lix/src/libutil/archive.cc

#include <cerrno>
#include <algorithm>
#include <string_view>
#include <vector>
#include <map>

#include <strings.h> // for strcasecmp

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <dirent.h>
#include <fcntl.h>

#include "archive.hh"
#include "file-system.hh"
#include "finally.hh"
#include "serialise.hh"
#include "config.hh"
#include "logging.hh"
#include "signals.hh"

namespace nix {

struct ArchiveSettings : Config
{
    Setting<bool> useCaseHack{this,
        #if __APPLE__
            true,
        #else
            false,
        #endif
        "use-case-hack",
        "Whether to enable a Darwin-specific hack for dealing with file name collisions."};
    Setting<bool> preallocateContents{this, false, "preallocate-contents",
        "Whether to preallocate files when writing objects with known size."};
};

static ArchiveSettings archiveSettings;

static GlobalConfig::Register rArchiveSettings(&archiveSettings);

PathFilter defaultPathFilter = [](const Path &) { return true; };


static WireFormatGenerator dumpContents(const Path & path, off_t size)
{
    co_yield "contents";
    co_yield size;

    AutoCloseFD fd{open(path.c_str(), O_RDONLY | O_CLOEXEC)};
    if (!fd) throw SysError("opening file '%1%'", path);

    std::vector<char> buf(65536);
    size_t left = size;

    while (left > 0) {
        auto n = std::min(left, buf.size());
        readFull(fd.get(), buf.data(), n);
        left -= n;
        co_yield std::span{buf.data(), n};
    }

    co_yield SerializingTransform::padding(size);
}


static WireFormatGenerator dump(const Path & path, time_t & mtime, PathFilter & filter)
{
    checkInterrupt();

    auto st = lstat(path);
    mtime = st.st_mtime;

    co_yield "(";

    if (S_ISREG(st.st_mode)) {
        co_yield "type";
        co_yield "regular";
        if (st.st_mode & S_IXUSR) {
            co_yield "executable";
            co_yield "";
        }
        co_yield dumpContents(path, st.st_size);
    }

    else if (S_ISDIR(st.st_mode)) {
        co_yield "type";
        co_yield "directory";

        /* If we're on a case-insensitive system like macOS, undo
           the case hack applied by restorePath(). */
        std::map<std::string, std::string> unhacked;
        for (auto & i : readDirectory(path))
            if (archiveSettings.useCaseHack) {
                std::string name(i.name);
                size_t pos = i.name.find(caseHackSuffix);
                if (pos != std::string::npos) {
                    debug("removing case hack suffix from '%1%'", path + "/" + i.name);
                    name.erase(pos);
                }
                if (!unhacked.emplace(name, i.name).second)
                    throw Error("file name collision in between '%1%' and '%2%'",
                       (path + "/" + unhacked[name]),
                       (path + "/" + i.name));
            } else
                unhacked.emplace(i.name, i.name);

        for (auto & i : unhacked)
            if (filter(path + "/" + i.first)) {
                co_yield "entry";
                co_yield "(";
                co_yield "name";
                co_yield i.first;
                co_yield "node";
                time_t tmp_mtime;
                co_yield dump(path + "/" + i.second, tmp_mtime, filter);
                if (tmp_mtime > mtime) {
                    mtime = tmp_mtime;
                }
                co_yield ")";
            }
    }

    else if (S_ISLNK(st.st_mode)) {
        co_yield "type";
        co_yield "symlink";
        co_yield "target";
        co_yield readLink(path);
    }

    else throw Error("file '%1%' has an unsupported type", path);

    co_yield ")";
}


WireFormatGenerator dumpPathAndGetMtime(Path path, time_t & mtime, PathFilter & filter)
{
    co_yield narVersionMagic1;
    co_yield dump(path, mtime, filter);
}

WireFormatGenerator dumpPath(Path path, PathFilter & filter)
{
    time_t ignored;
    co_yield dumpPathAndGetMtime(path, ignored, filter);
}


WireFormatGenerator dumpString(std::string_view s)
{
    co_yield narVersionMagic1;
    co_yield "(";
    co_yield "type";
    co_yield "regular";
    co_yield "contents";
    co_yield s;
    co_yield ")";
}


static SerialisationError badArchive(const std::string & s)
{
    return SerialisationError("bad archive: " + s);
}


#if 0
static void skipGeneric(Source & source)
{
    if (readString(source) == "(") {
        while (readString(source) != ")")
            skipGeneric(source);
    }
}
#endif


struct CaseInsensitiveCompare
{
    bool operator() (const std::string & a, const std::string & b) const
    {
        return strcasecmp(a.c_str(), b.c_str()) < 0;
    }
};

namespace nar {

static Generator<Entry> parseObject(Source & source, const Path & path)
{
#define EXPECT(raw, kind)                              \
    do {                                               \
        const auto s = readString(source);             \
        if (s != (raw)) {                                \
            throw badArchive("expected " kind " tag"); \
        }                                              \
        co_yield MetadataString{s};                    \
    } while (0)

    EXPECT("(", "open");
    EXPECT("type", "type");

    checkInterrupt();

    const auto t = readString(source);
    co_yield MetadataString{t};

    if (t == "regular") {
        auto contentsOrFlag = readString(source);
        co_yield MetadataString{contentsOrFlag};
        const bool executable = contentsOrFlag == "executable";
        if (executable) {
            auto s = readString(source);
            co_yield MetadataString{s};
            if (s != "") {
                throw badArchive("executable marker has non-empty value");
            }
            contentsOrFlag = readString(source);
            co_yield MetadataString{contentsOrFlag};
        }
        if (contentsOrFlag == "contents") {
            const uint64_t size = readLongLong(source);
            co_yield MetadataRaw{SerializingTransform()(size)};
            auto makeReader = [](Source & source, uint64_t & left) -> Generator<Bytes> {
                std::array<char, 65536> buf;

                while (left) {
                    checkInterrupt();
                    auto n = std::min<uint64_t>(buf.size(), left);
                    source(buf.data(), n);
                    co_yield std::span{buf.data(), n};
                    left -= n;
                }
            };
            auto left = size;
            co_yield File{path, executable, size, makeReader(source, left)};
            // we could drain the remainder of the file, but coroutines being interruptible
            // at any time makes this difficult. for files this is not that hard, but being
            // consistent with directories is more important than handling the simple case.
            assert(left == 0);
            readPadding(size, source);
            co_yield MetadataRaw{SerializingTransform::padding(size)};
        } else {
            throw badArchive("file without contents found: " + path);
        }
    } else if (t == "directory") {
        auto makeReader = [](Source & source, const Path & path, bool & completed
                          ) -> Generator<Entry> {
            std::map<Path, int, CaseInsensitiveCompare> names;
            std::string prevName;

            while (1) {
                checkInterrupt();

                {
                    const auto s = readString(source);
                    co_yield MetadataString{s};
                    if (s == ")") {
                        completed = true;
                        co_return;
                    } else if (s != "entry") {
                        throw badArchive("expected entry tag");
                    }
                    EXPECT("(", "open");
                }

                EXPECT("name", "name");
                auto name = readString(source);
                co_yield MetadataString{name};
                if (name.empty() || name == "." || name == ".."
                    || name.find('/') != std::string::npos
                    || name.find((char) 0) != std::string::npos
                    // The case hack is a thing that only exists on the
                    // filesystem.
                    // Unpacking one appearing in a NAR is super
                    // sketchy because it will at minimum cause corruption at
                    // the time of repacking the NAR.
                    || name.find(caseHackSuffix) != std::string::npos)
                {
                    throw Error("NAR contains invalid file name '%1%'", name);
                }
                if (name <= prevName) {
                    throw Error("NAR directory is not sorted");
                }
                prevName = name;
                if (archiveSettings.useCaseHack) {
                    auto i = names.find(name);
                    if (i != names.end()) {
                        debug("case collision between '%1%' and '%2%'", i->first, name);
                        name += caseHackSuffix;
                        name += std::to_string(++i->second);
                    } else {
                        names[name] = 0;
                    }
                }

                EXPECT("node", "node");
                co_yield parseObject(source, path + "/" + name);
                EXPECT(")", "close");
            }
        };
        bool completed = false;
        co_yield Directory{path, makeReader(source, path, completed)};
        // directories may nest, so to drain a directory properly we'd have to add a Finally
        // argument to the generator to ensure that the draining code is always run. this is
        // usually not necessary, hard to follow, and rather error-prone on top of all that.
        assert(completed);
        // directories are terminated already, don't try to read another ")"
        co_return;
    } else if (t == "symlink") {
        EXPECT("target", "target");
        std::string target = readString(source);
        co_yield MetadataString{target};
        co_yield Symlink{path, target};
    } else {
        throw badArchive("unknown file type " + t);
    }

    EXPECT(")", "close");

#undef EXPECT
}

Generator<Entry> parse(Source & source)
{
    std::string version;
    try {
        version = readString(source, narVersionMagic1.size());
        co_yield MetadataString{version};
    } catch (SerialisationError & e) {
        /* This generally means the integer at the start couldn't be
           decoded.  Ignore and throw the exception below. */
    }
    if (version != narVersionMagic1)
        throw badArchive("input doesn't look like a Nix archive");
    co_yield parseObject(source, "");
}

}


static WireFormatGenerator restore(NARParseVisitor & sink, Generator<nar::Entry> nar)
{
    while (auto entry = nar.next()) {
        co_yield std::visit(
            overloaded{
                [](nar::MetadataString m) -> WireFormatGenerator {
                    co_yield m.data;
                },
                [](nar::MetadataRaw r) -> WireFormatGenerator {
                    co_yield r.raw;
                },
                [&](nar::File f) {
                    return [](auto f, auto & sink) -> WireFormatGenerator {
                        auto handle = sink.createRegularFile(f.path, f.size, f.executable);

                        while (auto block = f.contents.next()) {
                            handle->receiveContents(std::string_view{block->data(), block->size()});
                            co_yield *block;
                        }
                        handle->close();
                    }(std::move(f), sink);
                },
                [&](nar::Symlink sl) {
                    return [](auto sl, auto & sink) -> WireFormatGenerator {
                        sink.createSymlink(sl.path, sl.target);
                        co_return;
                    }(std::move(sl), sink);
                },
                [&](nar::Directory d) {
                    return [](auto d, auto & sink) -> WireFormatGenerator {
                        sink.createDirectory(d.path);
                        return restore(sink, std::move(d.contents));
                    }(std::move(d), sink);
                },
            },
            std::move(*entry)
        );
    }
}

WireFormatGenerator parseAndCopyDump(NARParseVisitor & sink, Source & source)
{
    return restore(sink, nar::parse(source));
}

void parseDump(NARParseVisitor & sink, Source & source)
{
    auto parser = parseAndCopyDump(sink, source);
    while (parser.next()) {
        // ignore the actual item
    }
}

/*
 * Note [NAR restoration security]:
 * It's *critical* that NAR restoration will never overwrite anything even if
 * duplicate filenames are passed in. It is inevitable that not all NARs are
 * fit to actually successfully restore to the target filesystem; errors may
 * occur due to collisions, and this *must* cause the NAR to be rejected.
 *
 * Although the filenames are blocked from being *the same bytes* by a higher
 * layer, filesystems have other ideas on every platform:
 * - The store may be on a case-insensitive filesystem like APFS, ext4 with
 *   casefold directories, zfs with casesensitivity=insensitive
 * - The store may be on a Unicode normalizing (or normalization-insensitive)
 *   filesystem like APFS (where files are looked up by
 *   hash(normalize(fname))), HFS+ (where file names are always normalized to
 *   approximately NFD), or zfs with normalization=formC, etc.
 *
 * It is impossible to know the version of Unicode being used by the underlying
 * filesystem, thus it is *impossible* to stop these collisions.
 *
 * Overwriting files as a result of invalid NARs will cause a security bug like
 * CppNix's CVE-2024-45593 (GHSA-h4vv-h3jq-v493)
 */

/**
 * This code restores NARs from disk.
 *
 * See Note [NAR restoration security] for security invariants in this procedure.
 *
 */
struct NARRestoreVisitor : NARParseVisitor
{
    Path dstPath;

private:
    class MyFileHandle : public FileHandle
    {
        AutoCloseFD fd;

        MyFileHandle(AutoCloseFD && fd, uint64_t size, bool executable) : FileHandle(), fd(std::move(fd))
        {
            if (executable) {
                makeExecutable();
            }

            maybePreallocateContents(size);
        }

        void makeExecutable()
        {
            struct stat st;
            if (fstat(fd.get(), &st) == -1)
                throw SysError("fstat");
            if (fchmod(fd.get(), st.st_mode | (S_IXUSR | S_IXGRP | S_IXOTH)) == -1)
                throw SysError("fchmod");
        }

        void maybePreallocateContents(uint64_t len)
        {
            if (!archiveSettings.preallocateContents)
                return;

#if HAVE_POSIX_FALLOCATE
            if (len) {
                errno = posix_fallocate(fd.get(), 0, len);
                /* Note that EINVAL may indicate that the underlying
                   filesystem doesn't support preallocation (e.g. on
                   OpenSolaris).  Since preallocation is just an
                   optimisation, ignore it. */
                if (errno && errno != EINVAL && errno != EOPNOTSUPP && errno != ENOSYS)
                    throw SysError("preallocating file of %1% bytes", len);
            }
#endif
        }

    public:

        ~MyFileHandle() = default;

        virtual void close() override
        {
            /* Call close explicitly to make sure the error is checked */
            fd.close();
        }

        void receiveContents(std::string_view data) override
        {
            writeFull(fd.get(), data);
        }

        friend struct NARRestoreVisitor;
    };

public:
    void createDirectory(const Path & path) override
    {
        Path p = dstPath + path;
        if (mkdir(p.c_str(), 0777) == -1)
            throw SysError("creating directory '%1%'", p);
    };

    std::unique_ptr<FileHandle> createRegularFile(const Path & path, uint64_t size, bool executable) override
    {
        Path p = dstPath + path;
        AutoCloseFD fd = AutoCloseFD{open(p.c_str(), O_CREAT | O_EXCL | O_WRONLY | O_CLOEXEC, 0666)};
        if (!fd) throw SysError("creating file '%1%'", p);

        return std::unique_ptr<MyFileHandle>(new MyFileHandle(std::move(fd), size, executable));
    }

    void createSymlink(const Path & path, const std::string & target) override
    {
        Path p = dstPath + path;
        nix::createSymlink(target, p);
    }
};


void restorePath(const Path & path, Source & source)
{
    NARRestoreVisitor sink;
    sink.dstPath = path;
    parseDump(sink, source);
}


WireFormatGenerator copyNAR(Source & source)
{
    // FIXME: if 'source' is the output of dumpPath() followed by EOF,
    // we should just forward all data directly without parsing.

    static NARParseVisitor parseSink; /* null sink; just parse the NAR */

    return parseAndCopyDump(parseSink, source);
}

}