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' calls ftruncate(2) shrinking the file. fuse_do_setattr()
sends shrinking FUSE_SETATTR request to the server and updates local i_size
by i_size_write(inode, outarg.attr.size).
3. Process 'A' resumes execution of fuse_file_fallocate() and calls
fuse_write_update_size(inode, offset + length). But 'offset + length' was
obsoleted by ftruncate from previous step.

Changed in v2 (thanks Brian and Anand for suggestions):
 - made relation between mutex_lock() and fuse_set_nowrite(inode) more
   explicit and clear.
 - updated patch description to use ftruncate(2) in example

Signed-off-by: Maxim V. Patlasov <MPatlasov@parallels.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

1 files changed, 8 insertions, 4 deletions

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index e570081f9f76..35f281033142 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2470,13 +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);
-                fuse_set_nowrite(inode);
+                if (mode & FALLOC_FL_PUNCH_HOLE)
+                        fuse_set_nowrite(inode);
         }
 
         req = fuse_get_req_nopages(fc);
@@ -2511,8 +2514,9 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
         fuse_invalidate_attr(inode);
 
 out:
-        if (mode & FALLOC_FL_PUNCH_HOLE) {
-                fuse_release_nowrite(inode);
+        if (lock_inode) {
+                if (mode & FALLOC_FL_PUNCH_HOLE)
+                        fuse_release_nowrite(inode);
                 mutex_unlock(&inode->i_mutex);
         }