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

Gustavo A. R. Silva
2020-05-07T18:49:00Z

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>
---
include/linux/hyperv.h | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 692c89ccf5df..ce2c27440e17 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -117,7 +117,7 @@ struct hv_ring_buffer {
* Ring data starts here + RingDataStartOffset
* !!! DO NOT place any fields below this !!!
*/
- u8 buffer[0];
+ u8 buffer[];
} __packed;

struct hv_ring_buffer_info {
@@ -313,7 +313,7 @@ struct vmadd_remove_transfer_page_set {
struct gpa_range {
u32 byte_count;
u32 byte_offset;
- u64 pfn_array[0];
+ u64 pfn_array[];
};

/*
@@ -563,7 +563,7 @@ struct vmbus_channel_gpadl_header {
u32 gpadl;
u16 range_buflen;
u16 rangecount;
- struct gpa_range range[0];
+ struct gpa_range range[];
} __packed;

/* This is the followup packet that contains more PFNs. */
@@ -571,7 +571,7 @@ struct vmbus_channel_gpadl_body {
struct vmbus_channel_message_header header;
u32 msgnumber;
u32 gpadl;
- u64 pfn[0];
+ u64 pfn[];
} __packed;

struct vmbus_channel_gpadl_created {
@@ -672,7 +672,7 @@ struct vmbus_channel_msginfo {
* The channel message that goes out on the "wire".
* It will contain at minimum the VMBUS_CHANNEL_MESSAGE_HEADER header
*/
- unsigned char msg[0];
+ unsigned char msg[];
};

struct vmbus_close_msg {