Re: [RFC PATCH 1/2] fscrypt: new helper function - __fscrypt_prepare_atomic_open()

[Eric Biggers]

On Thu, Mar 09, 2023 at 12:19:09PM +0000, Luís Henriques wrote:
> This patch introduces a new helper function which prepares an atomic_open.
> Because atomic open can act as a lookup if handed a dentry that is negative,
> we need to set DCACHE_NOKEY_NAME if the key for the parent isn't available.
> 
> The reason for getting the encryption info before checking if the directory
> has the encryption key is because we may have the key available but the
> encryption info isn't yet set (maybe due to a drop_caches).  The regular
> open path will use fscrypt_file_open for that but in the atomic open a
> different approach is required.
> 
> Signed-off-by: Luís Henriques <lhenriques@xxxxxxx>
> ---
>  fs/crypto/hooks.c       | 14 ++++++++++++++
>  include/linux/fscrypt.h |  6 ++++++
>  2 files changed, 20 insertions(+)
> 
> diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
> index 7b8c5a1104b5..cbb828ecc5eb 100644
> --- a/fs/crypto/hooks.c
> +++ b/fs/crypto/hooks.c
> @@ -117,6 +117,20 @@ int __fscrypt_prepare_readdir(struct inode *dir)
>  }
>  EXPORT_SYMBOL_GPL(__fscrypt_prepare_readdir);
>  
> +int __fscrypt_prepare_atomic_open(struct inode *dir, struct dentry *dentry)

Anything exported to filesystems should have a kerneldoc comment.  That would be
a good place to put some of the explanation that you currently have only in the
commit message.

Also, double-underscored functions are not for use by filesystems directly.
Normally the pattern would be to make fscrypt_prepare_atomic_open() an inline
function that checks IS_ENCRYPTED() and calls an out-of-line function
__fscrypt_prepare_atomic_open().  But if it happens to be simpler to make the
caller handle the IS_ENCRYPTED() check in this case, then there should simply be
one function: fscrypt_prepare_atomic_open() (no leading underscores).

> +{
> +	int err = fscrypt_get_encryption_info(dir, true);
> +
> +	if (err || (!err && !fscrypt_has_encryption_key(dir))) {
> +		spin_lock(&dentry->d_lock);
> +		dentry->d_flags |= DCACHE_NOKEY_NAME;
> +		spin_unlock(&dentry->d_lock);
> +	}

Why does DCACHE_NOKEY_NAME need to be set on error?

Also note that the '!err &&' part has no effect.

- Eric