fuse: hold i_mutex in fuse_file_fallocate()

Changing size of a file on server and local update (fuse_write_update_size) should be always protected by inode->i_mutex. Otherwise a race like this is possible:

1. Process 'A' calls fallocate(2) to extend file (~FALLOC_FL_KEEP_SIZE). fuse_file_fallocate() sends FUSE_FALLOCATE request to the server.
2. Process 'B' performs ordinary buffered write(2) with a length big enough to extend the file beyond "offset + length" of fallocate call.
3. Process 'A' resumes execution of fuse_file_fallocate() and calls fuse_write_update_size(inode, offset + length). But 'offset + length' was obsoleted by write from previous step.

Signed-off-by Maxim V. Patlasov <[email protected]>

---
 fs/fuse/file.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

fs/fuse/file.c +6 -4

index e570081..8dfbf7d 100644
@@ -2470,14 +2470,16 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 		.mode = mode
 	};
 	int err;
+	bool lock_inode = !(mode & FALLOC_FL_KEEP_SIZE) ||
+			   (mode & FALLOC_FL_PUNCH_HOLE);
 
 	if (fc->no_fallocate)
 		return -EOPNOTSUPP;
 
-	if (mode & FALLOC_FL_PUNCH_HOLE) {
+	if (lock_inode)
 		mutex_lock(&inode->i_mutex);
+	if (mode & FALLOC_FL_PUNCH_HOLE)
 		fuse_set_nowrite(inode);
-	}
 
 	req = fuse_get_req_nopages(fc);
 	if (IS_ERR(req)) {

@@ -2511,10 +2513,10 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	fuse_invalidate_attr(inode);
 
 out:
-	if (mode & FALLOC_FL_PUNCH_HOLE) {
+	if (mode & FALLOC_FL_PUNCH_HOLE)
 		fuse_release_nowrite(inode);
+	if (lock_inode)
 		mutex_unlock(&inode->i_mutex);
-	}
 
 	return err;
 }

Quoted 4 lines · Maxim Patlasov

Changing size of a file on server and local update (fuse_write_update_size) should be always protected by inode->i_mutex. Otherwise a race like this is possible:

Quoted 8 lines

1. Process 'A' calls fallocate(2) to extend file (~FALLOC_FL_KEEP_SIZE). fuse_file_fallocate() sends FUSE_FALLOCATE request to the server.
2. Process 'B' performs ordinary buffered write(2) with a length big enough to extend the file beyond "offset + length" of fallocate call.
3. Process 'A' resumes execution of fuse_file_fallocate() and calls fuse_write_update_size(inode, offset + length). But 'offset + length' was obsoleted by write from previous step.

Hi Maxim,

Doesn't fuse_write_update_size() already handle this particular case by only ever extending the size?

fs/fuse/file.c static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,

Quoted 8 lines

fs/fuse/file.c +2 -2

-    if (mode & FALLOC_FL_PUNCH_HOLE) {
+    if (lock_inode)
          mutex_lock(&inode->i_mutex);
 
+    if (mode & FALLOC_FL_PUNCH_HOLE)
          fuse_set_nowrite(inode);
-    }

Just for clarity, can you make the condition to check FALLOC_FL_PUNCH_HOLE and call to fuse_set_nowrite() nested within the larger if (lock_inode) { .. } block? fuse_set_nowrite() should not be called without i_mutex acquired. The current style of calling mutex_lock() and fuse_set_nowrite() in separate conditions can potentially cause bugs in the future if they were to get re-ordered due to some unrelated patch. Nesting them makes the relation more explicit and clear.