efi: Replace zero-length array and use struct_size() helper

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

Lastly, make use of the sizeof_field() helper instead of an open-coded
version.

This issue was found with the help of Coccinelle and audited _manually_.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200527171425.GA4053@embeddedorSigned-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/efi.c

@@ -622,7 +622,8 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
 			rsv = (void *)(p + prsv % PAGE_SIZE);
 
 			/* reserve the entry itself */
-			memblock_reserve(prsv, EFI_MEMRESERVE_SIZE(rsv->size));
+			memblock_reserve(prsv,
+					 struct_size(rsv, entry, rsv->size));
 
 			for (i = 0; i < atomic_read(&rsv->count); i++) {
 				memblock_reserve(rsv->entry[i].base,

include/linux/efi.h

@@ -1236,14 +1236,11 @@ struct linux_efi_memreserve {
 	struct {
 		phys_addr_t	base;
 		phys_addr_t	size;
-	} entry[0];
+	} entry[];
 };
 
-#define EFI_MEMRESERVE_SIZE(count) (sizeof(struct linux_efi_memreserve) + \
-	(count) * sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
-
 #define EFI_MEMRESERVE_COUNT(size) (((size) - sizeof(struct linux_efi_memreserve)) \
-	/ sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
+	/ sizeof_field(struct linux_efi_memreserve, entry[0]))
 
 void __init efi_arch_mem_reserve(phys_addr_t addr, u64 size);