Re: [PATCH v5 02/15] util: Extend sock_probe_mem() to sock_probe_features()

[Stefano Brivio <sbrivio@redhat.com>]

On Tue,  2 Dec 2025 15:02:02 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> sock_probe_mem() currently checks whether we're able to allocate large
> socket buffers.  Extend it to also check whether the SO_BINDTODEVICE
> socket option is available.  Rename to sock_probe_features() to reflect the
> new functionality.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  passt.c |  2 +-
>  passt.h |  2 ++
>  util.c  | 19 +++++++++++++++++--
>  util.h  |  2 +-
>  4 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/passt.c b/passt.c
> index 4964427d..0b84ac6c 100644
> --- a/passt.c
> +++ b/passt.c
> @@ -381,7 +381,7 @@ int main(int argc, char **argv)
>  	if (setrlimit(RLIMIT_NOFILE, &limit))
>  		die_perror("Failed to set current limit for open files");
>  
> -	sock_probe_mem(&c);
> +	sock_probe_features(&c);
>  
>  	conf(&c, argc, argv);
>  	trace_init(c.trace);
> diff --git a/passt.h b/passt.h
> index 15801b44..79d01ddb 100644
> --- a/passt.h
> +++ b/passt.h
> @@ -204,6 +204,7 @@ struct ip6_ctx {
>   * @freebind:		Allow binding of non-local addresses for forwarding
>   * @low_wmem:		Low probed net.core.wmem_max
>   * @low_rmem:		Low probed net.core.rmem_max
> + * @no_bindtodevice:	Unprivileged SO_BINDTODEVICE not available
>   * @vdev:		vhost-user device
>   * @device_state_fd:	Device state migration channel
>   * @device_state_result: Device state migration result
> @@ -281,6 +282,7 @@ struct ctx {
>  
>  	int low_wmem;
>  	int low_rmem;
> +	int no_bindtodevice;
>  
>  	struct vu_dev *vdev;
>  
> diff --git a/util.c b/util.c
> index 347f34f5..bad38129 100644
> --- a/util.c
> +++ b/util.c
> @@ -233,12 +233,13 @@ int sock_unix(char *sock_path)
>  }
>  
>  /**
> - * sock_probe_mem() - Check if setting high SO_SNDBUF and SO_RCVBUF is allowed
> + * sock_probe_features() - Probe for socket features we might use
>   * @c:		Execution context
>   */
> -void sock_probe_mem(struct ctx *c)
> +void sock_probe_features(struct ctx *c)
>  {
>  	int v = INT_MAX / 2, s;
> +	const char lo[] = "lo";
>  	socklen_t sl;
>  
>  	s = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
> @@ -247,6 +248,7 @@ void sock_probe_mem(struct ctx *c)
>  		return;
>  	}
>  
> +	/* Check if setting high SO_SNDBUF and SO_RCVBUF is allowed */
>  	sl = sizeof(v);
>  	if (setsockopt(s, SOL_SOCKET, SO_SNDBUF, &v, sizeof(v))	||
>  	    getsockopt(s, SOL_SOCKET, SO_SNDBUF, &v, &sl) ||
> @@ -259,6 +261,19 @@ void sock_probe_mem(struct ctx *c)
>  	    (size_t)v < RCVBUF_BIG)
>  		c->low_rmem = 1;
>  
> +	/* Check if SO_BINDTODEVICE is available
> +	 *
> +	 * Supported since kernel version 5.7, commit c427bfec18f2 ("net: core:
> +	 * enable SO_BINDTODEVICE for non-root users").  Some distro kernels may
> +	 * have backports, of course.  Record whether we can use it so that we
> +	 * can give more useful diagnostics.
> +	 */
> +	if (setsockopt(s, SOL_SOCKET, SO_BINDTODEVICE, lo, sizeof(lo)-1)) {

Nit: our coding style uses spaces around arithmetic operators... I took
the liberty of fixing this up on merge, though.

-- 
Stefano