linux-kernel - [PATCH] efi/apple-properties: Replace zero-length array with flexible-array member

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Message-ID: <20200221152430.GA20788@embeddedor>
Date:   Fri, 21 Feb 2020 09:24:30 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] efi/apple-properties: Replace zero-length array with
 flexible-array member

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]

This issue was found with the help of Coccinelle.

[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 <gustavo@...eddedor.com>
---
 drivers/firmware/efi/apple-properties.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/efi/apple-properties.c b/drivers/firmware/efi/apple-properties.c
index 5ccf39986a14..084942846f4d 100644
--- a/drivers/firmware/efi/apple-properties.c
+++ b/drivers/firmware/efi/apple-properties.c
@@ -31,7 +31,7 @@ __setup("dump_apple_properties", dump_properties_enable);
 struct dev_header {
 	u32 len;
 	u32 prop_count;
-	struct efi_dev_path path[0];
+	struct efi_dev_path path[];
 	/*
 	 * followed by key/value pairs, each key and value preceded by u32 len,
 	 * len includes itself, value may be empty (in which case its len is 4)
@@ -42,7 +42,7 @@ struct properties_header {
 	u32 len;
 	u32 version;
 	u32 dev_count;
-	struct dev_header dev_header[0];
+	struct dev_header dev_header[];
 };
 
 static void __init unmarshal_key_value_pairs(struct dev_header *dev_header,
-- 
2.25.0