ftrace: Synchronize the initialization of ftrace_opsgrafted

There's some internal state that ftrace_ops needs to have set, but since
it can be declared outside of the ftrace.c code, it calls
ftrace_ops_init() on the ops in every global function. The issue is that
if two tasks call it on the same ops at the same time it is possible to
have the initialization of one corrupt the initialization of the other
call.

Create a ops_mutex to use to synchronize every initialization of the
ftrace_ops. The mutex is taken within checking the ftrace_ops flag that
states it was initializied but the flag is checked again after the mutex
has been taken. Checking first outside the mutex allows it to shortcut
having to take the mutex. But then the check needs to be done again after
the mute is taken in case of races.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260902095501.6b59af20@gandalf.local.home
Fixes: f04f24fb7e48d ("ftrace, kprobes: Fix a deadlock on ftrace_regex_lock")
Reported-by: sashiko-bot@kernel.org
Close: https://lore.kernel.org/all/20260829025528.49A831F000E9@smtp.kernel.org/
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 81 insertions, 0 deletions

diff --git a/drivers/md/dm-zero.c b/drivers/md/dm-zero.c
new file mode 100644
index 000000000..9a0bb623e
--- /dev/null
+++ b/drivers/md/dm-zero.c
@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2003 Jana Saout <jana@saout.de>
+ *
+ * This file is released under the GPL.
+ */
+
+#include <linux/device-mapper.h>
+
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/bio.h>
+
+#define DM_MSG_PREFIX "zero"
+
+/*
+ * Construct a dummy mapping that only returns zeros
+ */
+static int zero_ctr(struct dm_target *ti, unsigned int argc, char **argv)
+{
+	if (argc != 0) {
+		ti->error = "No arguments required";
+		return -EINVAL;
+	}
+
+	/*
+	 * Silently drop discards, avoiding -EOPNOTSUPP.
+	 */
+	ti->num_discard_bios = 1;
+	ti->discards_supported = true;
+
+	return 0;
+}
+
+/*
+ * Return zeros only on reads
+ */
+static int zero_map(struct dm_target *ti, struct bio *bio)
+{
+	switch (bio_op(bio)) {
+	case REQ_OP_READ:
+		if (bio->bi_opf & REQ_RAHEAD) {
+			/* readahead of null bytes only wastes buffer cache */
+			return DM_MAPIO_KILL;
+		}
+		zero_fill_bio(bio);
+		break;
+	case REQ_OP_WRITE:
+	case REQ_OP_DISCARD:
+		/* writes get silently dropped */
+		break;
+	default:
+		return DM_MAPIO_KILL;
+	}
+
+	bio_endio(bio);
+
+	/* accepted bio, don't make new request */
+	return DM_MAPIO_SUBMITTED;
+}
+
+static void zero_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	limits->max_hw_discard_sectors = UINT_MAX;
+	limits->discard_granularity = 512;
+}
+
+static struct target_type zero_target = {
+	.name   = "zero",
+	.version = {1, 2, 0},
+	.features = DM_TARGET_NOWAIT,
+	.module = THIS_MODULE,
+	.ctr    = zero_ctr,
+	.map    = zero_map,
+	.io_hints = zero_io_hints,
+};
+module_dm(zero);
+
+MODULE_AUTHOR("Jana Saout <jana@saout.de>");
+MODULE_DESCRIPTION(DM_NAME " dummy target returning zeros");
+MODULE_LICENSE("GPL");