Re: [PATCH v2] socket: Implement sockaddr_storage with an anonymous union

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 1/20/23 21:46, Bastien Roucariès wrote:
> Le vendredi 20 janvier 2023, 20:38:32 UTC Alejandro Colomar a écrit :
>> Hi Bastien,
>>
>> On 1/20/23 21:32, Bastien Roucariès wrote:
>> [...]
>>
>>>> diff --git a/bits/socket.h b/bits/socket.h
>>>> index aac8c49b00..c0c23b4e84 100644
>>>> --- a/bits/socket.h
>>>> +++ b/bits/socket.h
>>>> @@ -168,9 +168,14 @@ struct sockaddr
>>>>    
>>>>    struct sockaddr_storage
>>>>      {
>>>> -    __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
>>>> -    char __ss_padding[_SS_PADSIZE];
>>>> -    __ss_aligntype __ss_align;	/* Force desired alignment.  */
>>> no this is not correct you break ABI by reducing size
>>>> +    union
>>>> +      {
>>>> +        __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
>>>> +        struct sockaddr      sa;
>>>> +        struct sockaddr_in   sin;
>>>> +        struct sockaddr_in6  sin6;
>>>> +        struct sockaddr_un   sun;
>>>> +      };
>>>>      };
>>>
>>> Correct one structure is
>>>
>>> struct __private_sock_storage {
>>>       __SOCKADDR_COMMON (ssprivate_);   /* Address family, etc. */
>>>       char __ss_padding[_SS_PADSIZE];
>>>       __ss_aligntype __ss_align; /* Force desired alignment. */
>>> }
>>
>> What is this structure for?  I expect that it's for declaring a wide-enough and
>> correctly aligned type, but the union containing all the other types already
>> guarantees a size as wide as any other sockaddr_* and with the widest alignment.
>>
>> Also, any member that is necessary for superalignment or padding could be added
>> at the end of sockaddr_storage, after the anon union; you don't need the extra
>> struct, I guess.
> 
> No we need it, max of structure is struct sockaddr_un   sun and is size of 108.
> sizeof(sockaddr_storage) is 128...
> 
> Did you see the line of the kernel source I send you ? kernel expect size of 109 for un aka we should pad by a nul byte...

Yes, I saw it.  But that line from the kernel is already Undefined Behavior. 
The correct fix should go to sockaddr_un, not sockaddr_storage, IMO.

However, applying this change to sockaddr_storage would expose that kernel bug, 
so I think a prepatch to sockaddr_un that adds a padding byte to sockaddr_un 
would make sense.

struct sockaddr_un {
	__kernel_sa_family_t sun_family; /* AF_UNIX */
	char sun_path[UNIX_PATH_MAX];	/* pathname */
	char __null;  // make sure sun_path is terminated
};


> 
> I think it is safer in a first step, to keep the old structure... Maybe later simplify
> 
> Did you also see
> https://github.com/bminor/glibc/blob/master/socket/sys/socket.h#L63

Heh, I didn't see that one :)
I'll take it into account for a revision of the patch.

Cheers,

Alex


-- 
<http://www.alejandro-colomar.es/>

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