file: Factor files_lookup_fd_locked out of fcheck_files

To make it easy to tell where files->file_lock protection is being
used when looking up a file create files_lookup_fd_locked.  Only allow
this function to be called with the file_lock held.

Update the callers of fcheck and fcheck_files that are called with the
files->file_lock held to call files_lookup_fd_locked instead.

Hopefully this makes it easier to quickly understand what is going on.

The need for better names became apparent in the last round of
discussion of this set of changes[1].

[1] https://lkml.kernel.org/r/CAHk-=wj8BQbgJFLa+J0e=iT-1qpmCRTbPAJ8gd6MJQ=kbRPqyQ@mail.gmail.com
Link: https://lkml.kernel.org/r/20201120231441.29911-8-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

fs/file.c

@@ -1098,7 +1098,7 @@ static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 
 	spin_lock(&files->file_lock);
 	err = expand_files(files, newfd);
-	file = fcheck(oldfd);
+	file = files_lookup_fd_locked(files, oldfd);
 	if (unlikely(!file))
 		goto Ebadf;
 	if (unlikely(err < 0)) {

fs/locks.c

@@ -2539,14 +2539,15 @@ int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
 	 */
 	if (!error && file_lock->fl_type != F_UNLCK &&
 	    !(file_lock->fl_flags & FL_OFDLCK)) {
+		struct files_struct *files = current->files;
 		/*
 		 * We need that spin_lock here - it prevents reordering between
 		 * update of i_flctx->flc_posix and check for it done in
 		 * close(). rcu_read_lock() wouldn't do.
 		 */
-		spin_lock(&current->files->file_lock);
-		f = fcheck(fd);
-		spin_unlock(&current->files->file_lock);
+		spin_lock(&files->file_lock);
+		f = files_lookup_fd_locked(files, fd);
+		spin_unlock(&files->file_lock);
 		if (f != filp) {
 			file_lock->fl_type = F_UNLCK;
 			error = do_lock_file_wait(filp, cmd, file_lock);
@@ -2670,14 +2671,15 @@ int fcntl_setlk64(unsigned int fd, struct file *filp, unsigned int cmd,
 	 */
 	if (!error && file_lock->fl_type != F_UNLCK &&
 	    !(file_lock->fl_flags & FL_OFDLCK)) {
+		struct files_struct *files = current->files;
 		/*
 		 * We need that spin_lock here - it prevents reordering between
 		 * update of i_flctx->flc_posix and check for it done in
 		 * close(). rcu_read_lock() wouldn't do.
 		 */
-		spin_lock(&current->files->file_lock);
-		f = fcheck(fd);
-		spin_unlock(&current->files->file_lock);
+		spin_lock(&files->file_lock);
+		f = files_lookup_fd_locked(files, fd);
+		spin_unlock(&files->file_lock);
 		if (f != filp) {
 			file_lock->fl_type = F_UNLCK;
 			error = do_lock_file_wait(filp, cmd, file_lock);

fs/proc/fd.c

@@ -35,7 +35,7 @@ static int seq_show(struct seq_file *m, void *v)
 		unsigned int fd = proc_fd(m->private);
 
 		spin_lock(&files->file_lock);
-		file = fcheck_files(files, fd);
+		file = files_lookup_fd_locked(files, fd);
 		if (file) {
 			struct fdtable *fdt = files_fdtable(files);
 

include/linux/fdtable.h

@@ -91,6 +91,13 @@ static inline struct file *files_lookup_fd_raw(struct files_struct *files, unsig
 	return NULL;
 }
 
+static inline struct file *files_lookup_fd_locked(struct files_struct *files, unsigned int fd)
+{
+	RCU_LOCKDEP_WARN(!lockdep_is_held(&files->file_lock),
+			   "suspicious rcu_dereference_check() usage");
+	return files_lookup_fd_raw(files, fd);
+}
+
 static inline struct file *fcheck_files(struct files_struct *files, unsigned int fd)
 {
 	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&