linux-kernel - Re: [PATCH bpf] bpf: Mark bpf_arch_init_dispatcher_early() as __init_or

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Date:   Tue, 1 Nov 2022 08:57:11 +0100
From:   Jiri Olsa <olsajiri@...il.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, x86@...nel.org, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev
Subject: Re: [PATCH bpf] bpf: Mark bpf_arch_init_dispatcher_early() as
 __init_or_module

On Mon, Oct 31, 2022 at 10:38:19AM -0700, Nathan Chancellor wrote:
> After commit dbe69b299884 ("bpf: Fix dispatcher patchable function entry
> to 5 bytes nop"), building kernel/bpf/dispatcher.c in certain
> configurations with LLVM's integrated assembler results in a known
> recordmcount bug:
> 
>   Cannot find symbol for section 4: .init.text.
>   kernel/bpf/dispatcher.o: failed
> 
> This occurs when there are only weak symbols in a particular section in
> the translation unit; in this case, bpf_arch_init_dispatcher_early() is
> marked '__weak __init' and it is the only symbol in the .init.text
> section. recordmcount expects there to be a symbol for a particular
> section but LLVM's integrated assembler (and GNU as after 2.37) do not
> generated section symbols. This has been worked around in the kernel
> before in commit 55d5b7dd6451 ("initramfs: fix clang build failure")
> and commit 6e7b64b9dd6d ("elfcore: fix building with clang").
> 
> Fixing recordmcount has been brought up before but there is no clear
> solution that does not break ftrace outright.
> 
> Unfortunately, working around this issue by removing the '__init' from
> bpf_arch_init_dispatcher_early() is not an option, as the x86 version of
> bpf_arch_init_dispatcher_early() calls text_poke_early(), which is
> marked '__init_or_module', meaning that when CONFIG_MODULES is disabled,
> bpf_arch_init_dispatcher_early() has to be marked '__init' as well to
> avoid a section mismatch warning from modpost.
> 
> However, bpf_arch_init_dispatcher_early() can be marked
> '__init_or_module' as well, which would resolve the recordmcount warning
> for configurations that support modules (i.e., the vast majority of
> them) while not introducing any new warnings for all configurations. Do
> so to clear up the build failure for CONFIG_MODULES=y configurations.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/981
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>

LGTM but the whole thing might be actually going away:
  https://lore.kernel.org/bpf/Y2BD6xZ108lv3j7J@krava/T/#u

because it won't compile on gcc 7

jirka

> ---
>  arch/x86/net/bpf_jit_comp.c | 2 +-
>  include/linux/bpf.h         | 2 +-
>  kernel/bpf/dispatcher.c     | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 00127abd89ee..4145939bbb6a 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -389,7 +389,7 @@ static int __bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
>  	return ret;
>  }
>  
> -int __init bpf_arch_init_dispatcher_early(void *ip)
> +int __init_or_module bpf_arch_init_dispatcher_early(void *ip)
>  {
>  	const u8 *nop_insn = x86_nops[5];
>  
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 0566705c1d4e..4aa7bde406f5 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -971,7 +971,7 @@ 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);
> -int __init bpf_arch_init_dispatcher_early(void *ip);
> +int __init_or_module bpf_arch_init_dispatcher_early(void *ip);
>  
>  #define BPF_DISPATCHER_INIT(_name) {				\
>  	.mutex = __MUTEX_INITIALIZER(_name.mutex),		\
> diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c
> index 04f0a045dcaa..e14a68e9a74f 100644
> --- a/kernel/bpf/dispatcher.c
> +++ b/kernel/bpf/dispatcher.c
> @@ -91,7 +91,7 @@ int __weak arch_prepare_bpf_dispatcher(void *image, void *buf, s64 *funcs, int n
>  	return -ENOTSUPP;
>  }
>  
> -int __weak __init bpf_arch_init_dispatcher_early(void *ip)
> +int __weak __init_or_module bpf_arch_init_dispatcher_early(void *ip)
>  {
>  	return -ENOTSUPP;
>  }
> 
> base-commit: 8bdc2acd420c6f3dd1f1c78750ec989f02a1e2b9
> -- 
> 2.38.1
>