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Date: Mon, 10 Aug 2020 12:27:27 -0400
From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
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User-Agent: Mutt/1.5.21 (2010-09-15)
Subject: Re: [musl] Revisiting sigaltstack and implementation-internal signals

On Mon, Aug 10, 2020 at 05:45:06PM +0200, Olaf Flebbe wrote:
> 
> 
> > Am 10.08.2020 um 17:41 schrieb Szabolcs Nagy <nsz@port70.net>:
> > 
> > * Olaf Flebbe <of@oflebbe.de> [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.
> 
> 
> Deallocating an assigned sigaltstack without resetting sigaltstack  is undefined behaviour.

This is entirely correct and is not the relevant case.