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Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
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Date: Fri, 17 Feb 2017 09:34:07 -0800
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On Fri, 2017-02-17 at 02:55 +0000, Al Viro wrote:
> On Thu, Feb 16, 2017 at 07:56:30AM -0800, James Bottomley wrote:
> 
> > > Hi James,
> > > 
> > > Should it be "return d_splice_alias()" so that if we find an 
> > > alias it is returned back to caller and passed in dentry can be 
> > > freed. Though I don't know in what cases alias can be found. And 
> > > if alias is found how do we make sure alias_dentry->d_fsdata is 
> > > pointing to new (real dentry).
> > 
> > It probably should be for the sake of the pattern.  In our case I 
> > don't think we can have any root aliases because the root dentry is
> > always pinned in the cache, so cache lookup should always find it.
> 
> What does that have to do with root dentry?  The real reason why that 
> code works (FVerySVO) is that the damn thing allocates a new inode 
> every time. Including the hardlinks, BTW.

Yes, this is a known characteristic of stacked filesystems.  Is there
some magic I don't know about that would make it easier to reflect hard
links as aliases?

>   So d_splice_alias() will always return NULL - there's no way for 
> any dentries to be pointing to in-core struct inode you've
> just allocated.  Short of a use-after-free, that is...
> 
> Unless I'm missing something subtle, the whole thing is fucked
> in head wrt cache coherency - its dentries are blindly assumed to be
> forever valid, no matter what's happening with the underlying 
> filesystem.

Hopefully the patch in the previous email fixes this.

James