From: Ian Kent <raven@themaw.net>
To: Goldwyn Rodrigues <rgoldwyn@suse.de>, viro@zeniv.linux.org.uk
Cc: autofs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC] Don't propagate automount
Date: Fri, 27 Sep 2019 15:26:53 +0800 [thread overview]
Message-ID: <d163042ab8fffd975a6d460488f1539c5f619eaa.camel@themaw.net> (raw)
In-Reply-To: <e5fbf32668aea1b8143d15ff47bd1e4309d03b17.camel@themaw.net>
On Fri, 2019-09-27 at 15:09 +0800, Ian Kent wrote:
> On Fri, 2019-09-27 at 09:35 +0800, Ian Kent wrote:
> > On Thu, 2019-09-26 at 14:52 -0500, Goldwyn Rodrigues wrote:
> > > An access to automounted filesystem can deadlock if it is a bind
> > > mount on shared mounts. A user program should not deadlock the
> > > kernel
> > > while automount waits for propagation of the mount. This is
> > > explained
> > > at https://bugzilla.redhat.com/show_bug.cgi?id=1358887#c10
> > > I am not sure completely blocking automount is the best solution,
> > > so please reply with what is the best course of action to do
> > > in such a situation.
> > >
> > > Propagation of dentry with DCACHE_NEED_AUTOMOUNT can lead to
> > > propagation of mount points without automount maps and not under
> > > automount control. So, do not propagate them.
> >
> > Yes, I'm not sure my comments about mount propagation in that
> > bug are accurate.
> >
> > This behaviour has crept into the kernel in reasonably recent
> > times, maybe it's a bug or maybe mount propagation has been
> > "fixed", not sure.
> >
> > I think I'll need to come up with a more detailed description
> > of what is being done for Al to be able to offer advice.
> >
> > I'll get to that a bit later.
>
> To duplicate this problem use an autofs indirect map
> that uses bind mounts and has offsets:
>
> test / :/exports \
> /tmp :/exports/tmp \
> /lib :/exports/lib
>
> and add:
>
> /bind /etc/auto.exports
>
> to /etc/auto.master.
>
> Finally create the bind mount directories:
>
> mkdir -p /exports/lib /exports/tmp
>
> Then, on a broken kernel, eg. 4.13.9-300.fc27:
>
> ls /bind/test
>
> will result in:
>
> /etc/auto.exports on /bind type autofs
> (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,indirec
> t,pipe_ino=45485)
> /dev/mapper/fedora_f27-root on /bind/test type ext4
> (rw,relatime,seclabel,data=ordered)
> /etc/auto.exports on /bind/test/lib type autofs
> (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset,
> pipe_ino=45485)
> /etc/auto.exports on /exports/lib type autofs
> (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset,
> pipe_ino=45485)
> /etc/auto.exports on /bind/test/tmp type autofs
> (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset,
> pipe_ino=45485)
> /etc/auto.exports on /exports/tmp type autofs
> (rw,relatime,fd=5,pgrp=2981,timeout=300,minproto=5,maxproto=5,offset,
> pipe_ino=45485)
>
> these mount entries, not all of which have been mounted by autofs.
>
> Whereas on a kernel that isn't broken, eg. 4.11.8-300.fc26, the same
> ls command will result in:
>
> /etc/auto.exports on /bind type autofs
> (rw,relatime,fd=6,pgrp=2920,timeout=300,minproto=5,maxproto=5,indirec
> t,pipe_ino=42067)
> /etc/auto.exports on /bind/test/lib type autofs
> (rw,relatime,fd=6,pgrp=2920,timeout=300,minproto=5,maxproto=5,offset,
> pipe_ino=42067)
> /etc/auto.exports on /bind/test/tmp type autofs
> (rw,relatime,fd=6,pgrp=2920,timeout=300,minproto=5,maxproto=5,offset,
> pipe_ino=42067)
>
> these mount entries, all of which have been mounted by autofs (and
> are what's needed for these offset mount constructs).
>
> If the /bind mount is made propagation slave or private at mount
> by automount the problem doesn't happen and that is the workaround
> I implemented in autofs.
Actually that's not quite right, there should be a bind mount at
/bind/test as well but it's not present.
Although I expect this will happen with a rootless multi-mount
as well, aka.
test \
/tmp :/exports/tmp \
/lib :/exports/lib
Leave it with me while I investigate further.
>
> I initially thought this was the result of a "fix" in the mount
> propagation code but it occurred to me that propagation is meant
> to occur between mount trees not within them so this might be a
> bug.
>
> I probably should have worked out exactly what upstream kernel
> this started happening in and then done a bisect and tried to
> work out if the change was doing what it was supposed to.
>
> Anyway, I'll need to do that now for us to discuss this sensibly.
>
> > > Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> > >
> > > diff --git a/fs/pnode.c b/fs/pnode.c
> > > index 49f6d7ff2139..b960805d7954 100644
> > > --- a/fs/pnode.c
> > > +++ b/fs/pnode.c
> > > @@ -292,6 +292,9 @@ int propagate_mnt(struct mount *dest_mnt,
> > > struct
> > > mountpoint *dest_mp,
> > > struct mount *m, *n;
> > > int ret = 0;
> > >
> > > + if (source_mnt->mnt_mountpoint->d_flags &
> > > DCACHE_NEED_AUTOMOUNT)
> > > + return 0;
> > > +
> >
> > Possible problem with this is it will probably prevent mount
> > propagation in both directions which will break stuff.
> >
> > I had originally assumed the problem was mount propagation
> > back to the parent mount but now I'm not sure that this is
> > actually what is meant to happen.
> >
> > > /*
> > > * we don't want to bother passing tons of arguments to
> > > * propagate_one(); everything is serialized by namespace_sem,
> > >
next prev parent reply other threads:[~2019-09-27 7:27 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-26 19:52 [RFC] Don't propagate automount Goldwyn Rodrigues
2019-09-27 1:35 ` Ian Kent
2019-09-27 7:09 ` Ian Kent
2019-09-27 7:26 ` Ian Kent [this message]
2019-09-27 7:41 ` Ian Kent
2019-09-27 10:51 ` Ian Kent
2019-09-27 16:16 ` Goldwyn Rodrigues
2019-09-28 1:47 ` Ian Kent
2019-10-01 19:09 ` Goldwyn Rodrigues
2019-10-02 2:14 ` Ian Kent
2019-10-28 16:28 ` Goldwyn Rodrigues
2019-10-28 23:57 ` Ian Kent
2019-10-29 6:39 ` Ian Kent
2019-10-29 6:40 ` Ian Kent
2019-10-29 16:00 ` Goldwyn Rodrigues
2019-10-30 6:01 ` Ian Kent
2019-10-30 6:05 ` Ian Kent
2019-10-30 12:05 ` Ian Kent
2019-10-30 19:28 ` Goldwyn Rodrigues
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