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Date: Sun, 9 Aug 2020 18:28:17 -0600
From: Ariadne Conill <>
Subject: Re: Revisiting sigaltstack and implementation-internal signals


On 2020-08-08 18:39, Rich Felker wrote:
> It's come up again, via Go this time (see
>, that it would be nice to
> have musl use the alternate signal stack for implementation-internal
> signals. I've previously wanted to do this, but been unclear on (1)
> whether it's permissible for the implementation to touch the
> application-provided alternate stack when there is no signal delivered
> on it (possibly not even any signal handlers installed), and (2)
> whether we should care about breaking code that swaps off of and back
> onto the alternate signal stack with swapcontext.
> In regards to question (1), I believe this language from the
> specification of sigaltstack is sufficient to resolve it:
>      "The range of addresses starting at ss_sp up to but not including
>      ss_sp+ ss_size is available to the implementation for use as the
>      stack."
> I read "available to the implementation" as implying that the
> application can make no assumptions about values previously stored in
> the memory being retained.

This seems like a reasonable position.

> This still leaves (2) open, as well as whether there are any other
> reasons why we shouldn't have implementation-internal signals using
> the alternate stack.

In my opinion, mixing stacks with ucontext calls and sigaltstack is 
undefined behavior.  There is no way to guarantee the safety of such 
operations, or at least none that I can think of.

So personally, I think if people do that, they are basically asking for 
problems, and we have no obligation to fix those problems.


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