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, 129 insertions, 0 deletions

diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
new file mode 100644
index 000000000..4a6574b67
--- /dev/null
+++ b/lib/strnlen_user.c
@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/kernel.h>
+#include <linux/export.h>
+#include <linux/uaccess.h>
+#include <linux/mm.h>
+#include <linux/bitops.h>
+
+#include <asm/word-at-a-time.h>
+
+/*
+ * Do a strnlen, return length of string *with* final '\0'.
+ * 'count' is the user-supplied count, while 'max' is the
+ * address space maximum.
+ *
+ * Return 0 for exceptions (which includes hitting the address
+ * space maximum), or 'count+1' if hitting the user-supplied
+ * maximum count.
+ *
+ * NOTE! We can sometimes overshoot the user-supplied maximum
+ * if it fits in a aligned 'long'. The caller needs to check
+ * the return value against "> max".
+ */
+static __always_inline long do_strnlen_user(const char __user *src, unsigned long count, unsigned long max)
+{
+	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
+	unsigned long align, res = 0;
+	unsigned long c;
+
+	/*
+	 * Do everything aligned. But that means that we
+	 * need to also expand the maximum..
+	 */
+	align = (sizeof(unsigned long) - 1) & (unsigned long)src;
+	src -= align;
+	max += align;
+
+	unsafe_get_user(c, (unsigned long __user *)src, efault);
+	c |= aligned_byte_mask(align);
+
+	for (;;) {
+		unsigned long data;
+		if (has_zero(c, &data, &constants)) {
+			data = prep_zero_mask(c, data, &constants);
+			data = create_zero_mask(data);
+			return res + find_zero(data) + 1 - align;
+		}
+		res += sizeof(unsigned long);
+		/* We already handled 'unsigned long' bytes. Did we do it all ? */
+		if (unlikely(max <= sizeof(unsigned long)))
+			break;
+		max -= sizeof(unsigned long);
+		unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
+	}
+	res -= align;
+
+	/*
+	 * Uhhuh. We hit 'max'. But was that the user-specified maximum
+	 * too? If so, return the marker for "too long".
+	 */
+	if (res >= count)
+		return count+1;
+
+	/*
+	 * Nope: we hit the address space limit, and we still had more
+	 * characters the caller would have wanted. That's 0.
+	 */
+efault:
+	return 0;
+}
+
+/**
+ * strnlen_user: - Get the size of a user string INCLUDING final NUL.
+ * @str: The string to measure.
+ * @count: Maximum count (including NUL character)
+ *
+ * Context: User context only. This function may sleep if pagefaults are
+ *          enabled.
+ *
+ * Get the size of a NUL-terminated string in user space.
+ *
+ * Returns the size of the string INCLUDING the terminating NUL.
+ * If the string is too long, returns a number larger than @count. User
+ * has to check the return value against "> count".
+ * On exception (or invalid count), returns 0.
+ *
+ * NOTE! You should basically never use this function. There is
+ * almost never any valid case for using the length of a user space
+ * string, since the string can be changed at any time by other
+ * threads. Use "strncpy_from_user()" instead to get a stable copy
+ * of the string.
+ */
+long strnlen_user(const char __user *str, long count)
+{
+	unsigned long max_addr, src_addr;
+
+	if (unlikely(count <= 0))
+		return 0;
+
+	if (can_do_masked_user_access()) {
+		long retval;
+
+		str = masked_user_read_access_begin(str);
+		retval = do_strnlen_user(str, count, count);
+		user_read_access_end();
+		return retval;
+	}
+
+	max_addr = TASK_SIZE_MAX;
+	src_addr = (unsigned long)untagged_addr(str);
+	if (likely(src_addr < max_addr)) {
+		unsigned long max = max_addr - src_addr;
+		long retval;
+
+		/*
+		 * Truncate 'max' to the user-specified limit, so that
+		 * we only have one limit we need to check in the loop
+		 */
+		if (max > count)
+			max = count;
+
+		if (user_read_access_begin(str, max)) {
+			retval = do_strnlen_user(str, count, max);
+			user_read_access_end();
+			return retval;
+		}
+	}
+	return 0;
+}
+EXPORT_SYMBOL(strnlen_user);