libbpf: Fix no-args func prototype BTF dumping syntax

[ Upstream commit 189f1a976e426011e6a5588f1d3ceedf71fe2965 ]

For all these years libbpf's BTF dumper has been emitting not strictly
valid syntax for function prototypes that have no input arguments.

Instead of `int (*blah)()` we should emit `int (*blah)(void)`.

This is not normally a problem, but it manifests when we get kfuncs in
vmlinux.h that have no input arguments. Due to compiler internal
specifics, we get no BTF information for such kfuncs, if they are not
declared with proper `(void)`.

The fix is trivial. We also need to adjust a few ancient tests that
happily assumed `()` is correct.

Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion")
Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/bpf/20240712224442.282823-1-andrii@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>

2 files changed, 4 insertions, 4 deletions

diff --git a/tools/testing/selftests/bpf/progs/btf_dump_test_case_multidim.c b/tools/testing/selftests/bpf/progs/btf_dump_test_case_multidim.c
index ba97165bdb28..a657651eba52 100644
--- a/tools/testing/selftests/bpf/progs/btf_dump_test_case_multidim.c
+++ b/tools/testing/selftests/bpf/progs/btf_dump_test_case_multidim.c
@@ -14,9 +14,9 @@ typedef int *ptr_arr_t[6];
 
 typedef int *ptr_multiarr_t[7][8][9][10];
 
-typedef int * (*fn_ptr_arr_t[11])();
+typedef int * (*fn_ptr_arr_t[11])(void);
 
-typedef int * (*fn_ptr_multiarr_t[12][13])();
+typedef int * (*fn_ptr_multiarr_t[12][13])(void);
 
 struct root_struct {
 	arr_t _1;
diff --git a/tools/testing/selftests/bpf/progs/btf_dump_test_case_syntax.c b/tools/testing/selftests/bpf/progs/btf_dump_test_case_syntax.c
index fe43556e1a61..956b24ce8145 100644
--- a/tools/testing/selftests/bpf/progs/btf_dump_test_case_syntax.c
+++ b/tools/testing/selftests/bpf/progs/btf_dump_test_case_syntax.c
@@ -67,7 +67,7 @@ typedef void (*printf_fn_t)(const char *, ...);
  *   `int -> char *` function and returns pointer to a char. Equivalent:
  *   typedef char * (*fn_input_t)(int);
  *   typedef char * (*fn_output_outer_t)(fn_input_t);
- *   typedef const fn_output_outer_t (* fn_output_inner_t)();
+ *   typedef const fn_output_outer_t (* fn_output_inner_t)(void);
  *   typedef const fn_output_inner_t fn_ptr_arr2_t[5];
  */
 /* ----- START-EXPECTED-OUTPUT ----- */
@@ -94,7 +94,7 @@ typedef void (* (*signal_t)(int, void (*)(int)))(int);
 
 typedef char * (*fn_ptr_arr1_t[10])(int **);
 
-typedef char * (* (* const fn_ptr_arr2_t[5])())(char * (*)(int));
+typedef char * (* (* const fn_ptr_arr2_t[5])(void))(char * (*)(int));
 
 struct struct_w_typedefs {
 	int_t a;