[PATCH] prism54: Replace zero-length array with flexible-array

Gustavo A. R. Silva
2020-05-07T18:57:45Z

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>
---
drivers/net/wireless/intersil/prism54/isl_oid.h | 8 ++++----
drivers/net/wireless/intersil/prism54/islpci_mgt.h | 2 +-
2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/net/wireless/intersil/prism54/isl_oid.h b/drivers/net/wireless/intersil/prism54/isl_oid.h
index 5441c1f9f2fc..1afc2ccf94ca 100644
--- a/drivers/net/wireless/intersil/prism54/isl_oid.h
+++ b/drivers/net/wireless/intersil/prism54/isl_oid.h
@@ -37,7 +37,7 @@ struct obj_mlmeex {
u16 state;
u16 code;
u16 size;
- u8 data[0];
+ u8 data[];
} __packed;

struct obj_buffer {
@@ -68,12 +68,12 @@ struct obj_bss {

struct obj_bsslist {
u32 nr;
- struct obj_bss bsslist[0];
+ struct obj_bss bsslist[];
} __packed;

struct obj_frequencies {
u16 nr;
- u16 mhz[0];
+ u16 mhz[];
} __packed;

struct obj_attachment {
@@ -81,7 +81,7 @@ struct obj_attachment {
char reserved;
short id;
short size;
- char data[0];
+ char data[];
} __packed;

/*
diff --git a/drivers/net/wireless/intersil/prism54/islpci_mgt.h b/drivers/net/wireless/intersil/prism54/islpci_mgt.h
index d6bbbac46b4a..1f87d0aea60c 100644
--- a/drivers/net/wireless/intersil/prism54/islpci_mgt.h
+++ b/drivers/net/wireless/intersil/prism54/islpci_mgt.h
@@ -99,7 +99,7 @@ struct islpci_mgmtframe {
pimfor_header_t *header; /* payload header, points into buf */
void *data; /* payload ex header, points into buf */
struct work_struct ws; /* argument for schedule_work() */
- char buf[0]; /* fragment buffer */
+ char buf[]; /* fragment buffer */
};

int