Re: [PATCH] tcp: probe for SO_PEEK_OFF both in tcpv4 and tcp6

[Jon Maloy <jmaloy@redhat.com>]

My first approach to this was to condition the use of SO_PEEK_OFF with 
tcpv4, e.g., basically
a test like if (v4 && peek_offset_cap) {...} everywhere, but then I made 
an interesting discovery.

It turns out that, unless the ´-4' option is explicitly given on the 
command line, all sockets are
v6, even those that are later used as v4 sockets. So, the set_peek_off() 
call failed even
for supposedly v4 sockets.

I checked this by adding a printout to the tcp_listen_handler(), and 
noticed that all returns from
the accept4() call goes into the AF_INET6 branch, even when the client 
(iperf3) call is using an IPv4 address.
During traffic, the very same socket is marked as v4 in the tcp_tap_conn 
structure, and this seems to
have worked just fine until I added the set_peek_offset call().

I believe this is an issue that has been introduced during the last 
months, since I didn't start
using the ´-4' option consistently until some months ago, and then it 
worked.

Happy summer
///jon


On 2024-07-20 09:54, Jon Maloy wrote:
> The recently added socket option SO_PEEK_OFF is not supported for
> TCP/IPv6 sockets. Until we get that support into the kernel we need to
> test for support in both protocols to set the global 'peek_offset_cap´
> to true.
>
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>   tcp.c | 36 +++++++++++++++++++++++++-----------
>   1 file changed, 25 insertions(+), 11 deletions(-)
>
> diff --git a/tcp.c b/tcp.c
> index c5431f1..32026ca 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -2717,6 +2717,28 @@ static void tcp_sock_refill_init(const struct ctx *c)
>   	}
>   }
>   
> +/**
> + * tcp_probe_peek_offset_cap() - Check if SO_PEEK_OFF is supported by kernel
> + * @af:		Address family, IPv4 or IPv6
> + *
> + * Return: true if supported, false otherwise
> + */
> +bool tcp_probe_peek_offset_cap(int af)
> +{
> +	bool ret = false;
> +	int s, optv = 0;
> +
> +	s = socket(af, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
> +	if (s < 0) {
> +		warn_perror("Temporary TCP socket creation failed");
> +	} else {
> +		if (!setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &optv, sizeof(int)))
> +			ret = true;
> +		close(s);
> +	}
> +	return ret;
> +}
> +
>   /**
>    * tcp_init() - Get initial sequence, hash secret, initialise per-socket data
>    * @c:		Execution context
> @@ -2725,8 +2747,7 @@ static void tcp_sock_refill_init(const struct ctx *c)
>    */
>   int tcp_init(struct ctx *c)
>   {
> -	unsigned int b, optv = 0;
> -	int s;
> +	unsigned int b;
>   
>   	ASSERT(!c->no_tcp);
>   
> @@ -2752,15 +2773,8 @@ int tcp_init(struct ctx *c)
>   		NS_CALL(tcp_ns_socks_init, c);
>   	}
>   
> -	/* Probe for SO_PEEK_OFF support */
> -	s = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
> -	if (s < 0) {
> -		warn_perror("Temporary TCP socket creation failed");
> -	} else {
> -		if (!setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &optv, sizeof(int)))
> -			peek_offset_cap = true;
> -		close(s);
> -	}
> +	peek_offset_cap = tcp_probe_peek_offset_cap(AF_INET) &&
> +		tcp_probe_peek_offset_cap(AF_INET6);
>   	info("SO_PEEK_OFF%ssupported", peek_offset_cap ? " " : " not ");
>   
>   	return 0;