Fix fd_readdir to properly truncate directory entry names.

Previously, `fd_readdir` was truncating directory entry names based on the
calculation of `min(name_len, buf_len - bufused)`, but `bufused` was not being
updated after writing in the `dirent` structure to the buffer.

This allowed `bufused` to be incremented beyond `buf_len` and returned as the
number of bytes written to the buffer, which is invalid.

This fix adjusts `bufused` when the buffer is written to for the `dirent` so
that name truncation happens as expected.

Fixes #2618.

crates/wasi-common/src/snapshots/wasi_snapshot_preview1.rs

@@ -304,7 +304,6 @@ impl<'a> WasiSnapshotPreview1 for WasiCtx {
             let dirent_len: types::Size = dirent_raw.len().try_into()?;
             let name_raw = name.as_bytes();
             let name_len = name_raw.len().try_into()?;
-            let offset = dirent_len.checked_add(name_len).ok_or(Error::Overflow)?;
 
             // Copy as many bytes of the dirent as we can, up to the end of the buffer.
             let dirent_copy_len = min(dirent_len, buf_len - bufused);
@@ -318,6 +317,7 @@ impl<'a> WasiSnapshotPreview1 for WasiCtx {
             }
 
             buf = buf.add(dirent_copy_len)?;
+            bufused = bufused.checked_add(dirent_copy_len).ok_or(Error::Overflow)?;
 
             // Copy as many bytes of the name as we can, up to the end of the buffer.
             let name_copy_len = min(name_len, buf_len - bufused);
@@ -331,8 +331,7 @@ impl<'a> WasiSnapshotPreview1 for WasiCtx {
             }
 
             buf = buf.add(name_copy_len)?;
-
-            bufused += offset;
+            bufused = bufused.checked_add(name_copy_len).ok_or(Error::Overflow)?;
         }
 
         Ok(bufused)