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Date:   Fri, 8 Jan 2021 14:16:19 -0700
From:   Nathan Chancellor <natechancellor@gmail.com>
To:     Alexander Lobakin <alobakin@pm.me>
Cc:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Arnd Bergmann <arnd@arndb.de>,
        Kees Cook <keescook@chromium.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Fangrui Song <maskray@google.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Pei Huang <huangpei@loongson.cn>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Ralf Baechle <ralf@linux-mips.org>,
        Corey Minyard <cminyard@mvista.com>,
        linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, stable@vger.kernel.org,
        clang-built-linux@googlegroups.com
Subject: Re: [PATCH v4 mips-next 3/7] MIPS: properly stop .eh_frame generation
Message-ID: <20210108211619.GC2547542@ubuntu-m3-large-x86>
References: <20210107123331.354075-1-alobakin@pm.me>
 <20210107123428.354231-1-alobakin@pm.me>
 <20210107123428.354231-3-alobakin@pm.me>
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On Thu, Jan 07, 2021 at 12:35:01PM +0000, Alexander Lobakin wrote:
> Commit 866b6a89c6d1 ("MIPS: Add DWARF unwinding to assembly") added
> -fno-asynchronous-unwind-tables to KBUILD_CFLAGS to prevent compiler
> from emitting .eh_frame symbols.
> However, as MIPS heavily uses CFI, that's not enough. Use the
> approach taken for x86 (as it also uses CFI) and explicitly put CFI
> symbols into the .debug_frame section (except for VDSO).
> This allows us to drop .eh_frame from DISCARDS as it's no longer
> being generated.
> 
> Fixes: 866b6a89c6d1 ("MIPS: Add DWARF unwinding to assembly")
> Suggested-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

> ---
>  arch/mips/include/asm/asm.h    | 18 ++++++++++++++++++
>  arch/mips/kernel/vmlinux.lds.S |  1 -
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/include/asm/asm.h b/arch/mips/include/asm/asm.h
> index 3682d1a0bb80..908f6d6ae24b 100644
> --- a/arch/mips/include/asm/asm.h
> +++ b/arch/mips/include/asm/asm.h
> @@ -20,10 +20,27 @@
>  #include <asm/sgidefs.h>
>  #include <asm/asm-eva.h>
>  
> +#ifndef __VDSO__
> +/*
> + * Emit CFI data in .debug_frame sections, not .eh_frame sections.
> + * We don't do DWARF unwinding at runtime, so only the offline DWARF
> + * information is useful to anyone. Note we should change this if we
> + * ever decide to enable DWARF unwinding at runtime.
> + */
> +#define CFI_SECTIONS	.cfi_sections .debug_frame
> +#else
> + /*
> +  * For the vDSO, emit both runtime unwind information and debug
> +  * symbols for the .dbg file.
> +  */
> +#define CFI_SECTIONS
> +#endif
> +
>  /*
>   * LEAF - declare leaf routine
>   */
>  #define LEAF(symbol)					\
> +		CFI_SECTIONS;				\
>  		.globl	symbol;				\
>  		.align	2;				\
>  		.type	symbol, @function;		\
> @@ -36,6 +53,7 @@ symbol:		.frame	sp, 0, ra;			\
>   * NESTED - declare nested routine entry point
>   */
>  #define NESTED(symbol, framesize, rpc)			\
> +		CFI_SECTIONS;				\
>  		.globl	symbol;				\
>  		.align	2;				\
>  		.type	symbol, @function;		\
> diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
> index 16468957cba2..0f4e46ea4458 100644
> --- a/arch/mips/kernel/vmlinux.lds.S
> +++ b/arch/mips/kernel/vmlinux.lds.S
> @@ -225,6 +225,5 @@ SECTIONS
>  		*(.options)
>  		*(.pdr)
>  		*(.reginfo)
> -		*(.eh_frame)
>  	}
>  }
> -- 
> 2.30.0
> 
>