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Message-ID: <4481efb2-88d5-6b42-215d-6d7d96713274@redhat.com>
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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