Re: [RFC] Don't propagate automount

[Ian Kent <raven@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,
> > >