[SeaBIOS] varlow/extrastack vs code
Dr. David Alan Gilbert
dgilbert at redhat.com
Fri Jan 20 20:39:28 CET 2017
* Kevin O'Connor (kevin at koconnor.net) wrote:
> 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.
OK, I'll try and trace that.
> > 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?
Hangs, typically after/in display_uuid or kvm entry exceptions where
the EIP is totally bogus; they only happen sometimes on reboot,
and adding some debug can make them totally disappear.
So the thought of the code beign scribbled over by a stack sounded
like a reasonable explanation.
> > 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.
Hmm OK.
Dve
> -Kevin
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK
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