[ipxe-devel] [PATCH] [undi] Add stack segment logging

[Michael Brown]

On 09/06/16 12:53, Ladi Prosek wrote:
> On Thu, Jun 9, 2016 at 1:05 PM, Michael Brown <mcb30 at ipxe.org> wrote:
>> On 09/06/16 11:46, Ladi Prosek wrote:
>>> The UNDI ROM ID structure declares the minimum required stack segment
>>> size but the value is not used by iPXE.
>>
>> iPXE doesn't use any meaningful amount of real-mode stack space outside of
>> its own data segment.  Without checking, I think we should be using less
>> than 64 bytes before switching to our own stack.
>
> Is it not possible that what we start with is already less than what
> the NIC requires, though? I was surprised to see rm_sp pretty low on
> my system - 2F10. What guarantees that this will be enough for any
> UNDI ROM?

Sorry; I'd skim-read your patch and misread it as applying to our own 
UNDI loader (i.e. when we are being loaded by something external).

We provide a 4kB real-mode stack, which we use for any calls to an 
underlying PXE ROM.  The rm_sp value is somewhat meaningless to compare, 
since the stack is placed at an arbitrary (non-zero) offset within our 
data segment: this is a (very indirect) consequence of using gcc, which 
has no concept of segmentation and effectively enforces %ds=%ss.  We 
could potentially move the stack to the start of the data segment, but 
this would complicate our decompressor since we assume that 
initialised-data precedes uninitialised-data in each segment.

There is code (e.g. IBM Tivoli / Rembo-ia32) which provides less than 
512 bytes of stack space.  We should be safe offering 4kB.

Michael