btrfs: switch to library APIs for checksums

Make btrfs use the library APIs instead of crypto_shash, for all
checksum computations.  This has many benefits:

- Allows future checksum types, e.g. XXH3 or CRC64, to be more easily
  supported.  Only a library API will be needed, not crypto_shash too.

- Eliminates the overhead of the generic crypto layer, including an
  indirect call for every function call and other API overhead.  A
  microbenchmark of btrfs_check_read_bio() with crc32c checksums shows a
  speedup from 658 cycles to 608 cycles per 4096-byte block.

- Decreases the stack usage of btrfs by reducing the size of checksum
  contexts from 384 bytes to 240 bytes, and by eliminating the need for
  some functions to declare a checksum context at all.

- Increases reliability.  The library functions always succeed and
  return void.  In contrast, crypto_shash can fail and return errors.
  Also, the library functions are guaranteed to be available when btrfs
  is loaded; there's no longer any need to use module softdeps to try to
  work around the crypto modules sometimes not being loaded.

- Fixes a bug where blake2b checksums didn't work on kernels booted with
  fips=1.  Since btrfs checksums are for integrity only, it's fine for
  them to use non-FIPS-approved algorithms.

Note that with having to handle 4 algorithms instead of just 1-2, this
commit does result in a slightly positive diffstat.  That being said,
this wouldn't have been the case if btrfs had actually checked for
errors from crypto_shash, which technically it should have been doing.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 2 insertions, 4 deletions

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 4b3c2acac51a..f0974f4c0ae4 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -11,7 +11,6 @@
 #include <linux/bug.h>
 #include <linux/list.h>
 #include <linux/string_choices.h>
-#include <crypto/hash.h>
 #include "messages.h"
 #include "ctree.h"
 #include "discard.h"
@@ -1253,10 +1252,9 @@ static ssize_t btrfs_checksum_show(struct kobject *kobj,
 {
 	struct btrfs_fs_info *fs_info = to_fs_info(kobj);
 	u16 csum_type = btrfs_super_csum_type(fs_info->super_copy);
+	const char *csum_name = btrfs_super_csum_name(csum_type);
 
-	return sysfs_emit(buf, "%s (%s)\n",
-			  btrfs_super_csum_name(csum_type),
-			  crypto_shash_driver_name(fs_info->csum_shash));
+	return sysfs_emit(buf, "%s (%s-lib)\n", csum_name, csum_name);
 }
 
 BTRFS_ATTR(, checksum, btrfs_checksum_show);