linux-ext4 - Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d

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Date:   Fri, 24 Jan 2020 13:27:50 +0800
From:   Gao Xiang <hsiangkao@....com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Daniel Rosenberg <drosen@...gle.com>,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: [PATCH] ext4: fix race conditions in ->d_compare() and ->d_hash()

On Thu, Jan 23, 2020 at 09:16:01PM -0800, Eric Biggers wrote:
> On Fri, Jan 24, 2020 at 01:04:25PM +0800, Gao Xiang wrote:
> > > diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
> > > index 8964778aabefb..0129d14629881 100644
> > > --- a/fs/ext4/dir.c
> > > +++ b/fs/ext4/dir.c
> > > @@ -671,9 +671,11 @@ static int ext4_d_compare(const struct dentry *dentry, unsigned int len,
> > >  			  const char *str, const struct qstr *name)
> > >  {
> > >  	struct qstr qstr = {.name = str, .len = len };
> > > -	struct inode *inode = dentry->d_parent->d_inode;
> > > +	const struct dentry *parent = READ_ONCE(dentry->d_parent);
> > 
> > I'm not sure if we really need READ_ONCE d_parent here (p.s. d_parent
> > won't be NULL anyway), and d_seq will guard all its validity. If I'm
> > wrong, correct me kindly...
> > 
> > Otherwise, it looks good to me...
> > Reviewed-by: Gao Xiang <gaoxiang25@...wei.com>
> > 
> 
> While d_parent can't be set to NULL, it can still be changed concurrently.
> So we need READ_ONCE() to ensure that a consistent value is used.

If I understand correctly, unlazy RCU->ref-walk will be guarded by
seqlock, and for ref-walk we have d_lock (and even parent lock)
in relative paths. So I prematurely think no race of renaming or
unlinking evenually.

I'm curious about that if experts could correct me about this.

Thanks,
Gao Xiang

> 
> - Eric