[musl] Re: [PATCH] Make abort() AS-safe (Bug 26275).

[Florian Weimer]

* Rich Felker:

> On Thu, Oct 01, 2020 at 08:08:24AM +0200, Florian Weimer wrote:
>> * Rich Felker:
>> 
>> > Even without fork, execve and posix_spawn can also see the SIGABRT
>> > disposition change made by abort(), passing it on to a process that
>> > should have started with a disposition of SIG_IGN if you hit exactly
>> > the wrong spot in the race.
>> 
>> My feeling is that it's not worth bothering with this kind of leakage.
>> We've had this bug forever in glibc, and no one has complained about
>> it.
>> 
>> Carlos is investigating removal of the abort lock from glibc, I think.
>
> I don't think that's a good solution. The lock is really important in
> that it protects against serious wrong behavior *within the process*
> like an application-installed signal handler for SIGABRT getting
> called more than once.

I think glibc currently has this bug.  We only avoid it for abort, but
I'm not sure if it's a bug to handle the handler multiple times if abort
is called more than once.

But even for the more general case (threads call sigaction to install a
SIGABRT handler): Do we actually need a lock there?  We reach this state
only after raise (SIGABRT) has returned.  At this point, we can set a
flag (not a lock), and every other thread that calls signal or sigaction
would instead perform the late-stage SIG_DFL-for-SIGABRT part of abort?
It probably still needs some fiddling with sigprocmask.

Thanks,
Florian
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