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List:       glibc-alpha
Subject:    Re: [PATCH] Fix strict-aliasing warning in resolv/res_hconf.c
From:       Florian Weimer <fweimer () redhat ! com>
Date:       2015-05-27 11:16:46
Message-ID: 5565A79E.6010403 () redhat ! com
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On 05/27/2015 01:09 PM, Pedro Alves wrote:
> On 05/27/2015 11:50 AM, Florian Weimer wrote:
>> On 05/27/2015 11:19 AM, Pedro Alves wrote:
>>> On 05/20/2015 05:27 PM, Steve Ellcey wrote:
>>>
>>>> diff --git a/resolv/res_hconf.c b/resolv/res_hconf.c
>>>> index 73942e8..3b05287 100644
>>>> --- a/resolv/res_hconf.c
>>>> +++ b/resolv/res_hconf.c
>>>> @@ -444,13 +444,13 @@ _res_hconf_reorder_addrs (struct hostent *hp)
>>>>  
>>>>  	      ifaddrs[new_num_ifs].addrtype = AF_INET;
>>>>  	      ifaddrs[new_num_ifs].u.ipv4.addr =
>>>> -		((struct sockaddr_in *) &cur_ifr->ifr_addr)->sin_addr.s_addr;
>>>> +		cur_ifr->ifr_addr_in.sin_addr.s_addr;
>>>
>>> Without adding a new union member, isn't the simplest to just take a
>>> copy step?
>>
>> I think it's still undefined behavior.
> 
> How so?  AFAIK, it's implementation defined, and GCC allows type-punning provided
> the memory is accessed through the union type.
> 
>  https://gcc.gnu.org/onlinedocs/gcc-5.1.0/gcc/Optimize-Options.html#Type-punning

You mean, copy it to a union member of type struct sockaddr, and reading
from that union as struct sockaddr_in?

(Based on my reading of the standard, memcpy does not change effective
type if one has been assigned.)

-- 
Florian Weimer / Red Hat Product Security
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