[SeaBIOS] varlow/extrastack vs code

[Kevin O'Connor]

On Fri, Jan 20, 2017 at 06:40:44PM +0000, Dr. David Alan Gilbert wrote:
> Hi,
>   I turned the debug level up to 4 on our smaller (128k) ROM downstream
> build and seem to have hit a case where it's been layed out so that the
> 'ExtraStack' is at the same location as some code (display_uuid) which
> was causing some very random behaviour;
> 
> from an objdump disassemble of the rom.o that was produced:
> 
>    ef79d:       f3 ab                   rep stos %eax,%es:(%edi)
>    ef79f:       8d 43 08                lea    0x8(%ebx),%eax
>    ef7a2:       b1 10                   mov    $0x10,%cl
>    ef7a4:       8d 54 24 74             lea    0x74(%esp),%edx
>    ef7a8:       e8 50 46 ff ff          call   e3dfd <memcmp>
>    ef7ad:       85 c0                   test   %eax,%eax
>    ef7af:       0f 84 33 01 00 00       je     ef8e8 <ExtraStack+0x110>
>    ef7b5:       8b 15 0c 23 0f 00       mov    0xf230c,%edx
>    ef7bb:       80 7a 06 02             cmpb   $0x2,0x6(%edx)
>    ef7bf:       0f b6 43 17             movzbl 0x17(%ebx),%eax
>    ef7c3:       77 0c                   ja     ef7d1 <StackPos+0x1>
>    ef7c5:       0f 85 80 00 00 00       jne    ef84b <ExtraStack+0x73>
>    ef7cb:       80 7a 07 05             cmpb   $0x5,0x7(%edx)
>    ef7cf:       76 7a                   jbe    ef84b <ExtraStack+0x73>
> 
> Note the 'ExtraStack+...' where as a few lines before it's maininit
> for other jumps, then looking at a sorted output of the rom.o.objdump:
> 
> 000eddf2 l     F .text  000000e0 virtio_scsi_add_lun.constprop.113
> 000eded2 l     F .text  0000080d device_hardware_setup
> 000ee6df l     F .text  00001a17 maininit                       <-------------
> 000ef790 g       *ABS*  00000000 final_varlow_start
> 000ef790 g     O *ABS*  00000004 BootSequence
> 000ef794 g     O *ABS*  00000001 FloppyDOR
> 000ef798 g     O *ABS*  00000008 LastUSBkey
> 000ef7a0 g     O *ABS*  00000001 Ps2ctr
> 000ef7a4 g     O *ABS*  00000004 RTCusers
> 000ef7a8 g     O *ABS*  00000004 TimerLast
> 000ef7ac g     O *ABS*  00000001 HaveAttemptedReboot
> 000ef7ad g     O *ABS*  00000001 Century
> 000ef7b0 g     O *ABS*  00000010 CDRom_locks
> 000ef7c0 g     O *ABS*  00000010 DefaultDPTE
> 000ef7d0 g     O *ABS*  00000004 StackPos
> 000ef7d8 g     O *ABS*  00000801 ExtraStack                     <-------------
> 000effdc g     O *ABS*  00000018 Call16Data
> 000f0000 g       *ABS*  00000000 final_readonly_start
> 000f0000 g       *ABS*  00000000 zonefseg_start
> 000f00f6 g     F .text  0000038b dopost
> 000f0481 g     F .text  000001c2 handle_pmm
> 000f0644 l     O .text  00000014 CSWTCH.1353
> 000f0658 l     O .text  00000014 __func__.14607
> 000f066c l     O .text  00000011 __func__.14624
> 000f0680 l     O .text  00000010 __func__.14549
> 000f0690 l     O .text  0000000b __func__.14497

What I think you're seeing here is an artifact of seabios' code
self-relocation.  The objdump stores the final location of "varlow"
variables, and not the location of their pre-relocation initial
values.  After the code is self-relocated (in post.c:reloc_preinit() )
it's malloc.c:malloc_init() (see memmove call) that copies over that
area of memory.

> What's supposed to stop that happening?

The code in scripts/layoutrom.py is supposed to layout the rom without
conflicts.  It's not clear to me if that's malfunctioning or if the
underlying issue is something else - what is the "very random
behaviour" you are seeing?

> I'd chosen a debug level of 4 since that was the largest it would go
> without the build complaining it wouldn't fit, so I thought I was
> safe since something did complain if it got way too big.

It should have been safe - something must not be right.

-Kevin