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Date: Mon, 10 Aug 2020 19:04:36 +0200
From: Olaf Flebbe <>
Subject: Re: Revisiting sigaltstack and implementation-internal signals

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 .


> Am 10.08.2020 um 19:00 schrieb Rich Felker <>:
> 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 
>> 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 <>:
>>> 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

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