linux-kernel - Re: [PATCH v2 1/4] x86/signal: Introduce helpers to get the maximum signal frame size

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Date:   Wed, 25 Nov 2020 12:17:26 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     tglx@...utronix.de, mingo@...nel.org, luto@...nel.org,
        x86@...nel.org, len.brown@...el.com, dave.hansen@...el.com,
        hjl.tools@...il.com, Dave.Martin@....com, mpe@...erman.id.au,
        tony.luck@...el.com, ravi.v.shankar@...el.com,
        libc-alpha@...rceware.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] x86/signal: Introduce helpers to get the maximum
 signal frame size

On Thu, Nov 19, 2020 at 11:02:34AM -0800, Chang S. Bae wrote:
> Signal frames do not have a fixed format and can vary in size when a number
> of things change: support XSAVE features, 32 vs. 64-bit apps. Add the code
> to support a runtime method for userspace to dynamically discover how large
> a signal stack needs to be.
> 
> Introduce a new variable, max_frame_size, and helper functions for the
> calculation to be used in a new user interface. Set max_frame_size to a
> system-wide worst-case value, instead of storing multiple app-specific
> values.
> 
> Locate the body of the helper function -- fpu__get_fpstate_sigframe_size()
> in fpu/signal.c for its relevance.

This sentence is strange and not needed.

> diff --git a/arch/x86/include/asm/sigframe.h b/arch/x86/include/asm/sigframe.h
> index 84eab2724875..ac77f3f90bc9 100644
> --- a/arch/x86/include/asm/sigframe.h
> +++ b/arch/x86/include/asm/sigframe.h
> @@ -52,6 +52,15 @@ struct rt_sigframe_ia32 {
>  	char retcode[8];
>  	/* fp state follows here */
>  };
> +
> +#define SIZEOF_sigframe_ia32	sizeof(struct sigframe_ia32)
> +#define SIZEOF_rt_sigframe_ia32	sizeof(struct rt_sigframe_ia32)
> +
> +#else
> +
> +#define SIZEOF_sigframe_ia32	0
> +#define SIZEOF_rt_sigframe_ia32	0
> +
>  #endif /* defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION) */
>  
>  #ifdef CONFIG_X86_64
> @@ -81,8 +90,22 @@ struct rt_sigframe_x32 {
>  	/* fp state follows here */
>  };
>  
> +#define SIZEOF_rt_sigframe_x32	sizeof(struct rt_sigframe_x32)
> +
>  #endif /* CONFIG_X86_X32_ABI */
>  
> +#define SIZEOF_rt_sigframe	sizeof(struct rt_sigframe)
> +
> +#else
> +
> +#define SIZEOF_rt_sigframe	0
> +
>  #endif /* CONFIG_X86_64 */
>  
> +#ifndef SIZEOF_rt_sigframe_x32
> +#define SIZEOF_rt_sigframe_x32	0
> +#endif

Those are defined here to be used in only one place -
init_sigframe_size() - where there already is ifdeffery. Just use the
normal sizeof() operator there instead of adding more gunk here.

> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index a4ec65317a7f..9f009525f551 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -507,6 +507,26 @@ fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
>  
>  	return sp;
>  }
> +
> +unsigned long fpu__get_fpstate_sigframe_size(void)
> +{
> +	unsigned long xstate_size = xstate_sigframe_size();
> +	unsigned long fsave_header_size = 0;
> +
> +	/*
> +	 * This space is needed on (most) 32-bit kernels, or when a 32-bit
> +	 * app is running on a 64-bit kernel. To keep things simple, just
> +	 * assume the worst case and always include space for 'freg_state',
> +	 * even for 64-bit apps on 64-bit kernels. This wastes a bit of
> +	 * space, but keeps the code simple.
> +	 */
> +	if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
> +	     IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
> +		fsave_header_size = sizeof(struct fregs_state);
> +
> +	return fsave_header_size + xstate_size;
> +}

I guess this can be simplified to:

unsigned long fpu__get_fpstate_sigframe_size(void)
{
        unsigned long ret = xstate_sigframe_size();

        /*
         * This space is needed on (most) 32-bit kernels, or when a 32-bit
         * app is running on a 64-bit kernel. To keep things simple, just
         * assume the worst case and always include space for 'freg_state',
         * even for 64-bit apps on 64-bit kernels. This wastes a bit of
         * space, but keeps the code simple.
         */
        if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
             IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
                ret += sizeof(struct fregs_state);

        return ret;
}

Also, this function simply gives you the xstate size, there's no need
for "sigframe" in the name.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette