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

["Bastien Roucariès" <rouca@debian.org>]

[-- Attachment #1: Type: text/plain, Size: 6047 bytes --]

Le samedi 21 janvier 2023, 03:17:39 UTC Alejandro Colomar a écrit :
> 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;
> 	};
> };

No the solution is to avoid sockaddr and mark as deprecated. The problem it should be part of union without raising a warning each time we use a safe type...

The other solution is to render public  and ABI stable the type here
https://github.com/bminor/glibc/blob/ae612c45efb5e34713859a5facf92368307efb6e/socket/sys/socket.h#L78
under for instance sockaddr_ptr and sockaddr_const_ptr

Moreover this are some patch arch by arch
https://sourceware.org/legacy-ml/libc-alpha/2016-02/msg00340.html that should be made default 

Bastien



> 
> 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/>
> 


[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 833 bytes --]