Re: struct sockaddr_storage

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

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Hi Rick,

On 1/24/23 12:16, Rich Felker wrote:
> On Fri, Jan 20, 2023 at 12:06:50PM +0200, Stefan Puiu via Libc-alpha wrote:
>> Hi Alex,
>>
>> On Thu, Jan 19, 2023 at 4:14 PM Alejandro Colomar
>> <alx.manpages@gmail.com> wrote:
>>>
>>> Hi!
>>>
>>> I just received a report about struct sockaddr_storage in the man pages.  It
>>> reminded me of some concern I've always had about it: it doesn't seem to be a
>>> usable type.
>>>
>>> It has some alignment promises that make it "just work" most of the time, but
>>> it's still a UB mine, according to ISO C.
>>>
>>> According to strict aliasing rules, if you declare a variable of type 'struct
>>> sockaddr_storage', that's what you get, and trying to access it later as some
>>> other sockaddr_8 is simply not legal.  The compiler may assume those accesses
>>> can't happen, and optimize as it pleases.
>>
>> Can you detail the "is not legal" part? How about the APIs like
>> connect() etc that use pointers to struct sockaddr, where the
>> underlying type is different, why would that be legal while using
>> sockaddr_storage isn't?
> 
> Because they're specified to take different types. In C, any struct
> pointer type can legally point to any other struct type. You just
> can't dereference through it with the wrong type.

Yep.  Which basically means that users need to treat sockaddr structures 
as black boxes.  Otherwise, there's going to be undefined behavior at 
some point.  Because of course, you can't know the right type before 
reading the first field, which is already UB.

> How the
> implementation of connect etc. handle this is an implementation
> detail. You're allowed to pass pointers to struct sockaddr_in, etc. to
> connect etc. simply because the specification says you are.

While the implementation has some more freedom regarding UB, in this 
case it's waiting for a compiler optimization to break this code, so I'd 
go the safe way and use standard C techniques in libc so that there are 
no long-term UB issues.

As a side effect, user code that currently invokes UB could be changed 
to have defined behavior.

> 
> In any case, sockaddr_storage is a legacy thing designed by folks who
> didn't understand the rules of the C language. It should never appear
> in modern code except perhaps with sizeof for allocting buffers. There
> is no action that needs to be taken here except documenting that it
> should not be used (cannot be used meaningfully without UB).

I agree with you on this.  sockaddr_storage has been broken since day 0. 
  However, for designing a solution for libc using unions, it could be 
useful.

> 
> Rich

Cheers,

Alex

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


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