log better errors of local file creation and add option to use altera…

…ntive way (#2890)

* log better errors of local file creation and add option to use alternative way

* also pass errno

changelog.d/2889.changed.md

@@ -0,0 +1 @@
+log better errors of local file creation and add option to use alternative way

mirrord-schema.json

@@ -811,6 +811,14 @@
             "boolean",
             "null"
           ]
+        },
+        "use_dev_null": {
+          "title": "_experimental_ use_dev_null {#experimental-use_dev_null}",
+          "description": "Uses /dev/null for creating local fake files (should be better than using /tmp)",
+          "type": [
+            "boolean",
+            "null"
+          ]
         }
       },
       "additionalProperties": false

mirrord/config/src/experimental.rs

@@ -52,6 +52,12 @@ pub struct ExperimentalConfig {
     /// This option applies only on macOS.
     #[config(default = false)]
     pub disable_reuseaddr: bool,
+
+    /// ### _experimental_ use_dev_null {#experimental-use_dev_null}
+    ///
+    /// Uses /dev/null for creating local fake files (should be better than using /tmp)
+    #[config(default = false)]
+    pub use_dev_null: bool,
 }
 
 impl CollectAnalytics for &ExperimentalConfig {

mirrord/layer/src/error.rs

@@ -68,8 +68,8 @@ pub(crate) enum HookError {
     #[error("mirrord-layer: Failed converting `to_str` with `{0}`!")]
     Utf8(#[from] std::str::Utf8Error),
 
-    #[error("mirrord-layer: Failed creating local file for `remote_fd` `{0}`!")]
-    LocalFileCreation(u64),
+    #[error("mirrord-layer: Failed creating local file for `remote_fd` `{0}`! with errno `{1}`")]
+    LocalFileCreation(u64, i32),
 
     #[cfg(target_os = "macos")]
     #[error("mirrord-layer: SIP patch failed with error `{0}`!")]
@@ -289,7 +289,7 @@ impl From<HookError> for i64 {
             HookError::DNSNoName => libc::EFAULT,
             HookError::Utf8(_) => libc::EINVAL,
             HookError::NullPointer => libc::EINVAL,
-            HookError::LocalFileCreation(_) => libc::EINVAL,
+            HookError::LocalFileCreation(_, err) => err,
             #[cfg(target_os = "macos")]
             HookError::FailedSipPatch(_) => libc::EACCES,
             HookError::SocketUnsuportedIpv6 => libc::EAFNOSUPPORT,

mirrord/layer/src/file/ops.rs

@@ -142,22 +142,42 @@ fn get_remote_fd(local_fd: RawFd) -> Detour<u64> {
 /// Create temporary local file to get a valid local fd.
 #[mirrord_layer_macro::instrument(level = "trace", ret)]
 fn create_local_fake_file(remote_fd: u64) -> Detour<RawFd> {
+    if crate::setup().experimental().use_dev_null {
+        return create_local_devnull_file(remote_fd);
+    }
     let random_string = Alphanumeric.sample_string(&mut rand::thread_rng(), 16);
     let file_name = format!("{remote_fd}-{random_string}");
     let file_path = env::temp_dir().join(file_name);
     let file_c_string = CString::new(file_path.to_string_lossy().to_string())?;
     let file_path_ptr = file_c_string.as_ptr();
     let local_file_fd: RawFd = unsafe { FN_OPEN(file_path_ptr, O_RDONLY | O_CREAT) };
     if local_file_fd == -1 {
+        let error = errno::errno();
         // Close the remote file if creating a tmp local file failed and we have an invalid local fd
         close_remote_file_on_failure(remote_fd)?;
-        Detour::Error(HookError::LocalFileCreation(remote_fd))
+        Detour::Error(HookError::LocalFileCreation(remote_fd, error.0))
     } else {
         unsafe { unlink(file_path_ptr) };
         Detour::Success(local_file_fd)
     }
 }
 
+/// Open /dev/null to get a valid file fd
+#[mirrord_layer_macro::instrument(level = "trace", ret)]
+fn create_local_devnull_file(remote_fd: u64) -> Detour<RawFd> {
+    let file_c_string = CString::new("/dev/null")?;
+    let file_path_ptr = file_c_string.as_ptr();
+    let local_file_fd: RawFd = unsafe { FN_OPEN(file_path_ptr, O_RDONLY) };
+    if local_file_fd == -1 {
+        let error = errno::errno();
+        // Close the remote file if creating a tmp local file failed and we have an invalid local fd
+        close_remote_file_on_failure(remote_fd)?;
+        Detour::Error(HookError::LocalFileCreation(remote_fd, error.0))
+    } else {
+        Detour::Success(local_file_fd)
+    }
+}
+
 /// Close the remote file if the call to [`libc::shm_open`] failed and we have an invalid local fd.
 #[mirrord_layer_macro::instrument(level = "trace", ret)]
 fn close_remote_file_on_failure(fd: u64) -> Result<()> {