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Date: Wed, 16 Feb 2022 22:01:13 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Carlos O'Donell <carlos@...hat.com>
Cc: musl@...ts.openwall.com, Markus Wichmann <nullplan@....net>
Subject: Re: Is errno signal-safe?
* 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,
Florian
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