bpf: Move iterator functions into special init section

With upcoming changes to pahole [0], that change the way how and
which kernel functions are stored in BTF data, we need a way to
recognize iterator functions.

Iterator functions need to be in BTF data, but have no real body
and are currently placed in .init.text section, so they are freed
after kernel init and are filtered out of BTF data because of that.

The solution is to place these functions under new section:

  .init.bpf.preserve_type

And add 2 new symbols to mark that area:

  __init_bpf_preserve_type_begin
  __init_bpf_preserve_type_end

The code in pahole responsible for picking up the functions will be
able to recognize functions from this section and add them to the
BTF data and filter out all other .init.text functions.

  [0] 5a22c2de79fb ("btf_encoder: Change functions check due to broken dwarf")

Suggested-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201110154017.482352-1-jolsa@kernel.org

Author: olsajiri

include/asm-generic/vmlinux.lds.h

@@ -685,8 +685,21 @@
 	.BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) {			\
 		*(.BTF_ids)						\
 	}
+
+/*
+ * .init.bpf.preserve_type
+ *
+ * This section stores special BPF functions and marks them
+ * with begin/end symbols pair for the sake of pahole tool.
+ */
+#define INIT_BPF_PRESERVE_TYPE						\
+	__init_bpf_preserve_type_begin = .;                             \
+	*(.init.bpf.preserve_type)                                      \
+	__init_bpf_preserve_type_end = .;				\
+	MEM_DISCARD(init.bpf.preserve_type)
 #else
 #define BTF
+#define INIT_BPF_PRESERVE_TYPE
 #endif
 
 /*
@@ -741,7 +754,8 @@
 #define INIT_TEXT							\
 	*(.init.text .init.text.*)					\
 	*(.text.startup)						\
-	MEM_DISCARD(init.text*)
+	MEM_DISCARD(init.text*)						\
+	INIT_BPF_PRESERVE_TYPE
 
 #define EXIT_DATA							\
 	*(.exit.data .exit.data.*)					\

include/linux/bpf.h

@@ -1276,10 +1276,20 @@ struct bpf_link *bpf_link_get_from_fd(u32 ufd);
 int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
 int bpf_obj_get_user(const char __user *pathname, int flags);
 
+/* In case we generate BTF data, we need to group all iterator
+ * functions into special init section, so pahole can track them.
+ * Otherwise pure __init section is enough.
+ */
+#ifdef CONFIG_DEBUG_INFO_BTF
+#define __init_bpf_preserve_type __section(".init.bpf.preserve_type")
+#else
+#define __init_bpf_preserve_type __init
+#endif
+
 #define BPF_ITER_FUNC_PREFIX "bpf_iter_"
 #define DEFINE_BPF_ITER_FUNC(target, args...)			\
 	extern int bpf_iter_ ## target(args);			\
-	int __init bpf_iter_ ## target(args) { return 0; }
+	int __init_bpf_preserve_type bpf_iter_ ## target(args) { return 0; }
 
 struct bpf_iter_aux_info {
 	struct bpf_map *map;