linux-kernel - Re: [PATCH RFC 2/3] samples/bpf: Add define __EMITTING_BPF_

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Message-ID: <5943B1C6.7040809@iogearbox.net>
Date:   Fri, 16 Jun 2017 12:24:06 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     David Daney <david.daney@...ium.com>,
        Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
        ralf@...ux-mips.org, Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH RFC 2/3] samples/bpf: Add define __EMITTING_BPF__ when
 building BPF

On 06/16/2017 12:35 AM, David Daney wrote:
> ... this allows gating of inline assembly code that causes llvm to
> fail when emitting BPF.
>
> Signed-off-by: David Daney <david.daney@...ium.com>

I don't have a better idea at the moment, perhaps there could be
a clang rewrite plugin that would ignore all inline assembly code
since this is never used from BPF progs. Hmm. Really ugly that
we have to add this __EMITTING_BPF__ into arch asm files, but I
don't have a better idea for an immediate workaround right now ...
I would really prefer if we could avoid just for the sake of the
kernel samples going down the road of adding a !defined(__EMITTING_BPF__)
into a uapi asm header for mips, though. Is this coming from
networking sample code or rather tracing?

> ---
>   samples/bpf/Makefile | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index a0561dc762fe..4979e6b56662 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -193,12 +193,12 @@ $(src)/*.c: verify_target_bpf
>
>   $(obj)/tracex5_kern.o: $(obj)/syscall_nrs.h
>
> -# asm/sysreg.h - inline assembly used by it is incompatible with llvm.
> -# But, there is no easy way to fix it, so just exclude it since it is
> -# useless for BPF samples.
> +# __EMITTING_BPF__ used to exclude inline assembly, which cannot be
> +# emitted in BPF code.
>   $(obj)/%.o: $(src)/%.c
>   	$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
> -		-D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
> +		-D__KERNEL__ -D__EMITTING_BPF__ \
> +		-Wno-unused-value -Wno-pointer-sign \
>   		-Wno-compare-distinct-pointer-types \
>   		-Wno-gnu-variable-sized-type-not-at-end \
>   		-Wno-address-of-packed-member -Wno-tautological-compare \
>