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

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Message-ID: <FFC7041A-2CCA-4C7E-A9C9-6C2C30CE7D28@intel.com>
Date:   Mon, 30 Nov 2020 20:40:32 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "Dave.Martin@....com" <Dave.Martin@....com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "libc-alpha@...rceware.org" <libc-alpha@...rceware.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...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 Nov 25, 2020, at 20:17, Borislav Petkov <bp@...en8.de> wrote:
> 
> On Thu, Nov 19, 2020 at 11:02:34AM -0800, Chang S. Bae wrote:
>> 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.

[ Just want to clarify your comment. ]

Admittedly, this is an (ugly) workaround to avoid compile errors.

E.g. when code is written like this in the function:

	if (IS_ENABLED(CONFIG_X86_X32_ABI))
		size = max(size, sizeof(struct rt_sigframe_x32));

and compile with CONFIG_X86_X32_ABI=n, got such a message:

	"invalid application of 'sizeof' to incomplete type 'struct
	sigframe_ia32’"

While the coding-style doc [1] seems to mention this:

   "However, this approach still allows the C compiler to see the
    code inside the block, and check it for correctness (syntax, 
    types, symbol references, etc). Thus, you still have to use an 
    #ifdef if the code inside the block references symbols that 
    will not exist if the condition is not met.”

In general, putting #ifdef in a C file is advised to avoid. I wonder
whether it is okay to include #ifdef in the C file in this case.

Thanks,
Chang

[1] https://www.kernel.org/doc/html/v5.9/process/coding-style.html