Date: Wed, 16 Feb 2022 16:02:49 -0500 From: Carlos O'Donell <carlos@...hat.com> To: Florian Weimer <fweimer@...hat.com> Cc: musl@...ts.openwall.com, Markus Wichmann <nullplan@....net> Subject: Re: Is errno signal-safe? On 2/16/22 16:01, Florian Weimer wrote: > * Carlos O'Donell: > >> 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). > > glibc uses initial-exec TLS under the hood for storing the int variable, > so the variable access itself async-signal-safe (whether it goes > directly to errno@...BC_PRIVATE or via __errno_location, it doesn't > matter). Thanks. I'd forgotten about that quirk. Every *other* use of non-initial-exec TLS is still AS-unsafe. -- Cheers, Carlos.
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