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Date: Mon, 10 Aug 2020 21:29:52 +0200
From: Olaf Flebbe <>
Subject: Re: Revisiting sigaltstack and implementation-internal signals

Hi Rick,

Since the alternate stack is only used for signal handlers, one can limit to the allowed signal safe functions:
There is a list of async-safe-signal functions copied from the OpenGroup documents.

swapcontext is not mentioned here :)

Sounds to me moving the stack pointer like you are describing is not allowed.

Installing the same stack both as regular and alternate stack sound to me like asking for trouble as well.


> Am 10.08.2020 um 20:32 schrieb Rich Felker <>:
> 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 <>:
>>> 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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