From: Gustavo A. R. Silva <gustavoars@kernel.org>
Date: Thu, 7 May 2020 19:25:24 +0000 (-0500)
Subject: thunderbolt: Replace zero-length array with flexible-array
X-Git-Tag: v5.10.7~2412^2~8^2~2
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=913b99f70feb1cd9fb4bbcb302a029b4b9bee454;p=platform%2Fkernel%2Flinux-rpi.git

thunderbolt: Replace zero-length array with flexible-array

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.

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 <gustavoars@kernel.org>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
---

diff --git a/include/linux/thunderbolt.h b/include/linux/thunderbolt.h
index ece782e..ff397c0 100644
--- a/include/linux/thunderbolt.h
+++ b/include/linux/thunderbolt.h
@@ -80,7 +80,7 @@ struct tb {
 	int index;
 	enum tb_security_level security_level;
 	size_t nboot_acl;
-	unsigned long privdata[0];
+	unsigned long privdata[];
 };
 
 extern struct bus_type tb_bus_type;