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Date:   Fri, 12 Jul 2019 05:33:42 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Jeff Layton <jlayton@kernel.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
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Subject: Re: [PATCH v9 05/10] namei: O_BENEATH-style path resolution flags
Message-ID: <20190712043341.GI17978@ZenIV.linux.org.uk>
References: <20190706145737.5299-1-cyphar@cyphar.com>
 <20190706145737.5299-6-cyphar@cyphar.com>
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On Sun, Jul 07, 2019 at 12:57:32AM +1000, Aleksa Sarai wrote:

> @@ -1442,8 +1464,11 @@ static int follow_dotdot_rcu(struct nameidata *nd)
>  	struct inode *inode = nd->inode;
>  
>  	while (1) {
> -		if (path_equal(&nd->path, &nd->root))
> +		if (path_equal(&nd->path, &nd->root)) {
> +			if (unlikely(nd->flags & LOOKUP_BENEATH))
> +				return -EXDEV;

> @@ -1468,6 +1493,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
>  				return -ECHILD;
>  			if (&mparent->mnt == nd->path.mnt)
>  				break;
> +			if (unlikely(nd->flags & LOOKUP_XDEV))
> +				return -EXDEV;
>  			/* we know that mountpoint was pinned */
>  			nd->path.dentry = mountpoint;
>  			nd->path.mnt = &mparent->mnt;
> @@ -1482,6 +1509,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
>  			return -ECHILD;
>  		if (!mounted)
>  			break;
> +		if (unlikely(nd->flags & LOOKUP_XDEV))
> +			return -EXDEV;

Are you sure these failure exits in follow_dotdot_rcu() won't give
suprious hard errors?

> +	if (unlikely(nd->flags & LOOKUP_BENEATH)) {
> +		error = dirfd_path_init(nd);
> +		if (unlikely(error))
> +			return ERR_PTR(error);
> +		nd->root = nd->path;
> +		if (!(nd->flags & LOOKUP_RCU))
> +			path_get(&nd->root);
> +	}
>  	if (*s == '/') {
>  		if (likely(!nd->root.mnt))
>  			set_root(nd);
> @@ -2350,9 +2400,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>  			s = ERR_PTR(error);
>  		return s;
>  	}
> -	error = dirfd_path_init(nd);
> -	if (unlikely(error))
> -		return ERR_PTR(error);
> +	if (likely(!nd->path.mnt)) {

Is that a weird way of saying "if we hadn't already called dirfd_path_init()"?