[musl] [Bug?] getaddrinfo() fills .ai_canonname when AI_CANONNAME isn't passed

[musl] [Bug?] getaddrinfo() fills .ai_canonname when AI_CANONNAME isn't passed

[Michał Górny]

[-- Attachment #1: Type: text/plain, Size: 1804 bytes --]

Hello,

While debugging a test suite failure in dnspython [1], we've discovered
that the musl implementation of getaddrinfo() fills .ai_canonname field
even if flags do not include AI_CANONNAME.  We think that this could
incorrect.  Per POSIX:

> If nodename is not null, and if requested by the AI_CANONNAME flag,
                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> the ai_canonname field of the first returned addrinfo structure shall
> point to a null-terminated string containing the canonical name
> corresponding to the input nodename; [...]
>
> All fields in socket address structures returned by getaddrinfo() that
> are not filled in through an explicit argument (for example,
> sin6_flowinfo) shall be set to zero.  [2]

I think the correct behavior would be to set the field to zero (i.e.
null) when AI_CANONNAME is not present in flags.


I've reproduced the problem on Gentoo Linux amd64 with musl 1.2.4, using
the following test program:

```
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

int main(void)
{
    struct addrinfo hints = {0};
    struct addrinfo *res;
    struct addrinfo *p;

    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = 0;

    if (getaddrinfo("example.com", NULL, &hints, &res) == 0) {
        printf("%s\n", res->ai_canonname ? res->ai_canonname
                                         : "(null)");
        freeaddrinfo(res);
    }
    return 0;
}
```

On glibc system, it produces "(null)".  On musl system, it produces
"example.com".


[1] https://github.com/rthalley/dnspython/issues/1035
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html

-- 
Best regards,
Michał Górny


[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 512 bytes --]

Re: [musl] [Bug?] getaddrinfo() fills .ai_canonname when AI_CANONNAME isn't passed

[Markus Wichmann]

Hi,

I just had a look at the POSIX specification again and noticed that it
doesn't breathe a word about the setting of .ai_canonname in case
AI_CANONNAME is not set. Nor does it say anything about .ai_canonname in
any returned structure other than the first. The case is entirely
unspecified.

Therefore your application must not make assumptions about it in that
case. The test is wrong to assume anything about .ai_canonname if
AI_CANONNAME is not set.

Ciao,
Markus