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SubjectRe: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?
On Sat, Nov 19, 2016 at 8:52 PM, Andy Lutomirski <> wrote:
> This is a question for the old-timers here, since I can't find
> anything resembling an answer in the SDM.
> Suppose an exception happens (#UD in this case, but I assume it
> doesn't really matter). We're not in long mode, and the IDT is set up
> to deliver to a normal 32-bit kernel code segment. We're running in
> that very same code segment when the exception hits, so no CPL change
> occurs and the TSS doesn't particularly matter.
> The CPU will push EFLAGS, CS, and RIP. Here's the question: what
> happens to the high word of CS on the stack?
> The SDM appears to say nothing at all about this. Modern systems
> (e.g. my laptop running in 32-bit legacy mode under KVM) appear to
> zero-extend CS. But Matthew's 486DX appears to put garbage in the
> high bits (or maybe just leave whatever was already on the stack in
> place).
> Do any of you happen to know what's going on and when the behavior
> changed? I'd like to know just how big of a problem this is. Because
> if lots of CPUs work like Matthew's, we have lots of subtle bugs on
> them.
> --Andy

This came up a while back, and we was determined that we can't assume
zero-extension in 32-bit mode because older processors only do a
16-bit write even on a 32-bit push. So all segments have to be
treated as 16-bit values, or we have to explicitly zero-extend them.

All 64-bit capable processors do zero-extend segments, even in 32-bit mode.

Brian Gerst

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