Set up .local file before invoking LibFuzzer

src/agent/onefuzz-task/src/tasks/fuzz/libfuzzer/generic.rs

@@ -1,6 +1,8 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+use std::path::PathBuf;
+
 use anyhow::Result;
 use async_trait::async_trait;
 use onefuzz::libfuzzer::LibFuzzer;
@@ -37,7 +39,67 @@ impl common::LibFuzzerType for GenericLibFuzzer {
             config.common.machine_identity.clone(),
         ))
     }
+
+    async fn extra_setup(config: &common::Config<Self>) -> Result<()> {
+        // this is needed on Windows, but we do it unconditionally
+        let target_exe =
+            try_resolve_setup_relative_path(&config.common.setup_dir, &config.target_exe).await?;
+
+        // Set up a .local file on Windows before invoking the executable,
+        // so that all DLLs are resolved to the exe’s folder in preference to the Windows/system DLLs.
+        // The .local file is an empty file that tells DLL resolution to consider the same directory,
+        // even for system (or KnownDLL) files.
+        // See: https://learn.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-redirection#how-to-redirect-dlls-for-unpackaged-apps
+        let dotlocal_file = add_dotlocal_extension(target_exe);
+        if let Err(e) = tokio::fs::write(&dotlocal_file, &[]).await {
+            // ignore already-exists error, report anything else
+            if e.kind() != std::io::ErrorKind::AlreadyExists {
+                return Err(anyhow::Error::from(e).context("creating .local file"));
+            }
+        }
+
+        info!("Created .local file: {}", dotlocal_file.display());
+
+        Ok(())
+    }
+}
+
+fn add_dotlocal_extension(mut path: PathBuf) -> PathBuf {
+    if let Some(ext) = path.extension() {
+        let mut ext = ext.to_os_string();
+        ext.push(".local");
+        path.set_extension(ext);
+    } else {
+        path.set_extension("local");
+    }
+
+    path
 }
 
 pub type Config = common::Config<GenericLibFuzzer>;
 pub type LibFuzzerFuzzTask = common::LibFuzzerFuzzTask<GenericLibFuzzer>;
+
+#[cfg(test)]
+mod test {
+    use std::path::PathBuf;
+
+    use super::add_dotlocal_extension;
+
+    #[test]
+    fn dotlocal_with_extension() {
+        let path = PathBuf::from("executable.exe");
+        assert_eq!(
+            PathBuf::from("executable.exe.local"),
+            add_dotlocal_extension(path)
+        );
+    }
+
+    #[test]
+    fn dotlocal_without_extension() {
+        let path = PathBuf::from("executable");
+        assert_eq!(
+            PathBuf::from("executable.local"),
+            add_dotlocal_extension(path)
+        );
+    }
+}