Re: [PATCH v4] fs: introduce is_dot_or_dotdot helper for cleanup

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Dec 10, 2019 at 08:10:01PM +0800, Tiezhu Yang wrote:
> diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
> index 3da3707..ef7eba8 100644
> --- a/fs/crypto/fname.c
> +++ b/fs/crypto/fname.c
> @@ -11,21 +11,11 @@
>   * This has not yet undergone a rigorous security audit.
>   */
>  
> +#include <linux/namei.h>
>  #include <linux/scatterlist.h>
>  #include <crypto/skcipher.h>
>  #include "fscrypt_private.h"
>  
> -static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
> -{
> -	if (str->len == 1 && str->name[0] == '.')
> -		return true;
> -
> -	if (str->len == 2 && str->name[0] == '.' && str->name[1] == '.')
> -		return true;
> -
> -	return false;
> -}
> -
>  /**
>   * fname_encrypt() - encrypt a filename
>   *
> @@ -255,7 +245,7 @@ int fscrypt_fname_disk_to_usr(struct inode *inode,
>  	const struct qstr qname = FSTR_TO_QSTR(iname);
>  	struct fscrypt_digested_name digested_name;
>  
> -	if (fscrypt_is_dot_dotdot(&qname)) {
> +	if (is_dot_or_dotdot(qname.name, qname.len)) {

There's no need for the 'qname' variable anymore.  Can you please remove it and
do:

	if (is_dot_or_dotdot(iname->name, iname->len)) {

> diff --git a/include/linux/namei.h b/include/linux/namei.h
> index 7fe7b87..aba114a 100644
> --- a/include/linux/namei.h
> +++ b/include/linux/namei.h
> @@ -92,4 +92,14 @@ retry_estale(const long error, const unsigned int flags)
>  	return error == -ESTALE && !(flags & LOOKUP_REVAL);
>  }
>  
> +static inline bool is_dot_or_dotdot(const unsigned char *name, size_t len)
> +{
> +	if (unlikely(name[0] == '.')) {
> +		if (len < 2 || (len == 2 && name[1] == '.'))
> +			return true;
> +	}
> +
> +	return false;
> +}

This doesn't handle the len=0 case.  Did you check that none of the users pass
in zero-length names?  It looks like fscrypt_fname_disk_to_usr() can, if the
directory entry on-disk has a zero-length name.  Currently it will return
-EUCLEAN in that case, but with this patch it may think it's the name ".".

So I think there needs to either be a len >= 1 check added, *or* you need to
make an argument for why it's okay to not care about the empty name case.

- Eric