autofs: add ignore mount option

Add an autofs file system mount option that can be used to provide a
generic indicator to applications that the mount entry should be ignored
when displaying mount information.

In other OSes that provide autofs and that provide a mount list to user
space based on the kernel mount list a no-op mount option ("ignore" is
the one use on the most common OS) is allowed so that autofs file system
users can optionally use it.

The idea is that it be used by user space programs to exclude autofs
mounts from consideration when reading the mounts list.

Prior to the change to link /etc/mtab to /proc/self/mounts all I needed
to do to achieve this was to use mount(2) and not update the mtab but
now that no longer works.

I know the symlinking happened a long time ago and I considered doing
this then but, at the time I couldn't remember the commonly used option
name and thought persuading the various utility maintainers would be too
hard.

But now I have a RHEL request to do this for compatibility for a widely
used product so I want to go ahead with it and try and enlist the help
of some utility package maintainers.

Clearly, without the option nothing can be done so it's at least a
start.

Link: http://lkml.kernel.org/r/154725123970.11260.6113771566924907275.stgit@pluto-themaw-net
Signed-off-by: Ian Kent <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/autofs/autofs_i.h

···
 
 #define AUTOFS_SBI_CATATONIC	0x0001
 #define AUTOFS_SBI_STRICTEXPIRE 0x0002
+#define AUTOFS_SBI_IGNORE	0x0004
 
 struct autofs_sb_info {
 	u32 magic;

fs/autofs/inode.c

···
 		seq_printf(m, ",indirect");
 	if (sbi->flags & AUTOFS_SBI_STRICTEXPIRE)
 		seq_printf(m, ",strictexpire");
+	if (sbi->flags & AUTOFS_SBI_IGNORE)
+		seq_printf(m, ",ignore");
 #ifdef CONFIG_CHECKPOINT_RESTORE
 	if (sbi->pipe)
 		seq_printf(m, ",pipe_ino=%ld", file_inode(sbi->pipe)->i_ino);
···
 };
 
 enum {Opt_err, Opt_fd, Opt_uid, Opt_gid, Opt_pgrp, Opt_minproto, Opt_maxproto,
-	Opt_indirect, Opt_direct, Opt_offset, Opt_strictexpire};
+	Opt_indirect, Opt_direct, Opt_offset, Opt_strictexpire,
+	Opt_ignore};
 
 static const match_table_t tokens = {
 	{Opt_fd, "fd=%u"},
···
 	{Opt_direct, "direct"},
 	{Opt_offset, "offset"},
 	{Opt_strictexpire, "strictexpire"},
+	{Opt_ignore, "ignore"},
 	{Opt_err, NULL}
 };
 
···
 			break;
 		case Opt_strictexpire:
 			sbi->flags |= AUTOFS_SBI_STRICTEXPIRE;
+			break;
+		case Opt_ignore:
+			sbi->flags |= AUTOFS_SBI_IGNORE;
 			break;
 		default:
 			return 1;

include/uapi/linux/auto_fs.h

···
 #define AUTOFS_MIN_PROTO_VERSION	3
 #define AUTOFS_MAX_PROTO_VERSION	5
 
-#define AUTOFS_PROTO_SUBVERSION		4
+#define AUTOFS_PROTO_SUBVERSION		5
 
 /*
  * The wait_queue_token (autofs_wqt_t) is part of a structure which is passed