From: Carlos O'Donell <carlos@redhat.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: musl@lists.openwall.com, Markus Wichmann <nullplan@gmx.net>
Subject: Re: [musl] Is errno signal-safe?
Date: Wed, 16 Feb 2022 16:02:49 -0500 [thread overview]
Message-ID: <4481efb2-88d5-6b42-215d-6d7d96713274@redhat.com> (raw)
In-Reply-To: <87wnhuy19i.fsf@oldenburg.str.redhat.com>
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@GLIBC_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.
prev parent reply other threads:[~2022-02-16 21:03 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-16 19:40 Markus Wichmann
2022-02-16 19:56 ` Carlos O'Donell
2022-02-16 21:01 ` Florian Weimer
2022-02-16 21:02 ` Carlos O'Donell [this message]
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