sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 124 insertions, 0 deletions

diff --git a/crypto/crypto_null.c b/crypto/crypto_null.c
new file mode 100644
index 000000000..fea151df1
--- /dev/null
+++ b/crypto/crypto_null.c
@@ -0,0 +1,124 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Cryptographic API.
+ *
+ * Null algorithms, aka Much Ado About Nothing.
+ *
+ * These are needed for IPsec, and may be useful in general for
+ * testing & debugging.
+ *
+ * The null cipher is compliant with RFC2410.
+ *
+ * Copyright (c) 2002 James Morris <jmorris@intercode.com.au>
+ */
+
+#include <crypto/null.h>
+#include <crypto/internal/hash.h>
+#include <crypto/internal/skcipher.h>
+#include <crypto/scatterwalk.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/string.h>
+
+static int null_init(struct shash_desc *desc)
+{
+	return 0;
+}
+
+static int null_update(struct shash_desc *desc, const u8 *data,
+		       unsigned int len)
+{
+	return 0;
+}
+
+static int null_final(struct shash_desc *desc, u8 *out)
+{
+	return 0;
+}
+
+static int null_digest(struct shash_desc *desc, const u8 *data,
+		       unsigned int len, u8 *out)
+{
+	return 0;
+}
+
+static int null_hash_setkey(struct crypto_shash *tfm, const u8 *key,
+			    unsigned int keylen)
+{ return 0; }
+
+static int null_skcipher_setkey(struct crypto_skcipher *tfm, const u8 *key,
+				unsigned int keylen)
+{ return 0; }
+
+static int null_skcipher_crypt(struct skcipher_request *req)
+{
+	if (req->src != req->dst)
+		memcpy_sglist(req->dst, req->src, req->cryptlen);
+	return 0;
+}
+
+static struct shash_alg digest_null = {
+	.digestsize		=	NULL_DIGEST_SIZE,
+	.setkey   		=	null_hash_setkey,
+	.init   		=	null_init,
+	.update 		=	null_update,
+	.finup 			=	null_digest,
+	.digest 		=	null_digest,
+	.final  		=	null_final,
+	.base			=	{
+		.cra_name		=	"digest_null",
+		.cra_driver_name	=	"digest_null-generic",
+		.cra_blocksize		=	NULL_BLOCK_SIZE,
+		.cra_module		=	THIS_MODULE,
+	}
+};
+
+static struct skcipher_alg skcipher_null = {
+	.base.cra_name		=	"ecb(cipher_null)",
+	.base.cra_driver_name	=	"ecb-cipher_null",
+	.base.cra_priority	=	100,
+	.base.cra_blocksize	=	NULL_BLOCK_SIZE,
+	.base.cra_ctxsize	=	0,
+	.base.cra_module	=	THIS_MODULE,
+	.min_keysize		=	NULL_KEY_SIZE,
+	.max_keysize		=	NULL_KEY_SIZE,
+	.ivsize			=	NULL_IV_SIZE,
+	.setkey			=	null_skcipher_setkey,
+	.encrypt		=	null_skcipher_crypt,
+	.decrypt		=	null_skcipher_crypt,
+};
+
+MODULE_ALIAS_CRYPTO("digest_null");
+MODULE_ALIAS_CRYPTO("ecb(cipher_null)");
+
+static int __init crypto_null_mod_init(void)
+{
+	int ret = 0;
+
+	ret = crypto_register_shash(&digest_null);
+	if (ret < 0)
+		goto out;
+
+	ret = crypto_register_skcipher(&skcipher_null);
+	if (ret < 0)
+		goto out_unregister_shash;
+
+	return 0;
+
+out_unregister_shash:
+	crypto_unregister_shash(&digest_null);
+out:
+	return ret;
+}
+
+static void __exit crypto_null_mod_fini(void)
+{
+	crypto_unregister_shash(&digest_null);
+	crypto_unregister_skcipher(&skcipher_null);
+}
+
+module_init(crypto_null_mod_init);
+module_exit(crypto_null_mod_fini);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Null Cryptographic Algorithms");