fscrypto/f2fs: allow fs-specific key prefix for fs encryption

This patch allows fscrypto to handle a second key prefix given by filesystem.
The main reason is to provide backward compatibility, since previously f2fs
used "f2fs:" as a crypto prefix instead of "fscrypt:".
Later, ext4 should also provide key_prefix() to give "ext4:".

One concern decribed by Ted would be kinda double check overhead of prefixes.
In x86, for example, validate_user_key consumes 8 ms after boot-up, which turns
out derive_key_aes() consumed most of the time to load specific crypto module.
After such the cold miss, it shows almost zero latencies, which treats as a
negligible overhead.
Note that request_key() detects wrong prefix in prior to derive_key_aes() even.

Cc: Ted Tso <tytso@mit.edu>
Cc: stable@vger.kernel.org # v4.6
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

 Conflicts:
	fs/crypto/keyinfo.c

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/fscrypto.h b/include/linux/fscrypto.h
index 4c4bc8c05..7ddcd5bc8 100644
--- a/include/linux/fscrypto.h
+++ b/include/linux/fscrypto.h
@@ -174,6 +174,7 @@ struct fscrypt_name {
  */
 struct fscrypt_operations {
 	int (*get_context)(struct inode *, void *, size_t);
+	int (*key_prefix)(struct inode *, u8 **);
 	int (*prepare_context)(struct inode *);
 	int (*set_context)(struct inode *, const void *, size_t, void *);
 	int (*dummy_context)(struct inode *);