Re: git: 39fdad34e220 - main - stand: impose 510,000 byte limit for /boot/loader and /boot/pxeldr

On 8/11/22 11:18 AM, Warner Losh wrote:
> On Thu, Aug 11, 2022 at 10:56 AM John Baldwin <jhb@freebsd.org> wrote:
>> You really want to apply the size check to loader.bin, not loader.  The
>> memory
>> layout down in the first 1MB for boot loaders is roughly:
>>
>> 0x0000: real-mode IDT
>> 0x0400: BIOS data
>> 0x7c00: where BIOS loads boot loaders such as /boot/mbr, etc.
>> 0x1000: various BTX global data like GDT, TSS, IDT, stacks
>> 0x9000: BTX kernel
>> 0xa000: BTX client (loader.bin)
>> 0xa0000: top of BTX client stack (though this can be a bit lower for cases
>> like
>>            PXE booting)
>>
>> The real size constraint is on the BTX client (loader.bin) and the fact
>> that
>> it's text/data/bss plus stack need to fit into that 576k window (give or
>> take).
>> btxldr isn't stored in low memory, so its size isn't relevant, and BTX's
>> code
>> always takes up a full page even though it is much smaller.
>>
> 
> Where does 576k come from? That's 589824 bytes, but a0000-a000 is 614400
> bytes. The delta is 24k (24576). My 'observed' value of about 515,000 is
> another
> 75k below that, suggesting we are needing 100k of stack? Can that be right?
> I knew
> lua ate a lot of stack, but wow!

Hmm, I converted 0xa000 to 64k instead of 40k in my head which is where
the 576k came from.  Yeah, the total size is 600k.  100k stack does seem
like a lot.  It is possible that other BIOS ROMs besides just PXE might
steal data from the stack top.  You could maybe try looking at some of
your existing BIOS-boot machines to check the 16-bit word at physical
address 0x413.  That gives you the actual top of the 640k window.  On
my current desktop (albeit booted via UEFI and not BIOS) it is 629k:

% sudo dd if=/dev/mem bs=1 iseek=0x413 count=2 | hd
2+0 records in
2+0 records out
2 bytes transferred in 0.000026 secs (75643 bytes/sec)
00000000  75 02                                             |u.|
00000002
% gdb
...
(gdb) p/d 0x275
$2 = 629

Still, that's only 11k gone.

I do think that when people have ran into trouble in the past it has been
in situations that can recurse, e.g. using psuedo-filesystems like gzipfs
where the gzipfs open routine tries to open the underlying "foo.gz" file,
etc.

-- 
John Baldwin