Re: [PATCH v2] x86/boot: save fields explicitly, zero out everything else

[Chris Clayton <chris2553@googlemail.com>]

On 31/07/2019 06:46, john.hubbard@gmail.com wrote:
> From: John Hubbard <jhubbard@nvidia.com>
> 
> 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.
> 

I applied John's patch below to v5.3-rc3-285-gecb095bff5d4 and have been running the resultant kernel for two days now,
including 7 or 8 cold starts and reboots. The warnings that were produced by gcc9 are no longer emitted and, other than
a pre-existing problem (no network after resume from suspend or hibernate which I will investigate and, if necessary,
report later today), the kernel has supported my typical day to day activities (building software, email, browsing,
listening to music, watching video) without problem.

Tested-by: Chris Clayton <chris2553@googlemail.com>

> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Suggested-by: H. Peter Anvin <hpa@zytor.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.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));
> +		static struct boot_params scratch;
> +		char *bp_base = (char *)boot_params;
> +		char *save_base = (char *)&scratch;
> +		int i;
> +
> +		const struct boot_params_to_save to_save[] = {
> +			BOOT_PARAM_PRESERVE(screen_info),
> +			BOOT_PARAM_PRESERVE(apm_bios_info),
> +			BOOT_PARAM_PRESERVE(tboot_addr),
> +			BOOT_PARAM_PRESERVE(ist_info),
> +			BOOT_PARAM_PRESERVE(acpi_rsdp_addr),
> +			BOOT_PARAM_PRESERVE(hd0_info),
> +			BOOT_PARAM_PRESERVE(hd1_info),
> +			BOOT_PARAM_PRESERVE(sys_desc_table),
> +			BOOT_PARAM_PRESERVE(olpc_ofw_header),
> +			BOOT_PARAM_PRESERVE(efi_info),
> +			BOOT_PARAM_PRESERVE(alt_mem_k),
> +			BOOT_PARAM_PRESERVE(scratch),
> +			BOOT_PARAM_PRESERVE(e820_entries),
> +			BOOT_PARAM_PRESERVE(eddbuf_entries),
> +			BOOT_PARAM_PRESERVE(edd_mbr_sig_buf_entries),
> +			BOOT_PARAM_PRESERVE(edd_mbr_sig_buffer),
> +			BOOT_PARAM_PRESERVE(e820_table),
> +			BOOT_PARAM_PRESERVE(eddbuf),
> +		};
> +
> +		memset(&scratch, 0, sizeof(scratch));
> +
> +		for (i = 0; i < ARRAY_SIZE(to_save); i++)
> +			memcpy(save_base + to_save[i].start,
> +			       bp_base + to_save[i].start, to_save[i].len);
> +
> +		memcpy(boot_params, save_base, sizeof(*boot_params));
>  	}
>  }
>  
>