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

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Message-ID: <d7251d92-150b-5346-6237-52afc154bb00@rasmusvillemoes.dk>
Date:   Wed, 28 Jul 2021 13:24:01 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Cc:     linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 62/64] netlink: Avoid false-positive memcpy() warning

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@...omium.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