refactor: improve ephemeral ux

src-tauri/src/fs_secure.rs

@@ -0,0 +1,213 @@
+//! Best-effort secure deletion.
+//!
+//! "Best-effort" is load-bearing and is not a hedge. On copy-on-write
+//! filesystems (APFS, Btrfs, ZFS, ReFS) and on any SSD with wear levelling, the
+//! blocks holding the old contents may survive an overwrite entirely, because
+//! the write lands somewhere else. RAM-backed storage can also be paged out,
+//! and zeroing a file cannot reach the swap slot that held it. Treat these
+//! helpers as raising the cost of recovery, never as a guarantee of erasure.
+//!
+//! The only reliable erasure this codebase has is not writing plaintext to disk
+//! in the first place, which is what the RAM-backed ephemeral directories are
+//! for. These helpers exist for the paths where that failed.
+
+use std::fs;
+use std::io::Write;
+use std::path::Path;
+
+/// Zero a file's bytes and flush before unlinking, so the contents are not
+/// trivially recoverable from the freed blocks.
+///
+/// The overwrite must never gate the unlink. A write that fails part-way
+/// (ENOSPC on a copy-on-write volume, EIO) would otherwise leave the file both
+/// un-wiped and un-deleted, which is strictly worse than the plain remove this
+/// replaces, because callers report success either way and the data would
+/// silently survive.
+pub fn secure_remove_file(path: &Path) -> std::io::Result<()> {
+  if let Ok(meta) = fs::metadata(path) {
+    let len = meta.len();
+    if len > 0 {
+      if let Ok(mut f) = fs::OpenOptions::new().write(true).open(path) {
+        let zeros = vec![0u8; 64 * 1024];
+        let mut remaining = len;
+        while remaining > 0 {
+          let chunk = remaining.min(zeros.len() as u64) as usize;
+          if f.write_all(&zeros[..chunk]).is_err() {
+            break;
+          }
+          remaining -= chunk as u64;
+        }
+        // One flush per file, not per chunk: syncing every 64 KiB turns a
+        // profile teardown into thousands of barriers for no extra safety.
+        let _ = f.flush();
+        let _ = f.sync_all();
+      }
+    }
+  }
+  fs::remove_file(path)
+}
+
+/// Whether zeroing this file would even mean anything.
+///
+/// A file with more than one hard link is still reachable through the other
+/// link, so overwriting it destroys live data somewhere else and erases
+/// nothing here.
+#[cfg(unix)]
+fn is_last_link(meta: &fs::Metadata) -> bool {
+  use std::os::unix::fs::MetadataExt;
+  meta.nlink() <= 1
+}
+
+#[cfg(not(unix))]
+fn is_last_link(_meta: &fs::Metadata) -> bool {
+  true
+}
+
+/// Recursively delete a directory, optionally zeroing regular files first.
+///
+/// `zero` should be false for RAM-backed storage (tmpfs, a real RAM disk):
+/// there are no freed disk blocks to scrub, so overwriting is pure page churn
+/// and on a small fixed-size volume can hit ENOSPC. Pass true only when the
+/// tree is genuinely on disk.
+///
+/// Symlinks are unlinked, never followed and never zeroed: following one would
+/// destroy a target outside the tree.
+///
+/// Returns the number of files removed. The tree is removed even when
+/// individual steps fail, because leaving a half-wiped directory in place is
+/// the worst outcome available.
+pub fn secure_remove_dir_all(root: &Path, zero: bool) -> std::io::Result<u64> {
+  let mut removed = 0u64;
+  if !root.exists() {
+    return Ok(0);
+  }
+
+  remove_tree(root, zero, &mut removed);
+
+  // Unconditional backstop: a walk that failed part-way must still not leave
+  // the directory behind.
+  match fs::remove_dir_all(root) {
+    Ok(()) => Ok(removed),
+    Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(removed),
+    Err(e) => Err(e),
+  }
+}
+
+fn remove_tree(dir: &Path, zero: bool, removed: &mut u64) {
+  let entries = match fs::read_dir(dir) {
+    Ok(entries) => entries,
+    Err(e) => {
+      log::warn!("Secure erase could not read {}: {e}", dir.display());
+      return;
+    }
+  };
+
+  for entry in entries.flatten() {
+    let path = entry.path();
+    // file_type() on the DirEntry is lstat-based, so a symlink reports as a
+    // symlink rather than as whatever it points at.
+    let file_type = match entry.file_type() {
+      Ok(ft) => ft,
+      Err(_) => continue,
+    };
+
+    if file_type.is_symlink() {
+      let _ = fs::remove_file(&path);
+      *removed += 1;
+    } else if file_type.is_dir() {
+      remove_tree(&path, zero, removed);
+      let _ = fs::remove_dir(&path);
+    } else {
+      let should_zero = zero
+        && fs::symlink_metadata(&path)
+          .map(|m| is_last_link(&m))
+          .unwrap_or(false);
+      let outcome = if should_zero {
+        secure_remove_file(&path)
+      } else {
+        fs::remove_file(&path)
+      };
+      if outcome.is_ok() {
+        *removed += 1;
+      }
+    }
+  }
+}
+
+#[cfg(test)]
+mod tests {
+  use super::*;
+
+  #[test]
+  fn zeroing_erase_removes_a_nested_tree() {
+    let tmp = tempfile::tempdir().unwrap();
+    let nested = tmp.path().join("Default/Network");
+    fs::create_dir_all(&nested).unwrap();
+    let cookies = nested.join("Cookies");
+    fs::write(&cookies, b"session=supersecretvalue").unwrap();
+
+    let removed = secure_remove_dir_all(tmp.path(), true).unwrap();
+    assert!(
+      removed >= 1,
+      "expected at least the cookie file to be counted"
+    );
+    assert!(!tmp.path().exists());
+  }
+
+  #[test]
+  fn erase_without_zeroing_still_removes_everything() {
+    let tmp = tempfile::tempdir().unwrap();
+    fs::write(tmp.path().join("a"), b"x").unwrap();
+    fs::create_dir_all(tmp.path().join("d")).unwrap();
+    fs::write(tmp.path().join("d/b"), b"y").unwrap();
+
+    secure_remove_dir_all(tmp.path(), false).unwrap();
+    assert!(!tmp.path().exists());
+  }
+
+  #[test]
+  fn missing_root_is_not_an_error() {
+    let tmp = tempfile::tempdir().unwrap();
+    let absent = tmp.path().join("never-existed");
+    assert_eq!(secure_remove_dir_all(&absent, true).unwrap(), 0);
+  }
+
+  #[cfg(unix)]
+  #[test]
+  fn a_symlink_is_unlinked_without_touching_its_target() {
+    use std::os::unix::fs::symlink;
+
+    // The target lives OUTSIDE the tree being erased. Following the link would
+    // destroy a user's real file, which is the failure this guards against.
+    let outside = tempfile::tempdir().unwrap();
+    let target = outside.path().join("precious");
+    fs::write(&target, b"must survive intact").unwrap();
+
+    let tmp = tempfile::tempdir().unwrap();
+    symlink(&target, tmp.path().join("link")).unwrap();
+
+    secure_remove_dir_all(tmp.path(), true).unwrap();
+
+    assert!(!tmp.path().exists());
+    assert_eq!(fs::read(&target).unwrap(), b"must survive intact");
+  }
+
+  #[cfg(unix)]
+  #[test]
+  fn a_second_hard_link_is_not_zeroed_through() {
+    let tmp = tempfile::tempdir().unwrap();
+    let inside = tmp.path().join("shared");
+    fs::write(&inside, b"still referenced elsewhere").unwrap();
+
+    let outside = tempfile::tempdir().unwrap();
+    let other = outside.path().join("other-name");
+    fs::hard_link(&inside, &other).unwrap();
+
+    secure_remove_dir_all(tmp.path(), true).unwrap();
+
+    // The link inside the tree is gone, and the surviving link still holds the
+    // original bytes rather than a run of zeros.
+    assert!(!tmp.path().exists());
+    assert_eq!(fs::read(&other).unwrap(), b"still referenced elsewhere");
+  }
+}