linux-ext4 - Re: [PATCH v2 3/7] libfs: Validate negative dentries in case-insensitive directories

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Message-ID: <20230714050028.GC913@sol.localdomain>
Date:   Thu, 13 Jul 2023 22:00:28 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Gabriel Krisman Bertazi <krisman@...e.de>
Cc:     viro@...iv.linux.org.uk, brauner@...nel.org, tytso@....edu,
        jaegeuk@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH v2 3/7] libfs: Validate negative dentries in
 case-insensitive directories

On Fri, Apr 21, 2023 at 08:03:06PM -0400, Gabriel Krisman Bertazi wrote:
> diff --git a/fs/libfs.c b/fs/libfs.c
> index 4eda519c3002..f8881e29c5d5 100644
> --- a/fs/libfs.c
> +++ b/fs/libfs.c
> @@ -1467,9 +1467,43 @@ static int generic_ci_d_hash(const struct dentry *dentry, struct qstr *str)
>  	return 0;
>  }
>  
> +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;

A comment that explains why the !d_is_casefold_lookup() check is needed would be
helpful.  I know it's in the commit message, but that's not enough.

> +
> +			if (is_creation) {
> +				/*
> +				 * dentry->d_name won't change from under us in
> +				 * the is_creation path only, since d_revalidate
> +				 * during creation and renames is always called
> +				 * with the parent inode locked.  This isn't the
> +				 * case for all lookup callpaths, so it should
> +				 * not be accessed outside
> +				 * (LOOKUP_CREATE|LOOKUP_RENAME_TARGET) context.
> +				 */
> +				if (dentry->d_name.len != name->len ||
> +				    memcmp(dentry->d_name.name, name->name, name->len))
> +					return 0;
> +			}
> +		}
> +	}
> +	return 1;
> +}

I notice that the existing vfat_revalidate_ci() in fs/fat/namei_vfat.c behaves
differently in the 'flags == 0' case:


	/*
	 * This may be nfsd (or something), anyway, we can't see the
	 * intent of this. So, since this can be for creation, drop it.
	 */
	if (!flags)
		return 0;

I don't know whether that's really needed, but have you thought about this?

- Eric