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

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Date:   Thu, 23 Jan 2020 21:16:01 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Gao Xiang <hsiangkao@....com>
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 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.

- Eric