tracing: eprobe: read the complete FILTER_PTR_STRING pointer

For a char * element in an event, the FILTER_PTR_STRING filter type is
used. When the event occurs, a pointer is stored in the ringbuffer.

If an eprobe references such a char * element of a "base event", the
stored pointer is truncated when it's read from the ringbuffer.

$ cd /sys/kernel/tracing
$ echo 'e rcu.rcu_utilization $s:x64 $s:string' > dynamic_events
$ echo 1 > tracing_on
$ echo 1 > events/eprobes/enable
$ sleep 1
$ echo 0 > events/eprobes/enable
$ cat trace
   <idle>-0  ...: (rcu.rcu_utilization) arg1=0x4f arg2=(fault)
   <idle>-0  ...: (rcu.rcu_utilization) arg1=0x2 arg2=(fault)

The problem is in get_event_field

	val = (unsigned long)(*(char *)addr);

addr points to the position in the ringbuffer where the pointer was
stored. The assignment reads only the lowest byte of the pointer.

Fix the cast to read the whole pointer. The output of the test above
is now

<idle>-0 ... arg1=0xffffffff81c7d3f3 arg2="Start scheduler-tick"
<idle>-0 ... arg1=0xffffffff81c57340 arg2="End scheduler-tick"

Link: https://lore.kernel.org/all/20260620145339.3234726-1-martin@kaiser.cx/

Fixes: f04dec9346 ("tracing/eprobes: Fix reading of string fields")
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

kernel/trace/trace_eprobe.c

@@ -315,7 +315,7 @@ get_event_field(struct fetch_insn *code, void *rec)
 			val = (unsigned long)addr;
 			break;
 		case FILTER_PTR_STRING:
-			val = (unsigned long)(*(char *)addr);
+			val = *(unsigned long *)addr;
 			break;
 		default:
 			WARN_ON_ONCE(1);