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Date: Mon, 10 Aug 2020 17:41:23 +0200
From: Szabolcs Nagy <>
To: Olaf Flebbe <>
Subject: Re: Revisiting sigaltstack and implementation-internal signals

* Olaf Flebbe <> [2020-08-10 10:15:13 +0200]:
> 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).
> Tested with go and a glibc threaded setuid test tst-setuid3.c .

this will fail if an application calls sigaltstack,
then blocks all user signals that are SA_ONSTACK and
then deallocates the stack passed to sigaltstack.

it is important to discuss what an application may
or may not do, because the proposed change observably
modifies the behaviour.

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