netdev - RE: [PATCH] netfilter: fix stringop-overflow warning with UBSAN

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Date:   Tue, 1 Aug 2017 09:25:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...db.de>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>
CC:     Johannes Berg <johannes.berg@...el.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Aaron Conole <aconole@...heb.org>,
        "netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
        "coreteam@...filter.org" <coreteam@...filter.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] netfilter: fix stringop-overflow warning with UBSAN

From: Arnd Bergmann
> Sent: 31 July 2017 11:09
> Using gcc-7 with UBSAN enabled, we get this false-positive warning:
> 
> net/netfilter/ipset/ip_set_core.c: In function 'ip_set_sockfn_get':
> net/netfilter/ipset/ip_set_core.c:1998:3: error: 'strncpy' writing 32 bytes into a region of size 2
> overflows the destination [-Werror=stringop-overflow=]
>    strncpy(req_get->set.name, set ? set->name : "",
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     sizeof(req_get->set.name));
>     ~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This seems completely bogus, and I could not find a nice workaround.
> To work around it in a less elegant way, I change the ?: operator
> into an if()/else() construct.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  net/netfilter/ipset/ip_set_core.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/ipset/ip_set_core.c b/net/netfilter/ipset/ip_set_core.c
> index e495b5e484b1..d7ebb021003b 100644
> --- a/net/netfilter/ipset/ip_set_core.c
> +++ b/net/netfilter/ipset/ip_set_core.c
> @@ -1995,8 +1995,12 @@ ip_set_sockfn_get(struct sock *sk, int optval, void __user *user, int *len)
>  		}
>  		nfnl_lock(NFNL_SUBSYS_IPSET);
>  		set = ip_set(inst, req_get->set.index);
> -		strncpy(req_get->set.name, ,
> -			IPSET_MAXNAMELEN);
> +		if (set)
> +			strncpy(req_get->set.name, set->name,
> +				sizeof(req_get->set.name));
> +		else
> +			memset(req_get->set.name, '\0',
> +			       sizeof(req_get->set.name));

If you use strncpy() here, the compiler might optimise the code
back to 'how it was before'.

Or, maybe an explicit temporary: 'const char *name = set ? set->name : "";

	David