[syslinux] Maximum line length of APPEND option in PXELINUX

Jim Cromie jcromie at divsol.com
Fri Feb 25 06:47:42 PST 2005


H. Peter Anvin wrote:

> Jim Cromie wrote:
> >
>
>> perhaps its worth pointing out that 255 is a kernel limit - unless 
>> Documentation/i386/boot.txt
>> is oo-date:
>>
>> **** THE KERNEL COMMAND LINE
>>
>> The kernel command line is a null-terminated string up to 255
>> characters long, plus the final null.
>>
>> so any error msg from pxelinux is reflecting the fact that you wont 
>> succeed anyway.
>>
>
> Indeed it is.  I seem to remember it being an explicit request on this 
> mailing list that it didn't fail silently like it had been.
>
failsafe good.  mysterious-fail bad.

> Now, with bootproto 2.02+ it should be possible to extend the limit in 
> the kernel.  For syslinux 3.xx it should be pretty easy to extend the 
> limit in syslinux to match; for previous versions of syslinux it would 
> be much harder.
>
>     -hpa
>

IIUC, an rdev-like kernel image reading could know the kernel is 2.02+,
and (I infer from this thread) such kernels are able to handle >255 
command-lines.

to determine this property, youd fetch the kernel 1st, then look inside it,
and *then* prepare the command-line.  OR prepare cmdline before fetch as 
if >255 is ok,
and truncate and warn if bootproto <2.02.

in any case, virtually all 2.4.x kernels are 2.02+, and all 2.6.x are.

is that close enough, or did I get all crossed up ?

my $.02 - dont bother doing long-cmdline for 2.xx, its now legacy..




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