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Date:   Tue, 22 Dec 2020 01:53:02 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Liangyan <liangyan.peng@linux.alibaba.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>, linux-unionfs@vger.kernel.org,
        linux-kernel@vger.kernel.org, joseph.qi@linux.alibaba.com
Subject: Re: [PATCH v3] ovl: fix  dentry leak in ovl_get_redirect
Message-ID: <20201222015302.GR3579531@ZenIV.linux.org.uk>
References: <20201221183327.134077-1-liangyan.peng@linux.alibaba.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>
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On Tue, Dec 22, 2020 at 02:33:27AM +0800, Liangyan wrote:

> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 28a075b5f5b2..e9aa4a12ad82 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -973,6 +973,7 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>  	for (d = dget(dentry); !IS_ROOT(d);) {
>  		const char *name;
>  		int thislen;
> +		struct dentry *parent = NULL;
>  
>  		spin_lock(&d->d_lock);
>  		name = ovl_dentry_get_redirect(d);
> @@ -992,11 +993,10 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>  
>  		buflen -= thislen;
>  		memcpy(&buf[buflen], name, thislen);
> -		tmp = dget_dlock(d->d_parent);
>  		spin_unlock(&d->d_lock);
> -
> +		parent = dget_parent(d);
>  		dput(d);
> -		d = tmp;
> +		d = parent;
>  
>  		/* Absolute redirect: finished */
>  		if (buf[buflen] == '/')

FWIW, I'd suggest this:

dget_dlock(d->d_parent) is unsafe - it requires ->d_lock on
dentry we are grabbing and that's not what we are holding
here.  It's the wrong way to grab the parent anyway; use
dget_parent(d) instead.


diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 28a075b5f5b2..d1efa3a5a503 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -992,8 +992,8 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
 
 		buflen -= thislen;
 		memcpy(&buf[buflen], name, thislen);
-		tmp = dget_dlock(d->d_parent);
 		spin_unlock(&d->d_lock);
+		tmp = dget_parent(d);
 
 		dput(d);
 		d = tmp;