Messages in this thread | | | Date | Tue, 14 Feb 2023 09:05:52 -0800 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 2/2] x86/entry: Fix unwinding from kprobe on PUSH/POP instruction |
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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
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