<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/72334>72334</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            libFuzzer's runtime argument -stop_file doesn't work with empty files
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          markszabo
      </td>
    </tr>
</table>

<pre>
    libFuzzer has a runtime argument `-stop-file` with [this description](https://github.com/llvm/llvm-project/blob/72accbfd0a1023b3182202276904120524ff9200/compiler-rt/lib/fuzzer/FuzzerFlags.def#L76):

```
stop_file: "Stop fuzzing ASAP if this file exists"
```

However fuzzing only stops if the file exists and has some content. If the file is empty, the fuzzing continues.

## Steps to reproduce

1. Create a simple fuzz test, e.g.
   ```cpp
   #include <fuzzer/FuzzedDataProvider.h>
 #include <assert.h>
   
   extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
   {
 FuzzedDataProvider provider(data, size);
       int a = provider.ConsumeIntegral<int>();
       int b = provider.ConsumeIntegral<int>();
   
       assert(a+b == b+a);

 return 0;
   }
   ```
   Save it as `test.cpp`

2. Compile the test
   ```sh
   clang++ -g -fsanitize=address,fuzzer test.cpp -o test
   ```

3. Run the resulting binary with the `-stop_file` argument:
   ```sh
   ./test -stop_file=/tmp/stop-file
 ```

4. In a separate terminal create the stop-file as an empty file:
   ```sh
   touch /tmp/stop-file
   ```
   Observe that the fuzzing run continues.

4. Create a file with some content:
 ```sh
   rm /tmp/stop-file # optional
   echo abc > /tmp/stop-file
 ```
   Observe that the fuzzing stops quickly with a message like `Done 5036370 runs in 21 second(s)`.

## Expected behavior 

Either the flag description should say that the file can't be empty, or fuzzing should stop even if the file is empty.

## Potential cause of the issue

I believe [this is the part of the code](https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/fuzzer/FuzzerLoop.cpp#L878-L880) that stops the fuzzing loop if the file exists:

```cpp
    if (!Options.StopFile.empty() &&
        !FileToVector(Options.StopFile, 1, false).empty())
      break;
```

`FileToVector()` is implemented [here](https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/fuzzer/FuzzerIO.cpp#L41):

```cpp
Unit FileToVector(const std::string &Path, size_t MaxSize, bool ExitOnError) {
 std::ifstream T(Path, std::ios::binary);
  if (ExitOnError && !T) {
 Printf("No such directory: %s; exiting\n", Path.c_str());
 exit(1);
  }

  T.seekg(0, T.end);
  auto EndPos = T.tellg();
 if (EndPos < 0) return {};
  size_t FileLen = EndPos;
  if (MaxSize)
    FileLen = std::min(FileLen, MaxSize);

  T.seekg(0, T.beg);
  Unit Res(FileLen);
  T.read(reinterpret_cast<char *>(Res.data()), FileLen);
  return Res;
}
```

This returns `{}` if the file doesn't exist, but also if the file is empty, leading to this behavior.
</pre>
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