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

From: Gustavo A. R. Silva
Date: Wed May 27 2020 - 13:09:35 EST

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

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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
drivers/firmware/efi/efi.c | 3 ++-
include/linux/efi.h | 7 ++-----
2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 7f1657b6c30df..edc5d36caf54e 100644
--- a/drivers/firmware/efi/efi.c
+++ b/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++) {
diff --git a/include/linux/efi.h b/include/linux/efi.h
index c45ac969ea4eb..328cc52a5fd45 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1234,14 +1234,11 @@ 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);