Swatantra-66 opened a new pull request, #20820:
URL: https://github.com/apache/nuttx/pull/20820
## Summary
Ref: #20414
Adds `CONFIG_FS_DIRENT_LITE`, a new Kconfig option for flat-build
configurations that provides a lightweight direct-pointer directory stream API,
eliminating file descriptor allocation and `file` proxy structure overhead for
directory streams.
### Architecture & Motivation
In standard configurations, `opendir()` allocates a file descriptor and
binds a `struct file` proxy to `g_dir_inode`. On small embedded
microcontrollers with constrained memory or descriptor tables, this imposes
noticeable overhead:
1. Allocating an entry in the file descriptor table.
2. Holding descriptor slots that count against process limits.
3. Multiple layers of indirect traversal through the VFS descriptor table on
every directory operation.
With `CONFIG_FS_DIRENT_LITE`:
- Directory streams (`DIR`) store a lightweight `FAR struct fs_dirent_s *`
pointer directly.
- Operations dispatch to dedicated kernel routines: `nx_opendir()`,
`nx_readdir()`, `nx_closedir()`, `nx_rewinddir()`, `nx_seekdir()`, and
`nx_telldir()`.
- Standard POSIX wrappers (`opendir`, `readdir`, `closedir`, `rewinddir`,
`seekdir`, `telldir`, `dirfd`, `fdopendir`) transparently support the
lightweight mode.
### Key Implementation Details & CodeRabbit Fixes Addressed
- **Deadlock prevention in `nx_rewinddir`:** Inode references are taken
prior to unlocking, and previous inode release (`inode_release`) occurs outside
`inode_lock()` to prevent recursion deadlocks during inode destruction.
- **Memory safety on mountpoints:** Eliminated duplicate `fs_heap_free(dir)`
in `nx_closedir()`, properly delegating lifecycle management to the
filesystem's `closedir` driver hook.
- **Use-after-free & dangling pointer fixes:** In `nx_closedir()`, `fd_path`
is cleared before filesystem close and freed after close notifications.
- **Path correctness for `fs_notify`:** Absolute paths are resolved via
`inode_getpath()` during `nx_opendir()` so notifications remain valid across
working directory changes (`chdir`).
- **Signal safety in inotify:** `notify_read2()` checks
`TCB_FLAG_SIGNAL_ACTION` and masks with `notify_check_mask(IN_ACCESS)` prior to
taking `g_inotify.lock`, avoiding self-deadlock when `readdir()` is invoked
from signal handlers.
- **Input validation & POSIX error contracts:** `nx_opendir()` rejects
`NULL` path or dir pointers with `-EINVAL` prior to search setup.
- **Hard-link resolution:** Under `CONFIG_FS_LINKS`, hard links pointing to
pseudo-directories are dereferenced (`INODE_IS_HARDLINK`) so child entries of
target directories are enumerated correctly.
- **Kconfig documentation:** Clear documentation in `fs/Kconfig` that
`seekdir`/`telldir` set `errno = ENOSYS`, `open(..., O_DIRECTORY)` returns
`-ENOTSUP`, directory descriptors are not reclaimed automatically on task exit
without `closedir()`, and descriptor-based APIs (`dirfd`, `fdopendir`) return
`-ENOTSUP`.
---
## Impact
- **Is new feature added?** Yes (`CONFIG_FS_DIRENT_LITE`).
- **User Impact:** None when disabled (default `n`). When enabled on flat
builds, reduces memory footprint and descriptor usage for directory operations.
- **Build Process:** No impact. Compatible with both CMake/Ninja and
Makefile build systems.
- **Hardware:** Targets resource-constrained MCUs in flat builds
(`BUILD_FLAT`).
- **Compatibility:**
- Standard directory iteration (`opendir`, `readdir`, `closedir`,
`rewinddir`) works seamlessly.
- Descriptor-specific APIs (`dirfd()`, `fdopendir()`, `open(O_DIRECTORY)`)
return `-ENOTSUP`.
- `seekdir()` and `telldir()` set `errno = ENOSYS`.
- Directory streams must be explicitly closed with `closedir()` (not
auto-reclaimed on task termination).
---
## Testing
### Host Environment
- **Host OS:** Linux / WSL2 (Ubuntu, x86_64)
- **Compiler:** GCC 15.2.0 (`/usr/bin/gcc`)
- **Build System:** CMake 4.2.3 + Ninja 1.13.2
### Static Analysis & Coding Standards
- Verified all 17 modified C and header files with official Apache NuttX
coding standard linter (`nxstyle`):
```bash
tools/nxstyle fs/vfs/fs_dir.c
tools/nxstyle libs/libc/dirent/*.c
# Result: 0 errors, 0 warnings
```
### Build Verification Logs
1. **Standard Build (`sim:nsh`, non-lite default)**:
```text
cmake -B /var/tmp/nuttx-build -DBOARD_CONFIG=sim:nsh -GNinja
/path/to/nuttx
ninja -C /var/tmp/nuttx-build nuttx
[1260/1260] Linking C executable nuttx
Result: SUCCESS (0 errors, 0 warnings)
```
2. **Lite Build (`sim:nsh` with `CONFIG_FS_DIRENT_LITE=y`)**:
```text
sed -i 's/# CONFIG_FS_DIRENT_LITE is not set/CONFIG_FS_DIRENT_LITE=y/'
.config
ninja -C /var/tmp/nuttx-build nuttx
[1250/1250] Linking C executable nuttx
Result: SUCCESS (0 errors, 0 warnings)
```
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