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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jan 12, 2022, at 10:16 AM, Florian Weimer fw@deneb.enyo.de wrote:

> * 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.

The route I'm taking in my subsequent version of the patch is very close to
pushing a signal frame: on abort, skip the redzone, and push the abort-at-ip
pointer. Then abort handler is then expected to pop the abort-at-ip pointer
and unskip the redzone.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com