Vineet Gupta <vineetg@xxxxxxxxxxxx> writes: > On 2/14/23 08:50, Björn Töpel wrote: >> Andy Chiu <andy.chiu@xxxxxxxxxx> writes: >> >>> Hey Björn, >>> >>> On Tue, Feb 14, 2023 at 2:43 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote: >>>> So, two changes: >>>> >>>> 1. Disallow V-enablement if the existing altstack does not fit a V-sized >>>> frame. >>> This could potentially break old programs (non-V) that load new system >>> libraries (with V), If the program sets a small alt stack and takes >>> the fault in some libraries that use V. However, existing >>> implementation will also kill the process when the signal arrives, >>> finding insufficient stack frame in such cases. I'd choose the second >>> one if we only have these two options, because there is a chance that >>> the signal handler may not even run. >> I think we might have different views here. A process has a pre-V, a and >> post-V state. Is allowing a process to enter V without the correct >> preconditions a good idea? Allow to run with V turned on, but not able >> to correctly handle a signal (the stack is too small)? > > The requirement is sane, but the issue is user experience: User trying > to bring up some V code has no clue that deep in some startup code some > alt stack had been setup and causing his process to be terminated on > first V code. > >> >> This was the same argument that the Intel folks had when enabling >> AMX. Sure, AMX requires *explicit* enablement, but same rules should >> apply, no? >> >>>> 2. Sanitize altstack changes when V is enabled. >>> Yes, I'd like to have this. But it may be tricky when it comes to >>> deciding whether V is enabled, due to the first-use trap. If V is >>> commonly used in system libraries then it is likely that V will be >>> enabled before an user set an altstack. Sanitizing this case would be >>> easy and straightforward. > > Good. Lets have this in v14 as it seems reasonably easy to implement. > >>> But what if the user sets an altstack before >>> enabling V in the first-use trap? This could happen on a statically >>> program that has hand-written V routines. This takes us to the 1st >>> question above, should we fail the user program immediately if the >>> altstack is set too small? > > Please lets not cross threads. We discussed this already at top. While > ideally required, seems tricky so lets start with post-V alt stack check. > >> For me it's obvious to fail (always) "if the altstack is too small to >> enable V", because it allows to execute V without proper preconditions. >> >> Personally, I prefer a stricter model. Only enter V if you can, and >> after entering it disallow changing the altstack. >> >> Then again, this is *my* opinion and concern. What do other people >> think? I don't want to stall the series. > > I concur that the alt stack checking requirements are sensible in the > long run. We can add the obvious check for post-V case and see if there > is a sane way to flag pre-V case to. Reasonable. @Andy does this resonate with you as well? Björn
