Re: [PATCH v2 17/20] btrfs: reuse encrypted filename hash when possible.

David Sterba
2022-09-07T21:29:38Z

On Mon, Sep 05, 2022 at 08:35:32PM -0400, Sweet Tea Dorminy wrote:
> From: Omar Sandoval <osandov@osandov.com>
>
> For encrypted fscrypt_names, we can reuse fscrypt's precomputed hash of
> the encrypted name to generate our own hash, instead of rehashing the
> unencrypted name (which may not be possible if it's a nokey name).

Can you please describe how this works when the key is not available?

> Signed-off-by: Omar Sandoval <osandov@osandov.com>
> Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
> ---
> fs/btrfs/ctree.h | 21 ++++++++++++++++++---
> 1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index e8d000fcc85d..aa599518c057 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -2842,7 +2842,10 @@ static inline void btrfs_crc32c_final(u32 crc, u8 *result)
>
> static inline u64 btrfs_name_hash(const struct fscrypt_name *name)
> {
> - return crc32c((u32)~1, fname_name(name), fname_len(name));
> + if (fname_name(name))
> + return crc32c((u32)~1, fname_name(name), fname_len(name));
> + else
> + return name->hash | ((u64)name->minor_hash << 32);

So this is another on-disk format change.

> }
>
> /*
> @@ -2851,8 +2854,20 @@ static inline u64 btrfs_name_hash(const struct fscrypt_name *name)
> static inline u64 btrfs_extref_hash(u64 parent_objectid,
> const struct fscrypt_name *name)
> {
> - return (u64) crc32c(parent_objectid, fname_name(name),
> - fname_len(name));
> + /*
> + * If the name is encrypted and we don't have the key, we can use the
> + * fscrypt-provided hash instead of the normal name, and do the steps
> + * of crc32c() manually. Else, just hash the name, parent objectid,
> + * and name length.
> + */
> + if (fname_name(name))
> + return (u64) crc32c(parent_objectid, fname_name(name),
> + fname_len(name));
> + else
> + return (__crc32c_le_combine(parent_objectid,
> + name->hash,
> + fname_len(name)) ^
> + __crc32c_le_shift(~1, fname_len(name)));

How stable is the __crc32c_le_* API?

> }
>
> static inline gfp_t btrfs_alloc_write_mask(struct address_space *mapping)
> --
> 2.35.1