linux-kernel - Re: [PATCH] Add MS_BIND

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Date:	Thu, 14 Feb 2008 08:13:09 -0800
From:	"Paul Menage" <menage@...gle.com>
To:	"Miklos Szeredi" <miklos@...redi.hu>
Cc:	viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] Add MS_BIND_FLAGS mount flag

On Thu, Feb 14, 2008 at 8:03 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
>  > 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.
>
>  The way I imagined it, is that mnt_flags is a mask, and the operation
>  (determined by flags) is either:
>
>   - set bits in mask
>   - clear bits in mask (or not in mask)
>   - set flags to mask
>
>  It doesn't allow setting some bits, clearing some others, and leaving
>  alone the rest.  But I think such flexibility isn't really needed.

I think I'd suggest something like:

new_mnt->mnt_flags = (old_mnt->mnt_flags & ~arg_mask) | (arg_flags & mask)

>  Maybe instead of messing with masks, it's better to introduce a
>  get_flags() or a more general mount_stat() operation, and let
>  userspace deal with setting and clearing flags, just as we do for
>  stat/chmod?
>
>  So we'd have
>
>   mount_stat(path, stat);
>   mount_bind(from, to, flags);
>   mount_set_flags(path, flags);
>   mount_move(from, to);
>
>  and perhaps
>
>   mount_remount(path, opt_string, flags);

Sounds reasonable to me. But it wouldn't directly solve the "do a
recursive bind mount setting the MS_READONLY flag on all children"
problem, so we'd need some of the earlier suggestions too.

Paul
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