incfs/path.cpp

/*
 * Copyright (C) 2019 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "path.h"

#include <iterator>

#include <limits.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>

namespace android::incfs::path {

std::string normalize(std::string_view path) {
    if (path.empty()) {
        return {};
    }
    char buffer[PATH_MAX];
    if (!::realpath(path.data(), buffer)) {
        // need to return something
        return std::string{path};
    }
    return std::string(buffer);
}

std::string fromFd(int fd) {
    static constexpr char fdNameFormat[] = "/proc/self/fd/%d";
    char fdNameBuffer[std::size(fdNameFormat) + 11 + 1]; // max int length + '\0'
    snprintf(fdNameBuffer, std::size(fdNameBuffer), fdNameFormat, fd);

    std::string res;
    // lstat() is supposed to return us exactly the needed buffer size, but
    // somehow it may also return a smaller (but still >0) st_size field.
    // That's why let's only use it for the initial estimate.
    struct stat st = {};
    if (::lstat(fdNameBuffer, &st) || st.st_size == 0) {
        st.st_size = PATH_MAX;
    }
    auto bufSize = st.st_size;
    for (;;) {
        res.resize(bufSize + 1, '\0');
        auto size = ::readlink(fdNameBuffer, &res[0], res.size());
        if (size < 0) {
            return {};
        }
        if (size > bufSize) {
            // File got renamed in between lstat() and readlink() calls? Retry.
            bufSize *= 2;
            continue;
        }
        res.resize(size);
        return res;
    }
}

} // namespace android::incfs::path