cfg80211: 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>
Link: https://lore.kernel.org/r/20200507183909.GA12993@embeddedor
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

diff --git a/include/linux/ieee80211.h b/include/linux/ieee80211.h
index 1978441ded73f525e9d5118d9386c601d62607e9..e4d7488b63a52dc28f354efbc4df6aa3690d5336 100644 (file)
--- a/include/linux/ieee80211.h
+++ b/include/linux/ieee80211.h
@@ -805,7 +805,7 @@ struct ieee80211_msrment_ie {
        u8 token;
        u8 mode;
        u8 type;
-       u8 request[0];
+       u8 request[];
 } __packed;
 
 /**
@@ -1800,7 +1800,7 @@ struct ieee80211_he_operation {
        __le32 he_oper_params;
        __le16 he_mcs_nss_set;
        /* Optional 0,1,3,4,5,7 or 8 bytes: depends on @he_oper_params */
-       u8 optional[0];
+       u8 optional[];
 } __packed;
 
 /**
@@ -1812,7 +1812,7 @@ struct ieee80211_he_operation {
 struct ieee80211_he_spr {
        u8 he_sr_control;
        /* Optional 0 to 19 bytes: depends on @he_sr_control */
-       u8 optional[0];
+       u8 optional[];
 } __packed;
 
 /**