crypto: remove CONFIG_CRYPTO_STATS

Remove support for the "Crypto usage statistics" feature
(CONFIG_CRYPTO_STATS).  This feature does not appear to have ever been
used, and it is harmful because it significantly reduces performance and
is a large maintenance burden.

Covering each of these points in detail:

1. Feature is not being used

Since these generic crypto statistics are only readable using netlink,
it's fairly straightforward to look for programs that use them.  I'm
unable to find any evidence that any such programs exist.  For example,
Debian Code Search returns no hits except the kernel header and kernel
code itself and translations of the kernel header:
https://codesearch.debian.net/search?q=CRYPTOCFGA_STAT&literal=1&perpkg=1

The patch series that added this feature in 2018
(https://lore.kernel.org/linux-crypto/1537351855-16618-1-git-send-email-clabbe@baylibre.com/)
said "The goal is to have an ifconfig for crypto device."  This doesn't
appear to have happened.

It's not clear that there is real demand for crypto statistics.  Just
because the kernel provides other types of statistics such as I/O and
networking statistics and some people find those useful does not mean
that crypto statistics are useful too.

Further evidence that programs are not using CONFIG_CRYPTO_STATS is that
it was able to be disabled in RHEL and Fedora as a bug fix
(https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/2947).

Even further evidence comes from the fact that there are and have been
bugs in how the stats work, but they were never reported.  For example,
before Linux v6.7 hash stats were double-counted in most cases.

There has also never been any documentation for this feature, so it
might be hard to use even if someone wanted to.

2. CONFIG_CRYPTO_STATS significantly reduces performance

Enabling CONFIG_CRYPTO_STATS significantly reduces the performance of
the crypto API, even if no program ever retrieves the statistics.  This
primarily affects systems with a large number of CPUs.  For example,
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2039576 reported
that Lustre client encryption performance improved from 21.7GB/s to
48.2GB/s by disabling CONFIG_CRYPTO_STATS.

It can be argued that this means that CONFIG_CRYPTO_STATS should be
optimized with per-cpu counters similar to many of the networking
counters.  But no one has done this in 5+ years.  This is consistent
with the fact that the feature appears to be unused, so there seems to
be little interest in improving it as opposed to just disabling it.

It can be argued that because CONFIG_CRYPTO_STATS is off by default,
performance doesn't matter.  But Linux distros tend to error on the side
of enabling options.  The option is enabled in Ubuntu and Arch Linux,
and until recently was enabled in RHEL and Fedora (see above).  So, even
just having the option available is harmful to users.

3. CONFIG_CRYPTO_STATS is a large maintenance burden

There are over 1000 lines of code associated with CONFIG_CRYPTO_STATS,
spread among 32 files.  It significantly complicates much of the
implementation of the crypto API.  After the initial submission, many
fixes and refactorings have consumed effort of multiple people to keep
this feature "working".  We should be spending this effort elsewhere.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 0 insertions, 22 deletions

diff --git a/include/crypto/hash.h b/include/crypto/hash.h
index 5d61f576cfc8..0014bdd81ab7 100644
--- a/include/crypto/hash.h
+++ b/include/crypto/hash.h
@@ -24,26 +24,7 @@ struct crypto_ahash;
  */
 
 /*
- * struct crypto_istat_hash - statistics for has algorithm
- * @hash_cnt:		number of hash requests
- * @hash_tlen:		total data size hashed
- * @err_cnt:		number of error for hash requests
- */
-struct crypto_istat_hash {
-	atomic64_t hash_cnt;
-	atomic64_t hash_tlen;
-	atomic64_t err_cnt;
-};
-
-#ifdef CONFIG_CRYPTO_STATS
-#define HASH_ALG_COMMON_STAT struct crypto_istat_hash stat;
-#else
-#define HASH_ALG_COMMON_STAT
-#endif
-
-/*
  * struct hash_alg_common - define properties of message digest
- * @stat: Statistics for hash algorithm.
  * @digestsize: Size of the result of the transformation. A buffer of this size
  *	        must be available to the @final and @finup calls, so they can
  *	        store the resulting hash into it. For various predefined sizes,
@@ -60,8 +41,6 @@ struct crypto_istat_hash {
  *	  information.
  */
 #define HASH_ALG_COMMON {		\
-	HASH_ALG_COMMON_STAT		\
-					\
 	unsigned int digestsize;	\
 	unsigned int statesize;		\
 					\
@@ -243,7 +222,6 @@ struct shash_alg {
 	};
 };
 #undef HASH_ALG_COMMON
-#undef HASH_ALG_COMMON_STAT
 
 struct crypto_ahash {
 	bool using_shash; /* Underlying algorithm is shash, not ahash */