Re: Postmaster self-deadlock due to PLT linkage resolution

Hi,

On 2022-08-29 15:43:55 -0400, Tom Lane wrote:
> Buildfarm member mamba (NetBSD-current on prairiedog's former hardware)
> has failed repeatedly since I set it up. I have now run the cause of
> that to ground [1], and here's what's happening: if the postmaster
> receives a signal just before it first waits at the select() in
> ServerLoop, it can self-deadlock. During the postmaster's first use of
> select(), the dynamic loader needs to resolve the PLT branch table entry
> that the core executable uses to reach select() in libc.so, and it locks
> the loader's internal data structures while doing that. If we enter
> a signal handler while the lock is held, and the handler needs to do
> anything that also requires the lock, the postmaster is frozen.

Ick.

> The attached patch seems to fix the problem, by forcing resolution of
> the PLT link before we unblock signals. It depends on the assumption
> that another select() call appearing within postmaster.c will share
> the same PLT link, which seems pretty safe.

Hm, what stops the same problem from occuring with other functions?

Perhaps it'd be saner to default to building with -Wl,-z,now? That should fix
the problem too, right (and if we combine it with relro, it'd be a security
improvement to boot).

Greetings,

Andres Freund