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

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Date:   Tue, 1 Nov 2022 09:37:04 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Jiri Olsa <olsajiri@...il.com>
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 Tue, Nov 01, 2022 at 08:57:11AM +0100, Jiri Olsa wrote:
> 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

Ah, if that is the case, I suppose we can just wait until it is reverted
and replaced with the better fix. It seems like that will be the case
based on Bjorn's testing:

https://lore.kernel.org/87iljyyes6.fsf@all.your.base.are.belong.to.us/

Cheers,
Nathan

> > ---
> >  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
> >