[LEDE-DEV] Help me understand ubus extensions via C api

[Alexandru Ardelean]

On Thu, Jul 6, 2017 at 4:31 AM, Carlito Nueno <carlitonueno at gmail.com> wrote:
> Hi all,
>
> I want to customize a small feature in how hostapd responds to probe
> request and I was looking at
> https://github.com/lede-project/source/blob/master/package/network/services/hostapd/src/src/ap/ubus.c
>
> Please help me understand how it works? for example when I call `ubus
> call hostapd.wlan0-1 del_client "{....}" what happens?

the `ubus` cli will talk to ubusd
ubusd is the broker [or hub] for all things ubus between all processes
[that register with ubus[d] ]

hostapd has registered the `hostapd.wlan0-1` ubus object here
https://github.com/lede-project/source/blob/master/package/network/services/hostapd/src/src/ap/ubus.c#L457
via the ubus_add_object()

all registration to ubusd is done via libubus, which abstracts a lot
of the stuff to connect/talk to ubusd
all that you need to use [in a daemon/process with ubus] is in
libubus.h ; that header is exported at build time for other modules to
use
http://git.openwrt.org/?p=project/ubus.git;a=blob;f=libubus.h;h=4e45cb620a28befe23c27d3b8dc7eb1a1d12fb1a;hb=HEAD

it's important that all operations to ubus[d] be done on a
ubus_context object, which is a wrapper struct for a socket FD, some
buffer data, and other stuff

[ well, this was mostly ubus internals [in case it interests you ] ]

>
> I know that ubus_cli
> (http://git.openwrt.org/?p=project/ubus.git;a=blob;f=cli.c;h=19ccbb5093ce4c326010a9b2f504d7cd50798275;hb=HEAD)
> parses the input but how does ubus call the functions inside the above
> file (the ubus.c, ubus hostapd extension)?

the ubus cli tool uses libubus
but the code for cli, may not be a sufficient example to do other
operations [like in hostapd ]

>
> I see that there is this function, but how is that called
>
> static int hostapd_bss_del_client(struct ubus_context *ctx, struct
> ubus_object *obj, struct ubus_request_data *req, const char *method,
> struct blob_attr *msg);
>

Coming back to your "I want to customize a small feature in how hostapd"
I think that all you might need to do is register a new function here:
https://github.com/lede-project/source/blob/master/package/network/services/hostapd/src/src/ap/ubus.c#L419
or extend an existing one.

As I mentioned, the ubus cli talks to ubusd.
When doing `ubus call hostapd.wlan0-1 del_client "{....}"` , this
means that hostapd has already registered a ` hostapd.wlan0-1` with
ubusd
When you call "del_client", the hostapd_bss_del_client() callback will
be called [ubus cli will ask ubusd to access del_client from
hostapd.wlan0-1 ] , but before that, the `del_policy` object will be
used to validate the message format.
[ You may have noticed that some callbacks are UBUS_METHOD_NOARG() and
some are UBUS_METHOD() ; the ones without a policy arg will get
processed without any msg format validation ]

>
> Thank you!
>

Maybe this information was not too well structured.
But hope it helps.

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