Date:   Mon, 31 Oct 2022 16:21:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     David Laight <David.Laight@...lab.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Linux 6.1-rc3 build fail in include/linux/bpf.h

On Mon, Oct 31, 2022 at 01:17:16PM +0100, Jiri Olsa wrote:
> On Mon, Oct 31, 2022 at 11:14:31AM +0000, David Laight wrote:
> > The 6.1-rc3 sources fail to build because bpf.h unconditionally
> > #define BPF_DISPATCHER_ATTRIBUTES __attribute__((patchable_function_entry(5)))
> > for X86_64 builds.
> > 
> > I'm pretty sure that should depend on some other options
> > since the compiler isn't required to support it.
> > (The gcc 7.5.0 on my Ubunti 18.04 system certainly doesn't)
> > 
> > The only other reference to that attribute is in the definition
> > of 'notrace' in compiler.h.
> 
> I guess we need to make some __has_attribute check and make all that conditional
> 
> cc-ing Peter

Does something crazy like the below work? It compiles but is otherwise
totally untested.

---
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 9e7d46d16032..7d7a00306d19 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -953,6 +953,10 @@ struct bpf_dispatcher {
 	void *rw_image;
 	u32 image_off;
 	struct bpf_ksym ksym;
+#ifdef CONFIG_HAVE_STATIC_CALL
+	struct static_call_key *sc_key;
+	void *sc_tramp;
+#endif
 };
 
 static __always_inline __nocfi unsigned int bpf_dispatcher_nop_func(
@@ -970,6 +974,20 @@ 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);
+
+
+#ifdef CONFIG_HAVE_STATIC_CALL
+#define BPF_DISPATCH_CALL(name)	static_call(bpf_dispatcher_##name##_call)(ctx, insnsi, bpf_func)
+
+#define __BPF_DISPATCHER_SC_INIT(_name)				\
+	.sc_key = &STATIC_CALL_KEY(_name),			\
+	.sc_tramp = STATIC_CALL_TRAMP_ADDR(_name),
+
+#else
+#define BPF_DISPATCH_CALL(name)	bpf_func(ctx, insnsi)
+#define __BPF_DISPATCHER_SC_INIT(name)
+#endif
+
 #define BPF_DISPATCHER_INIT(_name) {				\
 	.mutex = __MUTEX_INITIALIZER(_name.mutex),		\
 	.func = &_name##_func,					\
@@ -981,32 +999,29 @@ int arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int num_func
 		.name  = #_name,				\
 		.lnode = LIST_HEAD_INIT(_name.ksym.lnode),	\
 	},							\
+	__BPF_DISPATCHER_SC_INIT(_name##_call)			\
 }
 
-#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				\
+	DEFINE_STATIC_CALL(bpf_dispatcher_##name##_call, bpf_dispatcher_nop_func); \
 	noinline __nocfi unsigned int bpf_dispatcher_##name##_func(	\
 		const void *ctx,					\
 		const struct bpf_insn *insnsi,				\
 		bpf_func_t bpf_func)					\
 	{								\
-		return bpf_func(ctx, insnsi);				\
+		return BPF_DISPATCH_CALL(name);				\
 	}								\
 	EXPORT_SYMBOL(bpf_dispatcher_##name##_func);			\
 	struct bpf_dispatcher bpf_dispatcher_##name =			\
 		BPF_DISPATCHER_INIT(bpf_dispatcher_##name);
+
 #define DECLARE_BPF_DISPATCHER(name)					\
 	unsigned int bpf_dispatcher_##name##_func(			\
 		const void *ctx,					\
 		const struct bpf_insn *insnsi,				\
 		bpf_func_t bpf_func);					\
 	extern struct bpf_dispatcher bpf_dispatcher_##name;
+
 #define BPF_DISPATCHER_FUNC(name) bpf_dispatcher_##name##_func
 #define BPF_DISPATCHER_PTR(name) (&bpf_dispatcher_##name)
 void bpf_dispatcher_change_prog(struct bpf_dispatcher *d, struct bpf_prog *from,
diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c
index fa64b80b8bca..1ca8bd6da6bb 100644
--- a/kernel/bpf/dispatcher.c
+++ b/kernel/bpf/dispatcher.c
@@ -4,6 +4,7 @@
 #include <linux/hash.h>
 #include <linux/bpf.h>
 #include <linux/filter.h>
+#include <linux/static_call.h>
 
 /* The BPF dispatcher is a multiway branch code generator. The
  * dispatcher is a mechanism to avoid the performance penalty of an
@@ -106,7 +107,6 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
 {
 	void *old, *new, *tmp;
 	u32 noff;
-	int err;
 
 	if (!prev_num_progs) {
 		old = NULL;
@@ -128,11 +128,10 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
 			return;
 	}
 
-	err = bpf_arch_text_poke(d->func, BPF_MOD_JUMP, old, new);
-	if (err || !new)
-		return;
+	__static_call_update(d->sc_key, d->sc_tramp, new ?: &bpf_dispatcher_nop_func);
 
-	d->image_off = noff;
+	if (new)
+		d->image_off = noff;
 }
 
 void bpf_dispatcher_change_prog(struct bpf_dispatcher *d, struct bpf_prog *from,

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