[libFuzzer] Diff 19 - Independent implementation of DirName for Windows.

[zturner]

Is libfuzzer using any shell api functions? If so, I kind of think it shouldn't. What Win32 API requires linking against this?

[amccarth]

This is fine by me. Some projects have a preference to do stuff like this in the build files rather than the code. I don't know if the Fuzzer folks have a preference.

I'll approve this to keep you from being blocked. It's easy to change later if necessary.

[amccarth]

@zach: It's for PathRemoveFileSpecA.

[mpividori]

Hi, I need that library because I am using the function PathRemoveFileSpec(). I couldn't find other option.
I use this pragma so when users link against libFuzzer, they don't have to explicitly add the Shlwapi.lib library.
Similar approaches in simplifying the user interface were considered when implementing other parts of the code. For example, in "FuzzerExtFunctions*" we use different implementation for Linux and Apple to avoid requiring extra flags when using libFuzzer.

[mpividori]

@zturner Yes, we can implement it. I considered that before, but as there are many cases to be considered: relative paths vs full paths, using UNC or disk designators, etc, I thought it would be better to use Win Api instead of implementing it from scratch. I didn't realize that it is implemented in the Support library. I will read that code.

[mpividori]

@zturner @amccarth
This is an option, using regex. It is simpler than the implementation in Support library, and probably more correct (I don't have much experience with Windows), but maybe not very efficient.
If we don't use regex, as Support library does, this will require many more lines of code. What do you think?
Thanks.

[mpividori]

It's based on the documentation for Naming Files, Paths, and Namespaces in: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx

[amccarth]

I appreciate the effort here, but now it looks like we have something complicated enough to need testing (and fuzzing!). I'm not sure that's the best use of Marcos's time.

I would prefer using the LLVM support library or the API from Shlwapi. Both of those are tested and don't require re-inventing the wheel. If we can't do either of those, I'll take a closer look at this.

[kcc]

remember that libFuzzer should not have *any* dependencies other than what's provided by the system by default,
i.e. LLVM support lib is out of discussion here.

[kcc]

We may cook something with std::string

[mpividori]

Hi, I implemented it from scratch. I tested with this cases and it works fine:

DirName("\\?\D:\SomeFile.txt") == "\\?\D:\"
DirName("\\?\D:\SomeDir\SomeFile.txt") == "\\?\D:\SomeDir"
DirName("\\?\UNC\Server\Share\") == "\\?\UNC\Server\Share\"
DirName("\\?\UNC\Server\Share\SomeFile.txt") == "\\?\UNC\Server\Share\"
DirName("SomeFile.txt") == "."
DirName("SomeDir\SomeFile.txt") == ".\SomeDir"
DirName("C:") == "C:"
DirName("C:SomeFile.txt") == "C:"
DirName("C:SomeDir\SomeFile.txt") == "C:SomeDir"
DirName("C:\") == "C:\"
DirName("C:\SomeFile.txt") == "C:\"
DirName("C:\SomeDir\SomeFile.txt") == "C:\SomeDir"
DirName("\") == "\"
DirName("\SomeFile.txt") == "\"
DirName("\SomeDir\SomeFile.txt") == "\SomeDir"
DirName("\\Server\Share\") == "\\Server\Share\"
DirName("\\Server\Share\SomeFile.txt") == "\\Server\Share\"

[mpividori]

[zturner]

Can you do a quick sanity check that we return similar output on Posix and Windows (i.e. run all the above tests through Linux's DirName function and verify that they're the same)? Obviously slashes will be different, and things like \\?\ syntax and UNC syntax won't apply. But for example, but the postconditions of the function should be the same on Posix and Windows, such as if you pass a simple filenmae like foo.txt to the function, then your example shows we return ., therefore Posix should return . as well otherwise someone might rely on it. Similarly, for foo\bar\file.txt, if we return foo\bar, then I would expect /foo/bar/file.txt on Posix would return /foo/bar without a trailing slash. (This is another argument for not relying on the Windows function in shlwapi I suppose, since we should try to match the postconditions as closely as possible.

[mpividori]

@zturner
I have checked this tests, and they output the same (Posix version and Windows):

WixDirName("") == "."
PosixDirName("") == "."

WinDirName("NoSlash") == "."
PosixDirName("NoSlash") == "."

WinDirName("\") == "\"
PosixDirName("/") == "/"

WinDirName("\SomeFile") == "\"
PosixDirName("/SomeFile") == "/"

WinDirName("\SomeDir\") == "\"
PosixDirName("/SomeDir/") == "/"

WinDirName("\SomeDir\SomeFile") == "\SomeDir"
PosixDirName("/SomeDir/SomeFile") == "/SomeDir"

[zturner]

I had imagined the algorithm starting from the end and looking backwards for the first separator. I can't find anything wrong, so it seems fine, but did you consider starting from the end and going backwards and decide it was more difficult and/or wouldn't work?

lib/Fuzzer/FuzzerIOWindows.cpp

149

Can we use a better name than In? I'm not sure what it's supposed to represent. Is this more like the starting offset? If so call it Offset perhaps.

190

What is Ad supposed to be short for? If it means something (Address?), I would just write it out as a full word.

203

should Ref be a const reference?

[mpividori]

@zturner I updated the code to include your suggestions.
The algorithm can't simply start from the end and look backwards for the first separator, because we need to preserve the prefix that represent the root location. We shouldn't remove anything there. In Windows we have many different options, like: \\Server\Share\ , \ , C: , C:\ , \\?\C:\ , \\?\UNC\Server\Share\
So, I need to start analyzing which kind of path we received, and then I can remove the separator in the rest of the path, if it exists.
Thanks.

[amccarth]

LGTM.

Please update the title and description of this patch to reflect the new direction.

lib/Fuzzer/FuzzerIOWindows.cpp

259

Nit: In these comments, I would say separator instead of '\'. It removes both slashes and backslashes, and '\' looks like a bad char literal.

[mpividori]

Done. Thanks.

lib/Fuzzer/FuzzerIOWindows.cpp

 int DuplicateFile(int Fd) {
   return _dup(Fd);
 }
 
 void RemoveFile(const std::string &Path) {
   _unlink(Path.c_str());
 }
 
+static bool IsSeparator(char C) {
+  return C == '\\' || C == '/';
+}
+
+// Parse disk designators, like "C:\". If Relative == true, also accepts: "C:".
+// Returns number of characters considered if successful.
+static size_t ParseDrive(const std::string &FileName, const size_t In,
+                         bool Relative = true) {
+  if (In + 1 >= FileName.size() || FileName[In + 1] != ':')
+    return 0;
+  if (In + 2 >= FileName.size() || !IsSeparator(FileName[In + 2])) {
+    if (!Relative) // Accept relative path?
+      return 0;
+    else
+      return 2;
+  }
+  return 3;
+}
+
+// Parse a file name, like: SomeFile.txt
+// Returns number of characters considered if successful.
+static size_t ParseFileName(const std::string &FileName, const size_t In) {
+  size_t Pos = In;
+  const size_t End = FileName.size();
+  for(; Pos < End && !IsSeparator(FileName[Pos]); ++Pos)
+    ;
+  return Pos - In;
+}
+
+// Parse a directory ending in separator, like: SomeDir\
+// Returns number of characters considered if successful.
+static size_t ParseDir(const std::string &FileName, const size_t In) {
+  size_t Pos = In;
+  const size_t End = FileName.size();
+  if (Pos >= End || IsSeparator(FileName[Pos]))
+    return 0;
+  for(; Pos < End && !IsSeparator(FileName[Pos]); ++Pos)
+    ;
+  if (Pos >= End)
+    return 0;
+  ++Pos; // Include separator.
+  return Pos - In;
+}
+
+// Parse a servername and share, like: SomeServer\SomeShare\
+// Returns number of characters considered if successful.
+static size_t ParseServerAndShare(const std::string &FileName, const size_t In) {
+  size_t Pos = In, Ad;
+  if (!(Ad = ParseDir(FileName, Pos)))
+    return 0;
+  Pos += Ad;
+  if (!(Ad = ParseDir(FileName, Pos)))
+    return 0;
+  Pos += Ad;
+  return Pos - In;
+}
+
+// Parse the given Ref string from the position In, to exactly match the given
+// string Patt.
+// Returns number of characters considered if successful.
+static size_t ParseCustomString(std::string Ref, size_t In, const char* Patt) {
+  size_t Len = strlen(Patt);
+  if (In + Len > Ref.size())
+    return 0;
+  return Ref.compare(In, Len, Patt) == 0 ? Len : 0;
+}
+
+// Parse a location, like:
+// \\?\UNC\Server\Share\  \\?\C:\  \\Server\Share\  \  C:\  C:
+// Returns number of characters considered if successful.
+static size_t ParseLocation(const std::string &FileName) {
+  size_t Pos = 0, Ad;
+
+  if ((Ad = ParseCustomString(FileName, Pos, R"(\\?\)"))) {
+    Pos += Ad;
+    if ((Ad = ParseCustomString(FileName, Pos, R"(UNC\)"))) {
+      Pos += Ad;
+      if ((Ad = ParseServerAndShare(FileName, Pos)))
+        return Pos + Ad;
+      return 0;
+    }
+    if ((Ad = ParseDrive(FileName, Pos, false)))
+      return Pos + Ad;
+    return 0;
+  }
+
+  if (Pos < FileName.size() && IsSeparator(FileName[Pos])) {
+    ++Pos;
+    if (Pos < FileName.size() && IsSeparator(FileName[Pos])) {
+      ++Pos;
+      if ((Ad = ParseServerAndShare(FileName, Pos)))
+        return Pos + Ad;
+      return 0;
+    }
+    return Pos;
+  }
+
+  if ((Ad = ParseDrive(FileName, Pos)))
+    return Pos + Ad;
+
+  return Pos;
+}
+
 std::string DirName(const std::string &FileName) {
-  std::string Res = FileName;
-  PathRemoveFileSpecA(&Res[0]);
-  Res.resize(std::strlen(Res.c_str()));
-  return Res;
+  size_t LocationLen = ParseLocation(FileName);
+  size_t DirLen = 0, Ad;
+  while ((Ad = ParseDir(FileName, LocationLen + DirLen)))
+    DirLen += Ad;
+  size_t FileLen = ParseFileName(FileName, LocationLen + DirLen);
+
+  if (LocationLen + DirLen + FileLen != FileName.size()) {
+    Printf("DirName() failed for \"%s\", invalid path.\n", FileName.c_str());
+    exit(1);
+  }
+
+  if (DirLen) {
+    --DirLen; // Remove trailing '\'.
+    if (!FileLen) { // Path ended in '\'.
+      assert(DirLen);
+      // Remove file name from Dir.
+      while (DirLen && !IsSeparator(FileName[LocationLen + DirLen - 1]))
+        --DirLen;
+      if (DirLen) // Remove trailing '\'.
+        --DirLen;
+    }
+  }
+
+  if (!LocationLen) { // Relative path.
+    if (!DirLen)
+      return ".";
+    return std::string(".\\").append(FileName, 0, DirLen);
+  }
+
+  return FileName.substr(0, LocationLen + DirLen);
 }
 
 }  // namespace fuzzer
 
 #endif // LIBFUZZER_WINDOWS