Re: [edk2-devel] [PATCH 2/2] MdeModulePkg/DxeCore: Fix stack overflow issue due to nested interrupts

["Paolo Bonzini" <pbonzini@redhat.com>]

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Il ven 1 mar 2024, 12:10 Michael Brown <mcb30@ipxe.org> ha scritto:

> It feels as though this should be able to be cleanly modelled with a
> single global state array
>
>    BOOLEAN mSavedInterruptState[TPL_HIGH_LEVEL]
>

Pretty much, yes. But you only have to write it when the interrupts are
already disabled, so the bitmask works and makes it easier to clear "all
values at NewTpl and above" with just an AND.

>
> (or possibly a bitmask, though using the array avoids having to disable
> interrupts just to write a value).
>
> I still need to think through the subtleties, to make sure it could cope
> with pathological edge cases such as
>
>    OldTpl = gBS->RaiseTPL (TPL_HIGH_LEVEL);
>
>    ...
>
>    gBS->RestoreTPL (OldTpl);
>    gBS->RestoreTPL (OldTpl);
>
> or
>
>    OldTpl = gBS->RaiseTPL (TPL_HIGH_LEVEL - 1);
>    gBS->RaiseTPL (TPL_HIGH_LEVEL);
>
>    ..
>
>    gBS->RestoreTPL (OldTpl);
>
> I think that at least one of the above pathological usage patterns would
> break the existing mInterruptedTplMask patches, since they currently
> clear state in RestoreTPL() and so will not correctly handle a duplicate
> call to RestoreTPL().
>

I think my patch works (modulo the 1 vs. 1U issue) for the second.
Declaring the first to be invalid is a good idea IMO. Also it would only
break in interrupt handlers and would revert to just causing a stack
overflow in the interrupt storm scenario, so it wouldn't be too bad...

Paolo


> I'll try to get a patch put together over the weekend.
>
> Thanks,
>
> Michael
>
>


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