tracing/probes: Prevent out-of-bounds write in __trace_probe_log_err()

commit a9d6fb2 upstream.

If trace_probe_log.argc is 0 in __trace_probe_log_err(), the loop
constructing the command string will not execute and p will remain equal to
command. Writing to *(p - 1) will cause an out-of-bounds access before
command. This should not happen, but better to be treated.

Reject if trace_probe_log.argc is 0.

Link: https://lore.kernel.org/all/178454233992.290363.18323091580600697731.stgit@devnote2/

Fixes: ab105a4 ("tracing: Use tracing error_log with probe events")
Cc: stable@vger.kernel.org
Assisted-by: Antigravity:gemini-3.5-flash
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: mhiramat

kernel/trace/trace_probe.c

@@ -186,7 +186,7 @@ void __trace_probe_log_err(int offset, int err_type)
 
 	lockdep_assert_held(&dyn_event_ops_mutex);
 
-	if (!trace_probe_log.argv)
+	if (!trace_probe_log.argv || !trace_probe_log.argc)
 		return;
 
 	/* Recalculate the length and allocate buffer */