[SeaBIOS] varlow/extrastack vs code

Dr. David Alan Gilbert dgilbert at redhat.com
Fri Jan 20 19:40:44 CET 2017

  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's supposed to stop that happening?

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.

(This is based off 1.9.1)

Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK

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