[FFmpeg-devel] [PATCH] Fix setsockopt IP_MULTICAST_TTL on OpenBSD

[Chad Fraleigh]

Since apparently linux will auto-detect (as mentioned by Marton Balint), based on the optlen parameter, just using unsigned char in all cases seems to be the cleanest. However, I would advise including a comment in the code to that effect which says to ignore the [outdated] linux documentation (so someone doesn't needlessly "correct" it in the future).

I looked at the kernel source and it does work both ways:

static int do_ip_setsockopt(struct sock *sk, int level, int optname,
                sockptr_t optval, unsigned int optlen)
{
 ...
        switch (optname) {
 ...
        case IP_MULTICAST_TTL:
 ...
                if (optlen >= sizeof(int)) {
                        if (copy_from_sockptr(&val, optval, sizeof(val)))
                                return -EFAULT;
                } else if (optlen >= sizeof(char)) {
                        unsigned char ucval;

                        if (copy_from_sockptr(&ucval, optval, sizeof(ucval)))
                                return -EFAULT;
                        val = (int) ucval;
                }
        }
...
}

This check has been in the kernel since at least 2.6.12-rc2 (from Apr 2005). It should work fine, unless newer ffmpeg builds support is needed on older systems. So the only question is how old are the kernels in IoT and android devices which might use the current ffmpeg?


On 1/24/2022 11:25 PM, lance.lmwang at gmail.com wrote:
> On Wed, Jan 12, 2022 at 12:13:13AM -0500, Brad Smith wrote:
>> Fix setsockopt() usage on OpenBSD with IP_MULTICAST_TTL. The field
>> type should be an unsigned char on anything but Linux.
>>
>>
>> diff --git a/libavformat/udp.c b/libavformat/udp.c
>> index 180d96a988..29aa865fff 100644
>> --- a/libavformat/udp.c
>> +++ b/libavformat/udp.c
>> @@ -163,7 +163,13 @@ static int udp_set_multicast_ttl(int sockfd, int mcastTTL,
>>  {
>>  #ifdef IP_MULTICAST_TTL
>>      if (addr->sa_family == AF_INET) {
>> -        if (setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_TTL, &mcastTTL, sizeof(mcastTTL)) < 0) {
>> +#ifdef __linux__
>> +        int ttl = mcastTTL;
>> +#else
>> +        unsigned char ttl = mcastTTL;
>> +#endif
> 
> 
> I don't have BSD system for test, but I prefer to use socklen_t, please try with my proposal patch:
> 
> ---
>  libavformat/udp.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/udp.c b/libavformat/udp.c
> index 83c042d079..b9baa0a803 100644
> --- a/libavformat/udp.c
> +++ b/libavformat/udp.c
> @@ -164,7 +164,9 @@ static int udp_set_multicast_ttl(int sockfd, int mcastTTL,
>  {
>  #ifdef IP_MULTICAST_TTL
>      if (addr->sa_family == AF_INET) {
> -        if (setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_TTL, &mcastTTL, sizeof(mcastTTL)) < 0) {
> +        socklen_t ttl = mcastTTL;
> +
> +        if (setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)) < 0) {
>              ff_log_net_error(logctx, AV_LOG_ERROR, "setsockopt(IP_MULTICAST_TTL)");
>              return ff_neterrno();
> 
> 
>> +
>> +        if (setsockopt(sockfd, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)) < 0) {
>>              ff_log_net_error(NULL, AV_LOG_ERROR, "setsockopt(IP_MULTICAST_TTL)");
>>              return ff_neterrno();
>>          }
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>