[patch] cope with buggy dhcp servers

[Cristian Ionescu-Idbohrn]

On Sat, 24 Nov 2007, Denys Vlasenko wrote:

> +#define UDHCP_UDP_PACKET_SIZE_MIN ((sizeof(struct udp_dhcp_packet)) - \
> +       (CONFIG_UDHCP_OPTIONS_SLACK_FOR_BUGGY_SERVERS))
> +
>  /* Let's see whether compiler understood us right */
>  struct BUG_bad_sizeof_struct_udp_dhcp_packet {
>         char BUG_bad_sizeof_struct_udp_dhcp_packet
> -                [sizeof(struct udp_dhcp_packet) != 576 ? -1 : 1];
> +               [sizeof(struct udp_dhcp_packet) !=
> +                ((UDHCP_UDP_PACKET_SIZE_MIN) +
> +                 (CONFIG_UDHCP_OPTIONS_SLACK_FOR_BUGGY_SERVERS)) ? -1 : 1];
>
> This check as written above can never fail.

Yes.

> The check should have literal 576 - we are checking than neither we nor
> compiler goof up.

That's fine.

> Maximum allowed slack size seems to be wrong, should be 1500 - 576 = 924.

Double checked, and you're right.

> See attached patch.
>
> I changed the default from 0 to 300, it doesn't seem to have any effect
> on code size:

That should not be necessary.  0 shuld be fine in most cases.

> $ make bloatcheck
> function                                             old     new   delta
> ------------------------------------------------------------------------------
> (add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0)                 Total: 0 bytes
>    text    data     bss     dec     hex filename
>  776551     929    9100  786580   c0094 busybox_old
>  776551     929    9100  786580   c0094 busybox_unstripped
>
> Please test whether patch works for you.

I'll do that as soon as possible.  I have to get my hands on one of those
routers.

This comment:

+            maximum size of entire IP packet, and sends packets which are
+            268 bytes too large.

does not seem to match the real behaviour.  The packet size difference
is _exactly_ 28 bytes:

I: sizeof(struct struct dhcpMessage): 548
I: sizeof(struct iphdr): 20
I: sizeof(struct udphdr): 8
I: sizeof(struct udp_dhcp_packet): 576

It looks like they confuse the dhcpMessage and udp_dhcp_packet sizes.

There also seems to be whitespace damage in Config.in (plain space
characters instead of 'TAB, space, space' in your patch).


Cheers,

-- 
Cristian