Le samedi 21 janvier 2023, 14:30:11 UTC Alejandro Colomar a écrit :
> Hi Bastien,
> 
> On 1/21/23 14:30, Bastien Roucariès wrote:
> 
> [...]
> 
> >> 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):
> >>
> 
> [...]
> 
> >>
> >> 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.
> 
> Declaring `sockaddr` as deprecated means deprecating also:
> 
> accept(2)
> accept4(2)
> bind(2)
> connect(2)
> getpeername(2)
> getsockname(2)
> recvfrom(2)
> sendto(2)
> getnameinfo(3)
> 
> which use the type in their prototype.
> 
> Also, other types such as `addrinfo`, which contain `sockaddr` would also need 
> to be deprecated, which would itself require deprecating:

No because this function will take a opaque transparent union pointer. I mean only raise 
a warning when user declare a variable (storage) of struct sockaddr...
> 
> getaddrinfo(3)
> freeaddrinfo(3)
> 
> Since addrinfo is itself contained in other structures such as `gaicb`, we would 
> also need to deprecate those, which would in turn require deprecating more APIs:
> 
> getaddrinfo_a(3)
> gai_error(3)
> gai_cancel(3)
> 
> And maybe I left some.  This feels like nuking the entire networking API, which 
> I don't see happening soon.
> 
> 
> Otherwise, we need to come up with a solution that keeps these APIs compatible 
> with whatever new type we suggest using.  Changing them to use `void*` instead 
> of `sockaddr*` would be possible, but would decrease type safety considerably, 
> so there should be a good reason for that.
> 
> Suggesting to use always `sockaddr_storage` but using `sockaddr` in the function 
> parameters keeps the current not-so-nice casting issues, which are not Undefined 
> Behavior per se, but not ideal either (in fact, I don't think `void*` is much 
> worse than code full of casts).  And it would also be error-prone, since users 
> could get the idea that `sockaddr` can be used safely, since it's what gets 
> passed as the parameter.
> 
> > The problem it should be part of union without raising a warning each time we use a safe type...
> 
> I don't understand this; please detail.

the transparent union will include sockaddr, thus even if we use it correctly raise a warning...
> 
> > 
> > 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
> 
> [[gnu::transparent_union]] (used in that link) seems like a "the design of this 
> interface is bad, sorry, this workaround will just make it work".  I guess it 
> just works, and probably it's the reason that so much undefined behavior hasn't 
> exploded so far.  However, if we can solve this using core language features, 
> I'd go that way.

It solve the problems and could be used without the  [[gnu::transparent_union]], c++17 support transparent union.

The transparent union should also include pointer to sockaddr_storage and bluetooth socket.

Bastien
> 
> > 
> > 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;
> >> 	};
> >> };
> 
> Hmm, this isn't still perfect.  Since the APIs get the sockaddr, this union 
> information would be lost.  `sockaddr` needs to be the type that is declared. 
> `sockaddr_storage` should just die; there's no way to make it compatible with 
> APIs getting a `sockaddr`.  The attribute `transparent_union` is the only way to 
> use is safely.
> 
> Cheers,
> 
> Alex
> 
> >>
> >>
> >> 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.
> 
> -- 
> <http://www.alejandro-colomar.es/>
>