If for some reason the trace_marker write does not have a nul byte for the
string, it will overflow the print:

  trace_seq_printf(s, ": %s", field->buf);

The field->buf could be missing the nul byte. To prevent overflow, add the
max size that the buf can be by using the event size and the field
location.

  int max = iter->ent_size - offsetof(struct print_entry, buf);

  trace_seq_printf(s, ": %*.s", max, field->buf);

Link: https://lore.kernel.org/linux-trace-kernel/20231212084444.4619b8ce@gandalf.local.home

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>