Re: [Qemu-devel] Stack corruption problem with SeaBIOS/gPXE under QEMU

[Naphtali Sprei]

Kevin O'Connor wrote:
> Hi,
> 
> On Thu, Nov 12, 2009 at 01:20:58PM +0200, Naphtali Sprei wrote:
>> I've found a problem with the usage of SeaBIOS/gPXE in Qemu.  The
>> scenario is when failing to boot from network and falling back to
>> booting from hard-disk (-boot nc).  The cause of the problem is that
>> both SeaBIOS and gPXE (in it's installation phase) uses same stack
>> area, 0x7c00.  The gPXE code corrupts the SeaBIOS stack, so when
>> gPXE returns to SeaBIOS chaos occurs.
>>
>> Output: "qemu: fatal: Trying to execute code outside RAM or ROM at 
>> 0x00000000eb300000"
> 
> Thanks for reporting this.
> 
> We can move the SeaBIOS stack, but it's not clear to me where to move
> it to.  Bochs bios puts the top of the stack at 0x10000, but this
> could potentially conflict with the OS load to 0x7c00.  So, in SeaBIOS
> the top of stack was moved to 0x7c00 to prevent this conflict.
> 
> Maybe the gPXE developers know where the bios typically places its
> stack.
> 
> However, I'm not sure why gPXE doesn't just use the stack it was
> given, or allocate the stack space it needs with PMM.
> 
>> A simple hack/patch (attached) solves this problem, but a proper
>> patch expected from the SeaBIOS guys.

Sorry for the misleading addressee. I should have addressed the request to the 
gPXE project, and not SeaBIOS project.
Since the gPXE uses the services of SeaBIOS, that where the changes should be.
Thanks for CC'ing them. 

>>
>>  Enjoy,
>>
>>   Naphtali
>>
>> Patch against current SeaBIOS git
> 
> The patch isn't against SeaBIOS.  Did you mean gPXE?

Sure, my mistake, against gPXE.

> 
> -Kevin