Re: [PATCH] THE LINUX/I386 BOOT PROTOCOL - Breaking the 256 limit (ping)

H. Peter Anvin
2006-08-28T20:43:28Z

Matt Domsch wrote:
> On Mon, Aug 28, 2006 at 12:00:37PM -0700, H. Peter Anvin wrote:
>> Matt Domsch wrote:
>>> No reason. I was just trying to be careful, not leaving data in the
>>> upper bits of those registers going uninitialized. If we know they're
>>> not being used ever, then it's not a problem. But I don't think
>>> that's the source of the command line size concern, is it?
>>>
>> No, it's treating the command line as a fixed buffer, as opposed to a
>> null-terminated string. This was always a bug, by the way.
>
> OK, I'll look at fixing that, and using %esi throughout.
>

There is a lot of weirdness in this code; it's broken in an enormous
amount of ways (sorry, Matt). This comment, for example:

pushl %esi
cmpl $0, %cs:cmd_line_ptr
jz done_cl
movl %cs:(cmd_line_ptr), %esi
# ds:esi has the pointer to the command line now

... doesn't handle the old boot protocol, and doesn't at all deal with
the fact that cmd_line_ptr is an absolute address, and not at all
relative to SETUPSEG, which is the normal value for %ds at this point.
For the old protocol, this is a 16-bit pointer which is relative to
INITSEG (not SETUPSEG), but this code just completely ignores it.

I'll hack up a patch for this.

-hpa