Re: [musl] Is errno signal-safe?

[Carlos O'Donell <carlos@redhat.com>]

On 2/16/22 14:40, Markus Wichmann wrote:
> Hi all,
> 
> today I had a flash of inspiration while staring at some code: errno is
> a global variable, right? OK, it is thread-local, but still a global
> variable in the context of one thread. And looking at a global variable
> while it may (or may not) be modified in a signal handler is not safe to
> do.

It is required that errno, if changed, must be restored by the signal handler before exit
(though note that for glibc the underlying lazy TLS allocation implementation makes errno
AS-unsafe for first use in a signal handler because calloc is used to allocate the storage).
 
> So now I have to wonder. There are a bunch of functions that set errno,
> that are on the ostensibly async-signal-safe list, like for example
> write(). And to my knowledge, changes to errno are not turned back by
> sigreturn(). So, are changes to errno made in a signal handler
> propagated to the main program? If so, how do I inspect errno correctly
> in the main program? I could block signals, but for one thing, doing so
> every time errno might be relevant is going to be overkill, and for two,
> if the system call I want the errno from is also blocking and I want to
> allow signals while the call is blocking, there is no way to do that
> without race condition.

You don't need to do any of that. A correctly written signal handler must save and
restore errno or not modify it all.

It has been discussed before by both glibc and musl developers that it might be a good
idea all around to wrap signal handlers and save and restore errno in the wrapper
to avoid this entire class of problems (but this is easier said than done).

> But then again, now that I thought of it, this is so obvious that surely
> someone else must have stumbled across it before, right? A solution must
> exist, right?

It's not entirely obvious (like [1] is not always obvious), but it has been discussed and
considered, and the solution is emergent given the standards requirements :}

-- 
Cheers,
Carlos.

[1] https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=dbb01cbbdb60c34a16d9d48cb58ed3680a5dd36d