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      1 /*
      2  * Copyright (c) 2003 Constantin S. Svintsoff <kostik (at) iclub.nsu.ru>
      3  *
      4  * Redistribution and use in source and binary forms, with or without
      5  * modification, are permitted provided that the following conditions
      6  * are met:
      7  * 1. Redistributions of source code must retain the above copyright
      8  *    notice, this list of conditions and the following disclaimer.
      9  * 2. Redistributions in binary form must reproduce the above copyright
     10  *    notice, this list of conditions and the following disclaimer in the
     11  *    documentation and/or other materials provided with the distribution.
     12  * 3. The names of the authors may not be used to endorse or promote
     13  *    products derived from this software without specific prior written
     14  *    permission.
     15  *
     16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     19  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
     20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     26  * SUCH DAMAGE.
     27  */
     28 
     29 #include "readlink.h"
     30 
     31 #include <string>
     32 
     33 #include <errno.h>
     34 #include <sys/param.h>
     35 #include <sys/stat.h>
     36 #include <unistd.h>
     37 
     38 /**
     39  * This differs from realpath(3) mainly in its behavior when a path element does not exist or can
     40  * not be searched. realpath(3) treats that as an error and gives up, but we have Java-compatible
     41  * behavior where we just assume the path element was not a symbolic link. This leads to a textual
     42  * treatment of ".." from that point in the path, which may actually lead us back to a path we
     43  * can resolve (as in "/tmp/does-not-exist/../blah.txt" which would be an error for realpath(3)
     44  * but "/tmp/blah.txt" under the traditional Java interpretation).
     45  *
     46  * This implementation also removes all the fixed-length buffers of the C original.
     47  */
     48 bool realpath(const char* path, std::string& resolved) {
     49     // 'path' must be an absolute path.
     50     if (path[0] != '/') {
     51         errno = EINVAL;
     52         return false;
     53     }
     54 
     55     resolved = "/";
     56     if (path[1] == '\0') {
     57         return true;
     58     }
     59 
     60     // Iterate over path components in 'left'.
     61     int symlinkCount = 0;
     62     std::string left(path + 1);
     63     while (!left.empty()) {
     64         // Extract the next path component.
     65         size_t nextSlash = left.find('/');
     66         std::string nextPathComponent = left.substr(0, nextSlash);
     67         if (nextSlash != std::string::npos) {
     68             left.erase(0, nextSlash + 1);
     69         } else {
     70             left.clear();
     71         }
     72         if (nextPathComponent.empty()) {
     73             continue;
     74         } else if (nextPathComponent == ".") {
     75             continue;
     76         } else if (nextPathComponent == "..") {
     77             // Strip the last path component except when we have single "/".
     78             if (resolved.size() > 1) {
     79                 resolved.erase(resolved.rfind('/'));
     80             }
     81             continue;
     82         }
     83 
     84         // Append the next path component.
     85         if (resolved[resolved.size() - 1] != '/') {
     86             resolved += '/';
     87         }
     88         resolved += nextPathComponent;
     89 
     90         // See if we've got a symbolic link, and resolve it if so.
     91         struct stat sb;
     92         if (lstat(resolved.c_str(), &sb) == 0 && S_ISLNK(sb.st_mode)) {
     93             if (symlinkCount++ > MAXSYMLINKS) {
     94                 errno = ELOOP;
     95                 return false;
     96             }
     97 
     98             std::string symlink;
     99             if (!readlink(resolved.c_str(), symlink)) {
    100                 return false;
    101             }
    102             if (symlink[0] == '/') {
    103                 // The symbolic link is absolute, so we need to start from scratch.
    104                 resolved = "/";
    105             } else if (resolved.size() > 1) {
    106                 // The symbolic link is relative, so we just lose the last path component (which
    107                 // was the link).
    108                 resolved.erase(resolved.rfind('/'));
    109             }
    110 
    111             if (!left.empty()) {
    112                 const char* maybeSlash = (symlink[symlink.size() - 1] != '/') ? "/" : "";
    113                 left = symlink + maybeSlash + left;
    114             } else {
    115                 left = symlink;
    116             }
    117         }
    118     }
    119 
    120     // Remove trailing slash except when the resolved pathname is a single "/".
    121     if (resolved.size() > 1 && resolved[resolved.size() - 1] == '/') {
    122         resolved.erase(resolved.size() - 1, 1);
    123     }
    124     return true;
    125 }
    126