[PATCH] Make file struct available to fchmod FS handlers.

[Jose Lopes <jabolopes@gmail.com>]

Syscall 'ftruncate' makes the 'file' struct available to filesystem
handlers. This makes it possible, e.g., for filesystems, such as,
FUSE, to access the file handle associated with the file descriptor
that was passed to 'ftruncate'. In the specific case of FUSE, this
also makes it possible for (userspace) FUSE-based filesystems to
distinguish between calls to 'truncate' and 'ftruncate'.

>From an implementation point of view, this is possible because the
'ftruncate' syscall passes the 'file' struct to the underlying
filesystem handlers via the 'ia_file' field and the 'ia_valid' field
mask.

Similarly to 'ftruncate', pass the 'file' struct to 'fchmod', which
allows filesystem handlers to get the file handle associated with the
file descriptor passed to 'fchmod' and allows FUSE-based filesystems
to distinguish between calls to 'chmod' and 'fchmod'.

In a future patch, make a similar change to the 'fchown' and
'futimens' syscalls.

Signed-off-by: Jose Lopes <jabolopes@gmail.com>
---
 fs/open.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index d3ed817..00214c5 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -516,7 +516,8 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
 	return error;
 }
 
-static int chmod_common(const struct path *path, umode_t mode)
+static int chmod_common(const struct path *path, umode_t mode,
+	struct file *filp)
 {
 	struct inode *inode = path->dentry->d_inode;
 	struct inode *delegated_inode = NULL;
@@ -533,6 +534,10 @@ static int chmod_common(const struct path *path, umode_t mode)
 		goto out_unlock;
 	newattrs.ia_mode = (mode & S_IALLUGO) | (inode->i_mode & ~S_IALLUGO);
 	newattrs.ia_valid = ATTR_MODE | ATTR_CTIME;
+	if (filp) {
+		newattrs.ia_file = filp;
+		newattrs.ia_valid |= ATTR_FILE;
+	}
 	error = notify_change(path->dentry, &newattrs, &delegated_inode);
 out_unlock:
 	inode_unlock(inode);
@@ -552,7 +557,7 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
 
 	if (f.file) {
 		audit_file(f.file);
-		err = chmod_common(&f.file->f_path, mode);
+		err = chmod_common(&f.file->f_path, mode, f.file);
 		fdput(f);
 	}
 	return err;
@@ -566,7 +571,7 @@ SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, umode_t, mode
 retry:
 	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (!error) {
-		error = chmod_common(&path, mode);
+		error = chmod_common(&path, mode, NULL);
 		path_put(&path);
 		if (retry_estale(error, lookup_flags)) {
 			lookup_flags |= LOOKUP_REVAL;
-- 
2.9.0