libmount: ignore "bind" from fstab on command line "remount"

The current code always apply all flags from /etc/fstab on remount.
Unfortunately remount+bind has special semantic and it's impossible
from command line to avoid interaction with the "bind" from fstab.

Example, fstab:

	/dev/sda1 /bar ext4 defaults 0 1
	/bar /foo none bind 0 0

Command line:

    # mount /foo -o remount,rw

produces:

    mount(... MS_REMOUNT|MS_BIND ) syscall

This changes the per-mountpoint (VFS) ro flag to rw, but doesn't
change the filesystem itself.

This patch forces libmount to ignore "bind" from fstab when "-o
remount" specified on command line. If you need remount+bind semantic
you have to specify the "bind" flag on command line. This allow to
differentiate between

    # mount /foo -o remount,bind,rw  --> mount(MS_REMOUNT|MS_BIND)
and

    # mount /foo -o remount,rw       --> mount(MS_REMOUNT)

Suggested-by: NeilBrown <neilb@suse.com>
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/sys-utils/mount.8 b/sys-utils/mount.8
index ac410640e..3bd278e6a 100644
--- a/sys-utils/mount.8
+++ b/sys-utils/mount.8
@@ -385,6 +385,20 @@ will be read-only.
 It's also possible to change nosuid, nodev, noexec, noatime, nodiratime and
 relatime VFS entry flags by "remount,bind" operation. It's impossible to change
 mount options recursively (for example with \fB-o rbind,ro\fR).
+
+.BR mount (8)
+since v2.31 ignores the \fBbind\fR flag from
+.I /etc/fstab
+on
+.B remount operation
+(if "-o remount" specified on command line). This is necessary to fully control
+mount options on remount by command line. In the previous versions the bind
+flag has been always applied and it was impossible to re-define mount options
+without interaction with the bind semantic. This
+.BR mount (8)
+behavior does not affect situations when "remount,bind" is specified in the
+.I /etc/fstab
+file.
 .RE
 
 .SS The move operation
@@ -1088,6 +1102,10 @@ Attempt to remount an already-mounted filesystem.  This is commonly
 used to change the mount flags for a filesystem, especially to make a
 readonly filesystem writable.  It does not change device or mount point.
 
+The remount operation together with the
+.B bind
+flag has special semantic. See above, the subsection \fBBind mounts\fR.
+
 The remount functionality follows the standard way the mount command works
 with options from fstab.  This means that \fBmount\fR does not
 read fstab (or mtab) only when both