bpf: Revert ("Fix dispatcher patchable function entry to 5 bytes nop")

Because __attribute__((patchable_function_entry)) is only available
since GCC-8 this solution fails to build on the minimum required GCC
version.

Undo these changes so we might try again -- without cluttering up the
patches with too many changes.

This is an almost complete revert of:

  dbe69b29 ("bpf: Fix dispatcher patchable function entry to 5 bytes nop")
  ceea991a ("bpf: Move bpf_dispatcher function out of ftrace locations")

(notably the arch/x86/Kconfig hunk is kept).
Reported-by: David Laight <David.Laight@aculab.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@kernel.org>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lkml.kernel.org/r/439d8dc735bb4858875377df67f1b29a@AcuMS.aculab.com
Link: https://lore.kernel.org/bpf/20221103120647.728830733@infradead.org

arch/x86/net/bpf_jit_comp.c

@@ -11,7 +11,6 @@
 #include <linux/bpf.h>
 #include <linux/memory.h>
 #include <linux/sort.h>
-#include <linux/init.h>
 #include <asm/extable.h>
 #include <asm/set_memory.h>
 #include <asm/nospec-branch.h>
@@ -389,18 +388,6 @@ static int __bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
 	return ret;
 }
 
-int __init bpf_arch_init_dispatcher_early(void *ip)
-{
-	const u8 *nop_insn = x86_nops[5];
-
-	if (is_endbr(*(u32 *)ip))
-		ip += ENDBR_INSN_SIZE;
-
-	if (memcmp(ip, nop_insn, X86_PATCH_SIZE))
-		text_poke_early(ip, nop_insn, X86_PATCH_SIZE);
-	return 0;
-}
-
 int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
 		       void *old_addr, void *new_addr)
 {

include/linux/bpf.h

@@ -27,7 +27,6 @@
 #include <linux/bpfptr.h>
 #include <linux/btf.h>
 #include <linux/rcupdate_trace.h>
-#include <linux/init.h>
 
 struct bpf_verifier_env;
 struct bpf_verifier_log;
@@ -971,8 +970,6 @@ struct bpf_trampoline *bpf_trampoline_get(u64 key,
 					  struct bpf_attach_target_info *tgt_info);
 void bpf_trampoline_put(struct bpf_trampoline *tr);
 int arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int num_funcs);
-int __init bpf_arch_init_dispatcher_early(void *ip);
-
 #define BPF_DISPATCHER_INIT(_name) {				\
 	.mutex = __MUTEX_INITIALIZER(_name.mutex),		\
 	.func = &_name##_func,					\
@@ -986,21 +983,7 @@ int __init bpf_arch_init_dispatcher_early(void *ip);
 	},							\
 }
 
-#define BPF_DISPATCHER_INIT_CALL(_name)					\
-	static int __init _name##_init(void)				\
-	{								\
-		return bpf_arch_init_dispatcher_early(_name##_func);	\
-	}								\
-	early_initcall(_name##_init)
-
-#ifdef CONFIG_X86_64
-#define BPF_DISPATCHER_ATTRIBUTES __attribute__((patchable_function_entry(5)))
-#else
-#define BPF_DISPATCHER_ATTRIBUTES
-#endif
-
 #define DEFINE_BPF_DISPATCHER(name)					\
-	notrace BPF_DISPATCHER_ATTRIBUTES				\
 	noinline __nocfi unsigned int bpf_dispatcher_##name##_func(	\
 		const void *ctx,					\
 		const struct bpf_insn *insnsi,				\
@@ -1010,9 +993,7 @@ int __init bpf_arch_init_dispatcher_early(void *ip);
 	}								\
 	EXPORT_SYMBOL(bpf_dispatcher_##name##_func);			\
 	struct bpf_dispatcher bpf_dispatcher_##name =			\
-		BPF_DISPATCHER_INIT(bpf_dispatcher_##name);		\
-	BPF_DISPATCHER_INIT_CALL(bpf_dispatcher_##name);
-
+		BPF_DISPATCHER_INIT(bpf_dispatcher_##name);
 #define DECLARE_BPF_DISPATCHER(name)					\
 	unsigned int bpf_dispatcher_##name##_func(			\
 		const void *ctx,					\

kernel/bpf/dispatcher.c

@@ -4,7 +4,6 @@
 #include <linux/hash.h>
 #include <linux/bpf.h>
 #include <linux/filter.h>
-#include <linux/init.h>
 
 /* The BPF dispatcher is a multiway branch code generator. The
  * dispatcher is a mechanism to avoid the performance penalty of an
@@ -91,11 +90,6 @@ int __weak arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int n
 	return -ENOTSUPP;
 }
 
-int __weak __init bpf_arch_init_dispatcher_early(void *ip)
-{
-	return -ENOTSUPP;
-}
-
 static int bpf_dispatcher_prepare(struct bpf_dispatcher *d, void *image, void *buf)
 {
 	s64 ips[BPF_DISPATCHER_MAX] = {}, *ipsp = &ips[0];