tracing/osnoise: Allow arbitrarily long CPU string

Allocate kernel memory for processing CPU string
(/sys/kernel/tracing/osnoise/cpus) also in osnoise_cpus_write to allow
the writing of a CPU string of an arbitrary length.

This replaces the 256-byte buffer, which is insufficient with the rising
number of CPUs. For example, if I wanted to measure on every even CPU
on a system with 256 CPUs, the string would be 456 characters long.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20250425091839.343289-1-tglozar@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: lenticularis39

kernel/trace/trace_osnoise.c

@@ -2302,7 +2302,7 @@ osnoise_cpus_read(struct file *filp, char __user *ubuf, size_t count,
  * osnoise_cpus_write - Write function for "cpus" entry
  * @filp: The active open file structure
  * @ubuf: The user buffer that contains the value to write
- * @cnt: The maximum number of bytes to write to "file"
+ * @count: The maximum number of bytes to write to "file"
  * @ppos: The current position in @file
  *
  * This function provides a write implementation for the "cpus"
@@ -2320,10 +2320,11 @@ osnoise_cpus_write(struct file *filp, const char __user *ubuf, size_t count,
 {
 	cpumask_var_t osnoise_cpumask_new;
 	int running, err;
-	char buf[256];
+	char *buf __free(kfree) = NULL;
 
-	if (count >= 256)
-		return -EINVAL;
+	buf = kmalloc(count, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
 
 	if (copy_from_user(buf, ubuf, count))
 		return -EFAULT;