Re: [PATCH 62/64] netlink: Avoid false-positive memcpy() warning

Rasmus Villemoes
2021-07-28T11:24:09Z

On 28/07/2021 07.49, Greg Kroah-Hartman wrote:
> On Tue, Jul 27, 2021 at 01:58:53PM -0700, Kees Cook wrote:
>> In preparation for FORTIFY_SOURCE performing compile-time and run-time
>> field bounds checking for memcpy(), memmove(), and memset(), avoid
>> intentionally writing across neighboring fields.
>>
>> Add a flexible array member to mark the end of struct nlmsghdr, and
>> split the memcpy() to avoid false positive memcpy() warning:
>>
>> memcpy: detected field-spanning write (size 32) of single field (size 16)
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> include/uapi/linux/netlink.h | 1 +
>> net/netlink/af_netlink.c | 4 +++-
>> 2 files changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
>> index 4c0cde075c27..ddeaa748df5e 100644
>> --- a/include/uapi/linux/netlink.h
>> +++ b/include/uapi/linux/netlink.h
>> @@ -47,6 +47,7 @@ struct nlmsghdr {
>> __u16 nlmsg_flags; /* Additional flags */
>> __u32 nlmsg_seq; /* Sequence number */
>> __u32 nlmsg_pid; /* Sending process port ID */
>> + __u8 contents[];
>
> Is this ok to change a public, userspace visable, structure?

At least it should keep using a nlmsg_ prefix for consistency and reduce
risk of collision with somebody having defined an object-like contents
macro. But there's no guarantees in any case, of course.

Rasmus