linux-kernel - RE: [PATCH v2] x86/boot: save fields explicitly, zero out everything else

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Message-ID: <531b38aaa15e4de79a5e27fd37c04351@AcuMS.aculab.com>
Date:   Wed, 7 Aug 2019 11:41:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'john.hubbard@...il.com'" <john.hubbard@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>
Subject: RE: [PATCH v2] x86/boot: save fields explicitly, zero out everything
 else

From: john.hubbard@...il.com
> Sent: 31 July 2019 06:46
> 
> Recent gcc compilers (gcc 9.1) generate warnings about an
> out of bounds memset, if you trying memset across several fields
> of a struct. This generated a couple of warnings on x86_64 builds.
> 
> Fix this by explicitly saving the fields in struct boot_params
> that are intended to be preserved, and zeroing all the rest.
> 
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Suggested-by: H. Peter Anvin <hpa@...or.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
>  arch/x86/include/asm/bootparam_utils.h | 62 +++++++++++++++++++-------
>  1 file changed, 47 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/include/asm/bootparam_utils.h b/arch/x86/include/asm/bootparam_utils.h
> index 101eb944f13c..514aee24b8de 100644
> --- a/arch/x86/include/asm/bootparam_utils.h
> +++ b/arch/x86/include/asm/bootparam_utils.h
> @@ -18,6 +18,20 @@
>   * Note: efi_info is commonly left uninitialized, but that field has a
>   * private magic, so it is better to leave it unchanged.
>   */
> +
> +#define sizeof_mbr(type, member) ({ sizeof(((type *)0)->member); })
> +
> +#define BOOT_PARAM_PRESERVE(struct_member)				\
> +	{								\
> +		.start = offsetof(struct boot_params, struct_member),	\
> +		.len   = sizeof_mbr(struct boot_params, struct_member),	\
> +	}
> +
> +struct boot_params_to_save {
> +	unsigned int start;
> +	unsigned int len;
> +};
> +
>  static void sanitize_boot_params(struct boot_params *boot_params)
>  {
>  	/*
> @@ -35,21 +49,39 @@ static void sanitize_boot_params(struct boot_params *boot_params)
>  	 * problems again.
>  	 */
>  	if (boot_params->sentinel) {
> -		/* fields in boot_params are left uninitialized, clear them */
> -		boot_params->acpi_rsdp_addr = 0;
> -		memset(&boot_params->ext_ramdisk_image, 0,
> -		       (char *)&boot_params->efi_info -
> -			(char *)&boot_params->ext_ramdisk_image);
> -		memset(&boot_params->kbd_status, 0,
> -		       (char *)&boot_params->hdr -
> -		       (char *)&boot_params->kbd_status);
> -		memset(&boot_params->_pad7[0], 0,
> -		       (char *)&boot_params->edd_mbr_sig_buffer[0] -
> -			(char *)&boot_params->_pad7[0]);
> -		memset(&boot_params->_pad8[0], 0,
> -		       (char *)&boot_params->eddbuf[0] -
> -			(char *)&boot_params->_pad8[0]);
> -		memset(&boot_params->_pad9[0], 0, sizeof(boot_params->_pad9));
...

How about replacing the above first using:
#define zero_struct_fields(ptr, from, to) memset(&ptr->from, 0, (char *)&ptr->to - (char *)&ptr->from)
	zero_struct_fields(boot_params, ext_ramdisk_image, efi_info);
	...
Which is absolutely identical to the original code.

The replacing the define with:
	#define so(s, m) offsetof(typeof(*s), m)
	#define zero_struct_fields(ptr, from, to) memset((char *)ptr + so(ptr, from), 0, so(ptr, to) - so(ptr, from))
which gcc probably doesn't complain about, but should generate identical code again.
There might be an existing define for so().

	David

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