Re: [PATCH 2/2] x86/entry: Fix unwinding from kprobe on PUSH/POP instruction

[Josh Poimboeuf]

On Tue, Feb 14, 2023 at 12:35:04PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 13, 2023 at 11:43:57PM +0900, Masami Hiramatsu wrote:
> 
> > > Fix it by annotating the #BP exception as a non-signal stack frame,
> > > which tells the ORC unwinder to decrement the instruction pointer before
> > > looking up the corresponding ORC entry.
> > 
> > Just to make it clear, this sounds like a 'hack' use of non-signal stack
> > frame. If so, can we change the flag name as 'literal' or 'non-literal' etc?
> > I concern that the 'signal' flag is used differently in the future.

Agreed, though I'm having trouble coming up with a succinct yet
scrutable name.  If length wasn't an issue it would be something like

  "decrement_return_address_when_looking_up_the_next_orc_entry"

> Oooh, bike-shed :-) Let me suggest trap=1, where a trap is a fault with
> a different return address, specifically the instruction after the
> faulting instruction.

I think "trap" doesn't work because

 1) It's more than just traps, it's also function calls.  We have
    traps/calls in one bucket (decrement IP); and everything else
    (faults, aborts, irqs) in the other (don't decrement IP).

 2) It's not necessarily all traps which need the flag, just those that
    affect a previously-but-now-overwritten stack-modifying instruction.
    So #OF (which we don't use?) and trap-class #DB don't seem to be
    affected.  In practice maybe this distinction doesn't matter, but
    for example there's no reason for ORC try to distinguish trap #DB
    from non-trap #DB at runtime.

-- 
Josh