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bpf: Move bpf_dispatcher function out of ftrace locations

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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
This commit is contained in:
Jiri Olsa 2022-09-03 15:11:54 +02:00 committed by Daniel Borkmann
parent 9440155ccb
commit ceea991a01
2 changed files with 8 additions and 0 deletions
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1
arch/x86/Kconfig
View file

@ -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

7
include/linux/bpf.h
View file

@ -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, \