efi: libstub: Clone memcmp() into the stub

We will no longer be able to call into the kernel image once we merge
the decompressor with the EFI stub, so we need our own implementation of
memcmp(). Let's add the one from lib/string.c and simplify it.

Signed-off-by: Ard Biesheuvel <[e-mail]>

arch/arm64/kernel/image-vars.h

···
  * linked at. The routines below are all implemented in assembler in a
  * position independent manner
  */
-PROVIDE(__efistub_memcmp		= __pi_memcmp);
 PROVIDE(__efistub_memchr		= __pi_memchr);
 PROVIDE(__efistub_strlen		= __pi_strlen);
 PROVIDE(__efistub_strnlen		= __pi_strnlen);

arch/loongarch/kernel/image-vars.h

···
 
 #ifdef CONFIG_EFI_STUB
 
-__efistub_memcmp		= memcmp;
 __efistub_memchr		= memchr;
 __efistub_strcat		= strcat;
 __efistub_strcmp		= strcmp;

arch/riscv/kernel/image-vars.h

···
  * linked at. The routines below are all implemented in assembler in a
  * position independent manner
  */
-__efistub_memcmp		= memcmp;
 __efistub_memchr		= memchr;
 __efistub_strlen		= strlen;
 __efistub_strnlen		= strnlen;

drivers/firmware/efi/libstub/intrinsics.c

···
 	efi_bs_call(set_mem, dst, len, c & U8_MAX);
 	return dst;
 }
+
+/**
+ * memcmp - Compare two areas of memory
+ * @cs: One area of memory
+ * @ct: Another area of memory
+ * @count: The size of the area.
+ */
+#undef memcmp
+int memcmp(const void *cs, const void *ct, size_t count)
+{
+	const unsigned char *su1, *su2;
+	int res = 0;
+
+	for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
+		if ((res = *su1 - *su2) != 0)
+			break;
+	return res;
+}

drivers/firmware/efi/libstub/zboot.c

···
 	log(L"error() called from decompressor library\n");
 }
 
-// Local version to avoid pulling in memcmp()
-static bool guids_eq(const efi_guid_t *a, const efi_guid_t *b)
-{
-	const u32 *l = (u32 *)a;
-	const u32 *r = (u32 *)b;
-
-	return l[0] == r[0] && l[1] == r[1] && l[2] == r[2] && l[3] == r[3];
-}
-
 static efi_status_t __efiapi
 load_file(efi_load_file_protocol_t *this, efi_device_path_protocol_t *rem,
 	  bool boot_policy, unsigned long *bufsize, void *buffer)
···
 	if (rem->type == EFI_DEV_MEDIA &&
 	    rem->sub_type == EFI_DEV_MEDIA_VENDOR) {
 		vendor_dp = container_of(rem, struct efi_vendor_dev_path, header);
-		if (!guids_eq(&vendor_dp->vendorguid, &LINUX_EFI_ZBOOT_MEDIA_GUID))
+		if (efi_guidcmp(vendor_dp->vendorguid, LINUX_EFI_ZBOOT_MEDIA_GUID))
 			return EFI_NOT_FOUND;
 
 		decompress = true;