From: Peter Zijlstra peterz@infradead.org

[ Upstream commit c86df29d11dfba27c0a1f5039cd6fe387fbf4239 ]

The dispatcher function is currently abusing the ftrace __fentry__ call location for its own purposes -- this obviously gives trouble when the dispatcher and ftrace are both in use.

A previous solution tried using __attribute__((patchable_function_entry())) which works, except it is GCC-8+ only, breaking the build on the earlier still supported compilers. Instead use static_call() -- which has its own annotations and does not conflict with ftrace -- to rewrite the dispatch function.

By using: return static_call()(ctx, insni, bpf_func) you get a perfect forwarding tail call as function body (iow a single jmp instruction). By having the default static_call() target be bpf_dispatcher_nop_func() it retains the default behaviour (an indirect call to the argument function). Only once a dispatcher program is attached is the target rewritten to directly call the JIT'ed image.

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/Y1/oBlK0yFk5c/Im@hirez.programming.kicks-ass.net Link: https://lore.kernel.org/bpf/20221103120647.796772565@infradead.org Signed-off-by: Vasiliy Kovalev kovalev@altlinux.org --- include/linux/bpf.h | 39 ++++++++++++++++++++++++++++++++++++++- kernel/bpf/dispatcher.c | 22 ++++++++-------------- 2 files changed, 46 insertions(+), 15 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 00c615fc8ec3c3..633b8842e617e8 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -23,6 +23,7 @@ #include <linux/slab.h> #include <linux/percpu-refcount.h> #include <linux/bpfptr.h> +#include <linux/static_call.h>

struct bpf_verifier_env; struct bpf_verifier_log; @@ -765,6 +766,10 @@ struct bpf_dispatcher { void *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( @@ -782,6 +787,34 @@ 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, s64 *funcs, int num_funcs); + +/* + * When the architecture supports STATIC_CALL replace the bpf_dispatcher_fn + * indirection with a direct call to the bpf program. If the architecture does + * not have STATIC_CALL, avoid a double-indirection. + */ +#ifdef CONFIG_HAVE_STATIC_CALL + +#define __BPF_DISPATCHER_SC_INIT(_name) \ + .sc_key = &STATIC_CALL_KEY(_name), \ + .sc_tramp = STATIC_CALL_TRAMP_ADDR(_name), + +#define __BPF_DISPATCHER_SC(name) \ + DEFINE_STATIC_CALL(bpf_dispatcher_##name##_call, bpf_dispatcher_nop_func) + +#define __BPF_DISPATCHER_CALL(name) \ + static_call(bpf_dispatcher_##name##_call)(ctx, insnsi, bpf_func) + +#define __BPF_DISPATCHER_UPDATE(_d, _new) \ + __static_call_update((_d)->sc_key, (_d)->sc_tramp, (_new)) + +#else +#define __BPF_DISPATCHER_SC_INIT(name) +#define __BPF_DISPATCHER_SC(name) +#define __BPF_DISPATCHER_CALL(name) bpf_func(ctx, insnsi) +#define __BPF_DISPATCHER_UPDATE(_d, _new) +#endif + #define BPF_DISPATCHER_INIT(_name) { \ .mutex = __MUTEX_INITIALIZER(_name.mutex), \ .func = &_name##_func, \ @@ -793,20 +826,23 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs); .name = #_name, \ .lnode = LIST_HEAD_INIT(_name.ksym.lnode), \ }, \ + __BPF_DISPATCHER_SC_INIT(_name##_call) \ }

#define DEFINE_BPF_DISPATCHER(name) \ + __BPF_DISPATCHER_SC(name); \ noinline __nocfi unsigned int bpf_dispatcher_##name##_func( \ const void *ctx, \ const struct bpf_insn *insnsi, \ unsigned int (*bpf_func)(const void *, \ const struct bpf_insn *)) \ { \ - return bpf_func(ctx, insnsi); \ + return __BPF_DISPATCHER_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, \ @@ -814,6 +850,7 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs); unsigned int (*bpf_func)(const void *, \ const struct bpf_insn *)); \ 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 2444bd15cc2d03..23042cfb5e809b 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 @@ -104,17 +105,11 @@ static int bpf_dispatcher_prepare(struct bpf_dispatcher *d, void *image)

static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs) { - void *old, *new; - u32 noff; - int err; - - if (!prev_num_progs) { - old = NULL; - noff = 0; - } else { - old = d->image + d->image_off; + void *new; + u32 noff = 0; + + if (prev_num_progs) noff = d->image_off ^ (PAGE_SIZE / 2); - }

new = d->num_progs ? d->image + noff : NULL; if (new) { @@ -122,11 +117,10 @@ 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; + __BPF_DISPATCHER_UPDATE(d, 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,