Re: [RFC PATCH] rseq: x86: implement abort-at-ip extension

[Florian Weimer <fw@deneb.enyo.de>]

* Mathieu Desnoyers:

> ----- On Jan 7, 2022, at 2:31 PM, Florian Weimer fw@deneb.enyo.de wrote:
>
>> * Mathieu Desnoyers:
>> 
>>> Allow rseq critical section abort handlers to optionally figure out at
>>> which instruction pointer the rseq critical section was aborted.
>>>
>>> This allows implementing rseq critical sections containing loops, in
>>> which case the commit side-effect cannot be the last instruction. This
>>> is useful to implement adaptative mutexes aware of preemption in
>>> user-space. (see [1])
>> 
>> Could you write the program counter to the rseq area instead?  This
>> would avoid discussing which register to clobber.
>
> Using the rseq area for that purpose would be problematic for nested signal
> handlers with rseq critical sections. If a signal happens to be delivered
> right after the abort ip adjustment, its signal handler containing a rseq
> critical section could overwrite the relevant "abort-at-ip" field in the
> rseq per-thread area before it has been read by the abort handler interrupted
> by the signal.
>
> Making this architecture-agnostic is indeed a laudable goal, but I don't
> think the rseq per-thread area is a good fit for this.
>
> I also though about making the clobbered register configurable on a
> per-critical-section basis, but I rather think that it would be
> overengineered: too much complexity for the gain. Unless there are
> very strong reasons for choosing one register over another on a per
> use-case basis ?

You could perhaps push a signal frame onto the stack.  It's going to
be expensive, but it's already in the context switch path, so maybe it
does not matter.