svn commit: r315653 - in head: lib/libstand sys/boot/common sys/boot/i386/libi386

[Kurt Lidl]

On 3/21/17 11:53 AM, Toomas Soome wrote:
>
>> On 21. märts 2017, at 17:15, Ian Lepore <ian at freebsd.org> wrote:
>>
>> On Tue, 2017-03-21 at 15:35 +0300, Slawa Olhovchenkov wrote:
>>> On Mon, Mar 20, 2017 at 10:20:17PM +0000, Toomas Soome wrote:
>>>
>>>>
>>>> Author: tsoome
>>>> Date: Mon Mar 20 22:20:17 2017
>>>> New Revision: 315653
>>>> URL: https://svnweb.freebsd.org/changeset/base/315653
>>>>
>>>> Log:
>>>>   loader: verify the value from dhcp.interface-mtu and use snprintf
>>>> to set mtu
>>>>
>>>>   Since the uset can set dhcp.interface-mtu, we need to try to
>>>> validate the
>>>>   value. So we verify if the conversion to int is successful and we
>>>> will not
>>>>   allow to set value greater than max IPv4 packet size.
>>>>
>>>> +				    tmp > USHRT_MAX) {
>>>> +					printf("%s: bad value:
>>>> \"%s\", "
>>>> +					    "ignoring\n",
>>>> +					    "dhcp.interface-mtu",
>>>> val);
>>> ===
>>> USHRT_MAX	Maximum value for an object of type unsigned short
>>> int
>>> 65535 (216-1) or greater*
>>>
>>> * the actual value depends on the particular system and library
>>> implementation, but shall reflect the limits of these types in the
>>> target platform.
>>> ===
>>>
>>> I mean IF_MAXMTU more correct.
>>>
>>
>> The context here is libstand; because it is standalone by design, the
>> code doesn't have access to IF_MAXMTU or other kernel/userland
>> constants.  There is also no question that a short is 16 bits or that
>> USHRT_MAX will be anything other than 65535 in that environment.  If
>> some platform did appear that had a different-sized short by default,
>> we would add whatever flags are necessary to force it back to 16 bits
>> in src/share/mk/bsd.stand.mk.
>>
>
>
> Also note the “upper” value is entirely fictional - we felt we need
> to  pick some “sane” default, with current (common 1Gb/s) ethernet hardware
> you will be in trouble long before reaching that value;)

All the world is not an VAX, nor is every network Ethernet.

HIPPI, not that you see much of it nowadays, had a MTU for
switched networks of 65280 (64K - 216 bytes of overhead).
Using USHRT_MAX is exactly the right thing here.

-Kurt