FileSystem: implement symlink support in zip archives, fix #83

Most of the code is taken from DarkPlaces source code

src/common/FileSystem.cpp

@@ -909,6 +909,20 @@ class ZipArchive {
 		return fileInfo.uncompressed_size;
 	}
 
+	// Check if currently open file is a symbolic link
+	bool CheckSymlink(std::error_code& err) const
+	{
+		unz_file_info64 fileInfo;
+		int result = unzGetCurrentFileInfo64(zipFile, &fileInfo, nullptr, 0, nullptr, 0, nullptr, 0);
+		if (result != UNZ_OK) {
+			SetErrorCodeZlib(err, result);
+			return false;
+		}
+		ClearErrorCode(err);
+		uLong attr = fileInfo.external_fa >> 16;
+		return ( ( attr & 0120000 ) == 0120000 );
+	}
+
 	// Read from the currently open file
 	size_t ReadFile(void* buffer, size_t length, std::error_code& err) const
 	{
@@ -1244,7 +1258,71 @@ const std::vector<LoadedPakInfo>& GetLoadedPaks()
 	return loadedPaks;
 }
 
-std::string ReadFile(Str::StringRef path, std::error_code& err)
+// expandSymlink
+// code adapted from DarkPlaces fs.c FS_OpenReadFile
+// commit 1c55fd95ff34d9aafb76c927215ea8fbab5210c6
+// Copyright 2008 divverent
+// GPLv2+
+std::string expandSymlink(Str::StringRef path, Str::StringRef content)
+{
+		const char *filename = path.c_str();
+
+		char linkbuf[MAX_QPATH];
+		strncpy(linkbuf, content.c_str(), MAX_QPATH);
+		int count = strlen(linkbuf);
+
+		const char *mergeslash;
+		char *mergestart;
+
+		// Now combine the paths...
+		mergeslash = strrchr(filename, '/');
+		mergestart = linkbuf;
+		if(!mergeslash) {
+			mergeslash = filename;
+		}
+		while(!strncmp(mergestart, "../", 3)) {
+			mergestart += 3;
+			while(mergeslash > filename) {
+				--mergeslash;
+				if(*mergeslash == '/') {
+					break;
+				}
+			}
+		}
+		// Now, mergestart will point to the path to be appended, and mergeslash points to where it should be appended
+		if(mergeslash == filename) {
+			// Either mergeslash == filename, then we just replace the name (done below)
+		}
+		else {
+			// Or, we append the name after mergeslash;
+			// or rather, we can also shift the linkbuf so we can put everything up to and including mergeslash first
+			int spaceNeeded = mergeslash - filename + 1;
+			int spaceRemoved = mergestart - linkbuf;
+			if(count - spaceRemoved + spaceNeeded >= MAX_QPATH)
+			{
+				Log::Debug("symlink: too long path rejected\n");
+				return "";
+			}
+			memmove(linkbuf + spaceNeeded, linkbuf + spaceRemoved, count - spaceRemoved);
+			memcpy(linkbuf, filename, spaceNeeded);
+			linkbuf[count - spaceRemoved + spaceNeeded] = 0;
+			mergestart = linkbuf;
+		}
+
+		Log::Debug("found symlink: %s → %s", path, mergestart);
+
+		/*
+		if(CheckNastyPath (mergestart, false))
+		{
+			Log::Debug("symlink: nasty path %s rejected\n", mergestart);
+			return "";
+		}
+		*/
+
+		return mergestart;
+}
+
+std::string ReadFile(Str::StringRef path, int symlinkLevels, std::error_code& err)
 {
 	auto it = fileMap.find(path);
 	if (it == fileMap.end()) {
@@ -1298,17 +1376,45 @@ std::string ReadFile(Str::StringRef path, std::error_code& err)
 		if (err)
 			return "";
 
-		// Close file and check for CRC errors
+		bool isSymLink = zipFile.CheckSymlink(err);
+		if (err) {
+			return "";
+		}
+
 		zipFile.CloseFile(err);
 		if (err)
 			return "";
 
+		if (isSymLink) {
+			if (symlinkLevels <= 0) {
+				Log::Debug("symlink: %s: too many levels of symbolic links", path);
+				return "";
+			}
+			out = expandSymlink(path, out);
+			if (out[0] == '\0') {
+				return "";
+			}
+			out = ReadFile(out, symlinkLevels -1, err);
+			if (err) {
+				return "";
+			}
+			return out;
+		}
+
+		// Close file and check for CRC errors
 		return out;
 	}
 
 	ASSERT_UNREACHABLE();
 }
 
+std::string ReadFile(Str::StringRef path, std::error_code& err)
+{
+	return ReadFile(path, 16, err);
+	if (err)
+		return "";
+}
+
 void CopyFile(Str::StringRef path, const File& dest, std::error_code& err)
 {
 	auto it = fileMap.find(path);