Re: [PATCH v2] conf: Unify several paths in conf_ports()

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 12 Mar 2025 14:43:59 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> In conf_ports() we have three different paths which actually do the setup
> of an individual forwarded port: one for the "all" case, one for the
> exclusions only case and one for the range of ports with possible
> exclusions case.
> 
> We can unify those cases using a new helper which handles a single range
> of ports, with a bitmap of exclusions.  Although this is slightly longer
> (largely due to the new helpers function comment), it reduces duplicated
> logic.  It will also make future improvements to the tracking of port
> forwards easier.
> 
> The new conf_ports_range_except() function has a pretty prodigious
> parameter list, but I still think it's an overall improvement in conceptual
> complexity.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  conf.c | 173 ++++++++++++++++++++++++++++++---------------------------
>  1 file changed, 90 insertions(+), 83 deletions(-)
> 
> v2:
>  * Commit message updated slightly, but otherwise unmodified.
> 
> 
> diff --git a/conf.c b/conf.c
> index 065e7201..4e0099ba 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -123,6 +123,75 @@ static int parse_port_range(const char *s, char **endptr,
>  	return 0;
>  }
>  
> +/**
> + * conf_ports_range_except() - Set up forwarding for a range of ports minus a
> + *                             bitmap of exclusions
> + * @c:		Execution context
> + * @optname:	Short option name, t, T, u, or U
> + * @optarg:	Option argument (port specification)
> + * @fwd:	Pointer to @fwd_ports to be updated
> + * @addr:	Listening address
> + * @ifname:	Listening interface
> + * @first:	First port to forward
> + * @last:	Last port to forward
> + * @exclude:	Bitmap of ports to exclude
> + * @to:		Port to translate @first to when forwarding
> + * @weak:	Ignore errors, as long as at least one port is mapped
> + */
> +static void conf_ports_range_except(const struct ctx *c, char optname,
> +				    const char *optarg, struct fwd_ports *fwd,
> +				    const union inany_addr *addr,
> +				    const char *ifname,
> +				    uint16_t first, uint16_t last,
> +				    const uint8_t *exclude, uint16_t to,
> +				    bool weak)
> +{
> +	bool bound_one = false;
> +	unsigned i;
> +	int ret;
> +
> +	if (first == 0) {
> +		die("Can't forward port 0 for option '-%c %s'",
> +		    optname, optarg);
> +	}

This introduces two subtle functional changes that are a bit unexpected
given the commit message. Before:

$ ./pasta -t 0
$

$ ./pasta -t 0-1025
Failed to bind port 1 (Permission denied) for option '-t 0-1025', exiting

After:

$ ./pasta -t 0
Can't forward port 0 for option '-t 0'

$ ./pasta -t 0-1025
Can't forward port 0 for option '-t 0-1025'

...anyway, I doubt anybody would use -t 0 on purpose (to get a port
automatically assigned), and while it probably works for TCP (check
bound ports after starting pasta, use the assigned one), it wouldn't
necessarily work as expected for UDP if the application relies on our
flow tracking.

For TCP, actually, -t 0 might be useful, see e.g. random_free_port() in
Podman tests (/test/system/helpers.network.bash). We should print the
port number that was bound, though, and document the feature.

More than that: that could actually be the only race-free possibility
of picking and forwarding a port where the number doesn't matter.

In any case, given that it works by mistake now and it's undocumented,
let me go ahead and apply this for the moment. We can add the
"functionality" back later if it makes sense.

-- 
Stefano