x86/boot: Save fields explicitly, zero out everything else

Recent gcc compilers (gcc 9.1) generate warnings about an out of bounds
memset, if the memset goes accross several fields of a struct. This
generated a couple of warnings on x86_64 builds in sanitize_boot_params().

Fix this by explicitly saving the fields in struct boot_params
that are intended to be preserved, and zeroing all the rest.

[ tglx: Tagged for stable as it breaks the warning free build there as well ]

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190731054627.5627-2-jhubbard@nvidia.com

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 +49,40 @@ 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));
 	}
 }