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SubjectRE: [PATCH] netfilter: fix stringop-overflow warning with UBSAN
Date
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@arndb.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

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