Re: [PATCH] Add MS_BIND_FLAGS mount flag

["Paul Menage" <menage@google.com>]

On Thu, Feb 14, 2008 at 12:30 AM, Miklos Szeredi <miklos@szeredi.hu> wrote:
>  > For recursive bind mounts, only the root of the tree being bound
>  > inherits the per-mount flags from the mount() arguments; sub-mounts
>  > inherit their per-mount flags from the source tree as usual.
>
>  This is rather strange behavior.  I think it would be much better, if
>  setting mount flags would work for recursive operations as well.  Also
>  what we really need is not resetting all the mount flags to some
>  predetermined values, but to be able to set or clear each flag
>  individually.

This is certainly true, but as you observe below it's a fair bit more
fiddly to specify in the API. I wasn't sure how much people recursive
bind mounts, so I figured I'd throw out this simpler version first.

>
>  For example, with the per-mount-read-only thing the most useful
>  application would be to just set the read-only flag and leave the
>  others alone.
>
>  And this is where we usually conclude, that a new userspace mount API
>  is long overdue.  So for starters, how about a new syscall for bind
>  mounts:
>
>  int mount_bind(const char *src, const char *dst, unsigned flags,
>                  unsigned mnt_flags);

The "flags" argument could be the same as for regular mount, and
contain the mnt_flags - so the extra argument could maybe usefully be
a "mnt_flags_mask", to indicate which flags we actually care about
overriding.

What would happen when an existing super-block flag changes to become
a per-mount flag (e.g. per-mount read-only)? I think that would just
fit in with the "mask" idea, as long as we complained if any bits in
mnt_flags_mask weren't actually per-mount settable.

Being able to mask/set mount flags might be useful on a remount too,
since there's no clean way to get the existing mount flags for a mount
other than by scanning /proc/mounts. So an alternative to a separate
system call would be a new mnt_flag_mask argument to mount() (whose
presence would be indicated by a flag bit being set in the main flags)
which would be used to control which bits were set cleared for
remount/bind calls. Seems a bit wasteful of bits though. If we turned
"flags" into an (optionally) 64-bit argument then we'd have plenty of
bits to be able to specify both a "set" bit and a "mask" bit for each,
without needing a new syscall.

Paul