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Date:   Fri, 19 Jun 2020 16:46:00 -0000
From:   "tip-bot2 for Gustavo A. R. Silva" <>
Cc:     "Gustavo A. R. Silva" <>,
        Kees Cook <>,
        Ard Biesheuvel <>, x86 <>,
        LKML <>
Subject: [tip: efi/urgent] efi: Replace zero-length array and use struct_size() helper

The following commit has been merged into the efi/urgent branch of tip:

Commit-ID:     2963795122f50b36ed16e3ba880c3ed2de1bda6e
Author:        Gustavo A. R. Silva <>
AuthorDate:    Wed, 27 May 2020 12:14:25 -05:00
Committer:     Ard Biesheuvel <>
CommitterDate: Mon, 15 Jun 2020 14:38:56 +02:00

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

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

[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <>
Reviewed-by: Kees Cook <>
Signed-off-by: Ard Biesheuvel <>
 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 7f1657b..edc5d36 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 2c6495f..c3449c9 100644
--- a/include/linux/efi.h
+++ b/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);

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