linux-ext4 - Re: [PATCH 3/7] libfs: Validate negative dentries in case-insensitive directories

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Date:   Sun, 26 Mar 2023 05:46:27 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     tytso@....edu, jaegeuk@...nel.org, ebiggers@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel@...labora.com
Subject: Re: [PATCH 3/7] libfs: Validate negative dentries in
 case-insensitive directories

On Wed, Jun 22, 2022 at 03:45:59PM -0400, Gabriel Krisman Bertazi wrote:

> +static inline int generic_ci_d_revalidate(struct dentry *dentry,
> +					  const struct qstr *name,
> +					  unsigned int flags)
> +{
> +	int is_creation = flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET);
> +
> +	if (d_is_negative(dentry)) {
> +		const struct dentry *parent = READ_ONCE(dentry->d_parent);
> +		const struct inode *dir = READ_ONCE(parent->d_inode);
> +
> +		if (dir && needs_casefold(dir)) {
> +			if (!d_is_casefold_lookup(dentry))
> +				return 0;

	In which conditions does that happen?

> +			if (is_creation &&
> +			    (dentry->d_name.len != name->len ||
> +			     memcmp(dentry->d_name.name, name->name, name->len)))
> +				return 0;
> +		}
> +	}
> +	return 1;
> +}

	Analysis of stability of ->d_name, please.  It's *probably* safe, but
the details are subtle and IMO should be accompanied by several asserts.
E.g. "we never get LOOKUP_CREATE in op->intent without O_CREAT in op->open_flag
for such and such reasons, and we verify that in such and such place"...

	A part of that would be "the call in lookup_dcache() can only get there
with non-zero flags when coming from __lookup_hash(), and that has parent locked,
stabilizing the name; the same goes for the call in __lookup_slow(), with the
only call chain with possibly non-zero flags is through lookup_slow(), where we
have the parent locked".  However, lookup_fast() and lookup_open() have the
flags come from nd->flags, and LOOKUP_CREATE can be found there in several areas.
I _think_ we are guaranteed the parent locked in all such call chains, but that
is definitely worth at least a comment.