Re: [musl] Revisiting sigaltstack and implementation-internal signals

[Rich Felker <dalias@libc.org>]

On Mon, Aug 10, 2020 at 07:04:36PM +0200, Olaf Flebbe wrote:
> Hi Rick,
> 
> Thanks for explanation, indeed: This might be a problem, if the
> business logic of the handler is under application control.
> But I was assuming that the handler context of __synccall is under
> musl control .

The handler in question is the one that's under application control
because the application installed it with intent for it to run on the
alternate stack. __synccall is the asynchronous clobbering of its
stack.

> > Am 10.08.2020 um 19:00 schrieb Rich Felker <dalias@libc.org>:
> > 
> > On Mon, Aug 10, 2020 at 06:57:21PM +0200, Olaf Flebbe wrote:
> >> Hi Rick ,
> >> 
> >> While the alternate stack is in use on cannot change the alternate stack.
> >> 
> >> See https://pubs.opengroup.org/onlinepubs/9699919799/ 
> >> EPERM Error.
> > 
> > No change of the alternate stack is described here. The minimal
> > example of the scenario only has one call to sigaltstack in the whole
> > program.
> > 
> > 
> >>> Am 10.08.2020 um 18:36 schrieb Rich Felker <dalias@libc.org>:
> >>> 
> >>> On Mon, Aug 10, 2020 at 10:15:13AM +0200, Olaf Flebbe wrote:
> >>>> Hi, 
> >>>> 
> >>>> I have some problems to follow the discussion here.
> >>>> 
> >>>> It is not about musl to create an alternate stack, it is to *honor* the alternate stack, if the application installed one, for a reason.
> >>>> 
> >>>> I am proposing smthg like
> >>>> 
> >>>> --- /oss/musl-1.2.1/src/thread/synccall.c
> >>>> +++ /work/musl/src/thread/synccall.c
> >>>> @@ -45,7 +45,7 @@
> >>>> {
> >>>> 	sigset_t oldmask;
> >>>> 	int cs, i, r;
> >>>> -	struct sigaction sa = { .sa_flags = SA_RESTART, .sa_handler = handler };
> >>>> +	struct sigaction sa = { .sa_flags = SA_RESTART|SA_ONSTACK, ....sa_handler = handler };
> >>>> 	pthread_t self = __pthread_self(), td;
> >>>> 	int count = 0;
> >>>> 
> >>>> This will fix the problem with dynamic stacks, like go implements it. 
> >>>> If the application does not install one, kernel will ignore
> >>>> SA_ONSTACK. (This is even specified by POSIX, since there is no
> >>>> error condition mentioned in man page specifically for this).
> >>> 
> >>> It's fundamental, since presence and identity of an alternate stack
> >>> are thread-local properties and SA_ONSTACK is global to the signal
> >>> disposition.
> >>> 
> >>> The behavior we're concerned about this alterring is not the case
> >>> where an application does not install an alternate stack; of course
> >>> that's unaffected. The interesting case is where an application does
> >>> install one, but expects (albeit IMO wrongly; that's what we're trying
> >>> to establish) that the stack memory is not touched/clobbered unless
> >>> there's actually an SA_ONSTACK signal handler present to run on it and
> >>> such a signal arrives. With the proposed change, the memory for the
> >>> alternate stack can be clobbered asynchronously with no such signal
> >>> handler existing. (In case it's not clear, the above code is *not a
> >>> signal handler* from the perspective that's relevant; it's an
> >>> implementation detail internal to the implementation.)
> >>> 
> >>> One way such clobbering could manifest is when a signal handler
> >>> running on the alternate stack temporarily moves the stack pointer to
> >>> somewhere else (not on the alternate stack), via swapcontext or some
> >>> other method. In this case, if a signal for cancellation or synccall
> >>> arrives, the kernel will consider the alt stack not in use, and will
> >>> start using it again from the beginning, clobbering the still-running
> >>> frames.
> >>> 
> >>> Rich
> >>