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

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

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Hi Zack,

On 1/21/23 03:38, Alejandro Colomar wrote:
> Hi Zack,
> 
> On 1/20/23 20:25, Alejandro Colomar wrote:
>> [CC += GCC]  // pun not intended :P
>>
>> Hi Zack,
>>
>> On 1/20/23 19:04, Zack Weinberg wrote:
>>> On Fri, Jan 20, 2023, at 8:40 AM, Alejandro Colomar wrote:
>>>> The historical design of `sockaddr_storage` makes it impossible to use
>>>> without breaking strict aliasing rules.  Not only this type is unusable,
>>>> but even the use of other `sockaddr_*` structures directly (when the
>>>> programmer only cares about a single address family) is also incorrect,
>>>> since at some point the structure will be accessed as a `sockaddr`, and
>>>> that breaks strict aliasing rules too.
>>>>
>>>> So, the only way for a programmer to not invoke Undefined Behavior is to
>>>> declare a union that includes `sockaddr` and any `sockaddr_*` structures
>>>> that are of interest, which allows later accessing as either the correct
>>>> structure or plain `sockaddr` for the sa_family.
>>>
>>> ...
>>>
>>>>      struct new_sockaddr_storage  nss;
>>>>
>>>>      // ... (initialize oss and nss)
>>>>
>>>>      inet_sockaddr2str(&nss.sa);  // correct (and has no casts)
>>>
>>> I think we need to move slowly here and be _sure_ that any proposed change
>>> does in fact reduce the amount of UB.
>>
>> Sure, I'm just sending the patch to polish the idea around some concrete code. 
>> While I checked that it compiles, I didn't add any tests about it or anything, 
>> to see that it's usable (and Joseph's email showed me that it's far from being 
>> finished).  I expect that this'll take some time.
>>
>>
>>>  This construct, in particular, might
>>> not actually be correct in practice: see https://godbolt.org/z/rn51cracn for
>>> a case where, if I'm reading it right, the compiler assumes that a write
>>> through a `struct fancy *` cannot alter the values accessible through a
>>> `struct simple *` even though both pointers point into the same union.
>>> (Test case provided by <https://stackoverflow.com/users/363751/supercat>;
>>
> 
> [...]
> 
> I was wrong in my guess; the correct output is 3/3; I think I had read it the 
> other way around.  So yes, I believe it's doing what you just wrote there, but 
> can't understand why.
> 
> I reduced @supercat's example to a smaller reproducer program (I couldn't 
> minimize it any more than this; any further simplification removes the incorrect 
> behavior):
> 
> #include <stdio.h>
> 
> struct a { int y[1];};
> struct b { int y[1];};
> union u  { struct a a; struct b b; };
> 
> 
> int read_a(struct a *a)
> {
>      return a->y[0];
> }
> 
> void write_b(struct b *b, int j)
> {
>      b->y[j] = 2;
> }
> 
> int use_union(union u *u, int j)
> {
>      if (u->a.y[0] == 0)
>          write_b(&u->b, j);
>          //write_b((struct b *)u, j);   // this has the same issue
>      return read_a(&u->a);
>      return read_a((struct a *)u);      // this has the same issue
> }
> 
> int (*volatile vtest)(union u *u, int j) = use_union;
> 
> int main(void)
> {
>      int       r1, r2;
>      union u   u;
>      struct b  b = {0};
> 
>      u.b = b;
>      r1 = vtest(&u, 0);
>      r2 = u.a.y[0];
> 
>      printf("%d/%d\n", r1, r2);
> }


Ahh, indeed it seems to be UB.  It's in the same 6.5.2.3/6:  there's a 
requirement that the information about the union is kept in the function in 
which it's accessed.

The standard presents an example, which is a bit ambiguous:

      The following is not a valid fragment (because the union type is not 
visible within function f):

           struct t1 { int m; };
           struct t2 { int m; };
           int f(struct t1 *p1, struct t2 *p2)
           {
                 if (p1->m < 0)
                         p2->m = -p2->m;
                 return p1->m;
           }
           int g()
           {
                 union {
                         struct t1 s1;
                         struct t2 s2;
                 } u;
                 /* ... */
                 return f(&u.s1, &u.s2);
           }

I don't know what's the intention if the union type was global but the variable 
`u` was still not seen by f().  But it seems GCC's interpretation is UB, 
according to the test we just saw.

The solution that I can see for that is making sockaddr also be a union.  That 
way, the union is kept across all calls (since they all use sockaddr).

struct sockaddr {
	union {
		struct {
			sa_family_t  sa_family;
			char         sa_data[14];  // why 14?
		}
		struct sockaddr_in   sin;
		struct sockaddr_in6  sin6;
		struct sockaddr_un   sun;
	};
};


struct sockaddr_storage {
	union {
		sa_family_t          ss_family;
		struct sockaddr      sa;
	};
};


With this, sockaddr_storage becomes useless, but still usable.  New code, could 
just use sockaddr and use the internal union members as necessary.  Union info 
is kept across all function boundaries.

Cheers,

Alex

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

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