linux-kernel - Re: [PATCH 0/6 resend] statfs: handle mount propagation

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Message-ID: <20180503134317.GA30522@ZenIV.linux.org.uk>
Date:   Thu, 3 May 2018 14:43:17 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christian Brauner <christian.brauner@...onical.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        hch@...radead.org, tglx@...utronix.de,
        kstewart@...uxfoundation.org, Greg KH <gregkh@...uxfoundation.org>,
        pombredanne@...b.com, Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 0/6 resend] statfs: handle mount propagation

On Thu, May 03, 2018 at 03:04:36PM +0200, Christian Brauner wrote:

> >From a userspace perspective we often run into the case where we simply
> want to know whether a given mountpoint is MS_SHARED or is MS_SLAVE.
> If it is we remount it as MS_PRIVATE to prevent any propagation from
> happening. We don't care about the peer relationship or how the
> propagation is exactly setup. We only want to prevent any propagation
> from happening.

So what's to stop you from doing exactly that --
	mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL)
without bothering with statfs() in the first place?  It's not like the
damn thing had been costly - it's O(mounts in your namespace) with not
to high constant, and in any case cheaper than allocating all of them
back when you did clone(2).  Confused...

> The above case is what I see most often. A more specific use-case is to
> differentiate between MS_SLAVE and MS_SHARED mountpoints.
> Mountpoints that are MS_SLAVE are kept intact and mountpoints that are
> MS_SHARED are made MS_PRIVATE.
> 
> For both cases the only way to do this right now is by parsing
> /proc/<pid>/mountinfo. Yes, it is doable but still it is somewhat costly
> and annoying as e.g. those mount propagation fields are optional.

Umm...  And how would you get the list of mountpoints to feed to statfs()
if not by parsing mountinfo?  IDGI...