tracing: Add task_prctl_unknown tracepoint

prctl() is a complex syscall which multiplexes its functionality based
on a large set of PR_* options. Currently we count 64 such options. The
return value of unknown options is -EINVAL, and doesn't distinguish from
known options that were passed invalid args that also return -EINVAL.

To understand if programs are attempting to use prctl() options not yet
available on the running kernel, provide the task_prctl_unknown
tracepoint.

Note, this tracepoint is in an unlikely cold path, and would therefore
be suitable for continuous monitoring (e.g. via perf_event_open).

While the above is likely the simplest usecase, additionally this
tracepoint can help unlock some testing scenarios (where probing
sys_enter or sys_exit causes undesirable performance overheads):

  a. unprivileged triggering of a test module: test modules may register a
     probe to be called back on task_prctl_unknown, and pick a very large
     unknown prctl() option upon which they perform a test function for an
     unprivileged user;

  b. unprivileged triggering of an eBPF program function: similar
     as idea (a).

Example trace_pipe output:

  test-380     [001] .....    78.142904: task_prctl_unknown: option=1234 arg2=101 arg3=102 arg4=103 arg5=104

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Link: https://lore.kernel.org/r/20241108113455.2924361-1-elver@google.com
Signed-off-by: Kees Cook <kees@kernel.org>

Author: melver

include/trace/events/task.h

@@ -56,6 +56,43 @@ TRACE_EVENT(task_rename,
 		__entry->newcomm, __entry->oom_score_adj)
 );
 
+/**
+ * task_prctl_unknown - called on unknown prctl() option
+ * @option:	option passed
+ * @arg2:	arg2 passed
+ * @arg3:	arg3 passed
+ * @arg4:	arg4 passed
+ * @arg5:	arg5 passed
+ *
+ * Called on an unknown prctl() option.
+ */
+TRACE_EVENT(task_prctl_unknown,
+
+	TP_PROTO(int option, unsigned long arg2, unsigned long arg3,
+		 unsigned long arg4, unsigned long arg5),
+
+	TP_ARGS(option, arg2, arg3, arg4, arg5),
+
+	TP_STRUCT__entry(
+		__field(	int,		option)
+		__field(	unsigned long,	arg2)
+		__field(	unsigned long,	arg3)
+		__field(	unsigned long,	arg4)
+		__field(	unsigned long,	arg5)
+	),
+
+	TP_fast_assign(
+		__entry->option = option;
+		__entry->arg2 = arg2;
+		__entry->arg3 = arg3;
+		__entry->arg4 = arg4;
+		__entry->arg5 = arg5;
+	),
+
+	TP_printk("option=%d arg2=%ld arg3=%ld arg4=%ld arg5=%ld",
+		  __entry->option, __entry->arg2, __entry->arg3, __entry->arg4, __entry->arg5)
+);
+
 #endif
 
 /* This part must be outside protection */

kernel/sys.c

@@ -75,6 +75,8 @@
 #include <asm/io.h>
 #include <asm/unistd.h>
 
+#include <trace/events/task.h>
+
 #include "uid16.h"
 
 #ifndef SET_UNALIGN_CTL
@@ -2810,6 +2812,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		error = arch_lock_shadow_stack_status(me, arg2);
 		break;
 	default:
+		trace_task_prctl_unknown(option, arg2, arg3, arg4, arg5);
 		error = -EINVAL;
 		break;
 	}