Re: [PATCH v2 12/13] x86: Add support for dynamic kernel stacks via FRED

[Thomas Gleixner <tglx@kernel.org>]

On Fri, Jun 26 2026 at 21:05, David Stevens wrote:
> On Fri, Jun 26, 2026 at 3:39 PM Thomas Gleixner <tglx@kernel.org> wrote:
> That is definitely a situation that could happen, I hadn't thought of
> it. Luckily, I don't think we need NMI-safe re-entrancy to handle
> this, since an NMI on SL0 couldn't have interrupted a dynamic stack
> fault.

Correct.

>> As a consequence you copy stack data around to the same place, which
>> should be benign, but it is well understood that memcpy() source and
>> destination areas _must_ not overlap. That's UB, no?
>>
>> I know that should not happen, but that doesn't make it less UB :)
>
> That sort of fault would probably end up either with a
> page_fault_oops() or risk a context switch with SL>0, but we shouldn't
> make debugging harder with UB.

Definitely not :)

You can avoid quite some of the problems right in the ASM entry code.
Instead of checking event type/vector and then going to the stack switch
logic, you can check the stack level on which the event happened
first. If it's not 0, then there is nothing to do.