data) {
// Write Path and Data.
std::string Fullpath = BaseDir + "/" + sys::path::convert_to_slash(Path);
// We do not want to include the same file more than once.
if (!Files.insert(Fullpath).second)
return;
StringRef Prefix;
StringRef Name;
if (splitUstar(Fullpath, Prefix, Name)) {
writeUstarHeader(OS, Prefix, Name, Data.size());
} else {
writePaxHeader(OS, Fullpath);
writeUstarHeader(OS, "", "", Data.size());
}
OS << Data;
pad(OS);
// POSIX requires tar archives end with two null blocks.
// Here, we write the terminator and then seek back, so that
// the file being output is terminated correctly at any moment.
uint64_t Pos = OS.tell();
OS << std::string(BlockSize * 2, '\0');
OS.seek(Pos);
OS.flush();
}
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