Date: Wed, 16 Feb 2022 14:56:12 -0500 From: Carlos O'Donell <carlos@...hat.com> To: musl@...ts.openwall.com, Markus Wichmann <nullplan@....net> Subject: Re: Is errno signal-safe? 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  is not always obvious), but it has been discussed and considered, and the solution is emergent given the standards requirements :} -- Cheers, Carlos.  https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=dbb01cbbdb60c34a16d9d48cb58ed3680a5dd36d
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