From: Alejandro Colomar <alx.manpages@gmail.com>
To: Zack Weinberg <zack@owlfolio.org>,
GNU libc development <libc-alpha@sourceware.org>,
gcc@gcc.gnu.org
Cc: "Alejandro Colomar" <alx@kernel.org>,
'linux-man' <linux-man@vger.kernel.org>,
"Bastien Roucariès" <rouca@debian.org>,
"Eric Blake" <eblake@redhat.com>,
"Stefan Puiu" <stefan.puiu@gmail.com>,
"Igor Sysoev" <igor@sysoev.ru>
Subject: Re: [PATCH v2] socket: Implement sockaddr_storage with an anonymous union
Date: Sat, 21 Jan 2023 03:38:07 +0100 [thread overview]
Message-ID: <4d3a8505-1ec1-0e4c-299a-1b56e3525410@gmail.com> (raw)
In-Reply-To: <0e6c7856-549b-5014-fb37-bc5925660ffe@gmail.com>
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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);
}
Cheers,
Alex
--
<http://www.alejandro-colomar.es/>
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next prev parent reply other threads:[~2023-01-21 2:38 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-20 13:40 Alejandro Colomar
2023-01-20 17:49 ` Joseph Myers
2023-01-20 19:26 ` Alejandro Colomar
2023-01-20 18:04 ` Zack Weinberg
2023-01-20 19:25 ` Alejandro Colomar
2023-01-21 2:38 ` Alejandro Colomar [this message]
2023-01-21 3:17 ` Alejandro Colomar
2023-01-21 13:30 ` Bastien Roucariès
2023-01-21 14:30 ` Alejandro Colomar
2023-01-22 14:12 ` Bastien Roucariès
2023-01-20 20:32 ` Bastien Roucariès
2023-01-20 20:38 ` Alejandro Colomar
2023-01-20 20:46 ` Bastien Roucariès
2023-01-20 20:51 ` Alejandro Colomar
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