bpf: Move bpf_dispatcher function out of ftrace locations

The dispatcher function is attached/detached to trampoline by
dispatcher update function. At the same time it's available as
ftrace attachable function.

After discussion [1] the proposed solution is to use compiler
attributes to alter bpf_dispatcher_##name##_func function:

  - remove it from being instrumented with __no_instrument_function__
    attribute, so ftrace has no track of it

  - but still generate 5 nop instructions with patchable_function_entry(5)
    attribute, which are expected by bpf_arch_text_poke used by
    dispatcher update function

Enabling HAVE_DYNAMIC_FTRACE_NO_PATCHABLE option for x86, so
__patchable_function_entries functions are not part of ftrace/mcount
locations.

Adding attributes to bpf_dispatcher_XXX function on x86_64 so it's
kept out of ftrace locations and has 5 byte nop generated at entry.

These attributes need to be arch specific as pointed out by Ilya
Leoshkevic in here [2].

The dispatcher image is generated only for x86_64 arch, so the
code can stay as is for other archs.

  [1] https://lore.kernel.org/bpf/20220722110811.124515-1-jolsa@kernel.org/
  [2] https://lore.kernel.org/bpf/969a14281a7791c334d476825863ee449964dd0c.camel@linux.ibm.com/

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/bpf/20220903131154.420467-3-jolsa@kernel.org

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f9920f1..089c20c 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -284,6 +284,7 @@ config X86
        select PROC_PID_ARCH_STATUS             if PROC_FS
        select HAVE_ARCH_NODE_DEV_GROUP         if X86_SGX
        imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI
+       select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
 
 config INSTRUCTION_DECODER
        def_bool y

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 54178b9..e0dbe0c 100644 (file)
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -977,7 +977,14 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs);
        },                                                      \
 }
 
+#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,                          \