linux-kernel - Re: [PATCH 0/6 v1 resend] statfs: handle mount propagation

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Message-ID: <20180613140625.GA31082@mailbox.org>
Date:   Wed, 13 Jun 2018 16:06:25 +0200
From:   Christian Brauner <christian@...uner.io>
To:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, tglx@...utronix.de,
        kstewart@...uxfoundation.org, gregkh@...uxfoundation.org,
        pombredanne@...b.com, linux-api@...r.kernel.org,
        ebiederm@...ssion.com, seth.forshee@...onical.com,
        viro@...iv.linux.org.uk
Subject: Re: [PATCH 0/6 v1 resend] statfs: handle mount propagation

On Fri, May 25, 2018 at 02:48:19PM +0200, Christian Brauner wrote:
> From: Christian Brauner <christian.brauner@...ntu.com>
> 
> Hey,
> 
> This is v1 of this patchset. All changes from v0 to v1 are non-functional.
> Specifically, the commit messages and justification have been extended as
> requested by Linus and Al.

Hey everyone,

Just a ping whether the requested changes have made this series suitable
for inclusion. Would be excellent if someone could do another review. :)

Thanks!
Christian


> 
> This little series does the following:
> 
> - unify the definition of constants in statfs.h and fs.h:
>   The definitions for MS_* flags are currently a mixture between hex values
>   and bit-shifts. All higher values are already initialized with bit-shifts
>   for MS_* constants starting with (1<<16). This patch switches the
>   definitions for MS_* constants over to uniformly use bit-shifts and
>   alignes the definitions of ST_* flags too.
>   Initializing them identically let's userspace easily detect when flags
>   indicate the same property but use a different value in doing so.
> 
> - extend statfs to handle mount propagation:
>   For all 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.
>   1. prevent propagation from happening:
>      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.
>      These mountpoints might be known in advance so parsing
>      /proc/<pid>/mountinfo should not be needed.
>   2. 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. These mountpoint might be known in
>      advance so parsing /proc/<pid>/mountinfo should not be needed.
>   3. retrieve propagation properties when procfs is not mounted:
>      When the set of interesting mountpoints is known and /proc is not
>      mounted calling statfs() is the only good way to reliably determine
>      the propagation property of a mountpoint.
>   4. inspecting file descriptors to mountpoints for propagation
>      properties:
>      When file descriptors to mountpoints are passed around between
>      processes it is useful to have fstatvfs() handle mount propagation
>      properties too.
>   To this end the flags:
>   - ST_UNBINDABLE
>   - ST_SHARED
>   - ST_PRIVATE
>   - ST_SLAVE
>   are added. They have the same value as their MS_* counterparts.
> 
> - Testing:
>   I verified that now userspace can do e.g.
> 
>   int ret;
>   char *s = "/some/path";
>   struct statvfs sb;
> 
>   ret = statvfs(s, &sb);
>   if (ret < 0)
>           return false;
> 
>   if (sb.f_flag & ST_SHARED) {
>           ret = mount("", s, NULL, MS_SLAVE | MS_REC, NULL);
>           if (ret < 0)
>                   return -1;
>   }
> 
> Thanks!
> Christian
> 
> Christian Brauner (6):
>   fs: use << for MS_* flags
>   statfs: use << to align with fs header
>   statfs: add ST_UNBINDABLE
>   statfs: add ST_SHARED
>   statfs: add ST_SLAVE
>   statfs: add ST_PRIVATE
> 
>  fs/statfs.c             | 16 +++++++++++++++-
>  include/linux/statfs.h  | 30 +++++++++++++++++-------------
>  include/uapi/linux/fs.h | 33 +++++++++++++++++----------------
>  3 files changed, 49 insertions(+), 30 deletions(-)
> 
> -- 
> 2.17.0
>