selftests/bpf: Fix BPF_KRETPROBE macro and use it in attach_probe test

For kretprobes, there is no point in capturing input arguments from pt_regs,
as they are going to be, most probably, clobbered by the time probed kernel
function returns. So switch BPF_KRETPROBE to accept zero or one argument
(optional return result).

Fixes: ac065870d9 ("selftests/bpf: Add BPF_PROG, BPF_KPROBE, and BPF_KRETPROBE macros")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200229231112.1240137-4-andriin@fb.com

tools/testing/selftests/bpf/bpf_trace_helpers.h

@@ -96,15 +96,16 @@ typeof(name(0)) name(struct pt_regs *ctx)				    \
 static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
 
 #define ___bpf_kretprobe_args0() ctx
-#define ___bpf_kretprobe_argsN(x, args...) \
-	___bpf_kprobe_args(args), (void *)PT_REGS_RET(ctx)
+#define ___bpf_kretprobe_args1(x) \
+	___bpf_kretprobe_args0(), (void *)PT_REGS_RET(ctx)
 #define ___bpf_kretprobe_args(args...) \
-	___bpf_apply(___bpf_kretprobe_args, ___bpf_empty(args))(args)
+	___bpf_apply(___bpf_kretprobe_args, ___bpf_narg(args))(args)
 
 /*
- * BPF_KRETPROBE is similar to BPF_KPROBE, except, in addition to listing all
- * input kprobe arguments, one last extra argument has to be specified, which
- * captures kprobe return value.
+ * BPF_KRETPROBE is similar to BPF_KPROBE, except, it only provides optional
+ * return value (in addition to `struct pt_regs *ctx`), but no input
+ * arguments, because they will be clobbered by the time probed function
+ * returns.
  */
 #define BPF_KRETPROBE(name, args...)					    \
 name(struct pt_regs *ctx);						    \

tools/testing/selftests/bpf/progs/test_attach_probe.c

@@ -4,6 +4,7 @@
 #include <linux/ptrace.h>
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
+#include "bpf_trace_helpers.h"
 
 int kprobe_res = 0;
 int kretprobe_res = 0;
@@ -18,7 +19,7 @@ int handle_kprobe(struct pt_regs *ctx)
 }
 
 SEC("kretprobe/sys_nanosleep")
-int handle_kretprobe(struct pt_regs *ctx)
+int BPF_KRETPROBE(handle_kretprobe)
 {
 	kretprobe_res = 2;
 	return 0;

tools/testing/selftests/bpf/progs/test_overhead.c

@@ -17,11 +17,9 @@ int BPF_KPROBE(prog1, struct task_struct *tsk, const char *buf, bool exec)
 }
 
 SEC("kretprobe/__set_task_comm")
-int BPF_KRETPROBE(prog2,
-		  struct task_struct *tsk, const char *buf, bool exec,
-		  int ret)
+int BPF_KRETPROBE(prog2, int ret)
 {
-	return !PT_REGS_PARM1(ctx) && ret;
+	return ret;
 }
 
 SEC("raw_tp/task_rename")