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List:       gcc
Subject:    Re: [PATCH] sockaddr.3type: Document that sockaddr_storage is the API to be used
From:       Eric Blake via Gcc <gcc () gcc ! gnu ! org>
Date:       2023-04-21 15:27:07
Message-ID: sjomqddm5sik7zufnpozmagkbgnjiu55vbozruh4nsehg7hqqi () hl3jl5yde5pl
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On Fri, Apr 21, 2023 at 05:00:14PM +0200, Alejandro Colomar wrote:
> > 
> > The wording I see in <https://austingroupbugs.net/view.php?id=1641#c6262>
> > doesn't seem to cover the case of aliasing a sockaddr_storage as a
> > protocol-specific address for setting other members.
> > 
> > Aliasing rules don't allow one to declare an object of type
> > sockaddr_storage and then fill the structure as if it were another
> > structure, even if alignment and size are correct.  We would need
> > some wording that says something like:
> > 
> > When a pointer to a sockaddr_storage structure is first aliased as a
> > pointer to a protocol-specific address structure, the effective type
> > of the object will be set to the protocol-specific structure.

I'll add that as a comment to the Austin Group page; it seems like a
reasonable statement of intent (POSIX already says that struct
sockaddr_storage is sufficiently sized and aligned; all that remains
is for the compiler to be aware that we intend to use a
more-appropriate effective type once we have the storage allocated).

> > 
> > This is similar to what happens when malloc(3) is assigned to a
> > non-character type.  That's a big hammer, but it does the job.  Maybe
> > we would need some looser language?  I CCd GCC, in case they have
> > concerns about this wording.
> > 
> > Cheers,
> > Alex
> > 
> >>
> >> I quite like this way of putting it.  It subsumes both what I wrote and 
> >> the related potential headache with deciding whether the sa_family_t 
> >> field is considered an object or just a range of bytes within a larger 
> >> object.
> >>
> >> zw
> > 
> 
> For the man pages, I've rewritten it to the following:
> 
> 
> $ git diff
> diff --git a/man3type/sockaddr.3type b/man3type/sockaddr.3type
> index 2fdf56c59..e610aa0f5 100644
> --- a/man3type/sockaddr.3type
> +++ b/man3type/sockaddr.3type
> @@ -117,6 +117,14 @@ .SH HISTORY
>  was invented by POSIX.
>  See also
>  .BR accept (2).
> +.PP
> +These structures were invented before modern ISO C strict-aliasing rules.
> +If aliasing rules are applied strictly,
> +these structures would be impossible to use

Maybe "extremely difficult" instead of "impossible" to use (if I
understand this thread correctly, it is possible to memcpy() from one
struct into different storage of a different effective type where the
memcpy()'s intermediate aliasing through char* avoids the UB).

> +without invoking Undefined Behavior (UB).
> +POSIX Issue 8 will fix this by requiring that implementations
> +make sure that these structures
> +can be safely used as they were designed.
>  .SH NOTES
>  .I socklen_t
>  is also defined in
> 
> 
> I guess this is simple enough that it should work as documentation.

It seems fine from my perspective.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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