proc/fd: In proc_readfd_common use task_lookup_next_fd_rcu

When discussing[1] exec and posix file locks it was realized that none
of the callers of get_files_struct fundamentally needed to call
get_files_struct, and that by switching them to helper functions
instead it will both simplify their code and remove unnecessary
increments of files_struct.count.  Those unnecessary increments can
result in exec unnecessarily unsharing files_struct which breaking
posix locks, and it can result in fget_light having to fallback to
fget reducing system performance.

Using task_lookup_next_fd_rcu simplifies proc_readfd_common, by moving
the checking for the maximum file descritor into the generic code, and
by remvoing the need for capturing and releasing a reference on
files_struct.

As task_lookup_fd_rcu may update the fd ctx->pos has been changed
to be the fd +2 after task_lookup_fd_rcu returns.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.comSuggested-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Andy Lavr <andy.lavr@gmail.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-10-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-15-ebiederm@xmission.comSigned-off-by: Eric W. Biederman <ebiederm@xmission.com>

fs/proc/fd.c

@@ -217,7 +217,6 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
 			      instantiate_t instantiate)
 {
 	struct task_struct *p = get_proc_task(file_inode(file));
-	struct files_struct *files;
 	unsigned int fd;
 
 	if (!p)
@@ -225,22 +224,18 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
 
 	if (!dir_emit_dots(file, ctx))
 		goto out;
-	files = get_files_struct(p);
-	if (!files)
-		goto out;
 
 	rcu_read_lock();
-	for (fd = ctx->pos - 2;
-	     fd < files_fdtable(files)->max_fds;
-	     fd++, ctx->pos++) {
+	for (fd = ctx->pos - 2;; fd++) {
 		struct file *f;
 		struct fd_data data;
 		char name[10 + 1];
 		unsigned int len;
 
-		f = files_lookup_fd_rcu(files, fd);
+		f = task_lookup_next_fd_rcu(p, &fd);
+		ctx->pos = fd + 2LL;
 		if (!f)
-			continue;
+			break;
 		data.mode = f->f_mode;
 		rcu_read_unlock();
 		data.fd = fd;
@@ -249,13 +244,11 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx,
 		if (!proc_fill_cache(file, ctx,
 				     name, len, instantiate, p,
 				     &data))
-			goto out_fd_loop;
+			goto out;
 		cond_resched();
 		rcu_read_lock();
 	}
 	rcu_read_unlock();
-out_fd_loop:
-	put_files_struct(files);
 out:
 	put_task_struct(p);
 	return 0;