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 pointer 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@xxxxxxxxxx/
[2] https://lore.kernel.org/bpf/969a14281a7791c334d476825863ee449964dd0c.camel@xxxxxxxxxxxxx/
Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
arch/x86/Kconfig | 1 +
include/linux/bpf.h | 8 ++++++++
2 files changed, 9 insertions(+)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f9920f1341c8..089c20cefd2b 100644
--- 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 9c1674973e03..4ab4b0a1beb8 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -924,7 +924,15 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs);
}, \
}
+#ifdef CONFIG_X86_64
+#define BPF_DISPATCHER_ATTRIBUTES __attribute__((__no_instrument_function__)) \
+ __attribute__((patchable_function_entry(5)))
+#else
+#define BPF_DISPATCHER_ATTRIBUTES
+#endif
+
#define DEFINE_BPF_DISPATCHER(name) \
+ BPF_DISPATCHER_ATTRIBUTES \
noinline __nocfi unsigned int bpf_dispatcher_##name##_func( \
const void *ctx, \
const struct bpf_insn *insnsi, \
--
2.37.2
