mysqld should create its PID file before dropping privileges

Summary

The database server daemon should create its PID file before dropping
privileges. This represents a minor security issue; additional factors
are needed to make it exploitable.

Description

The purpose of the PID file is to hold the PID of the running daemon,
so that later it can be stopped, restarted, or otherwise signalled
(many daemons reload their configurations in response to a SIGHUP).
To fulfill that purpose, the contents of the PID file need to be
trustworthy. If the PID file is writable by a non-root user, then he
can replace its contents with the PID of a root process. Afterwards,
any attempt to signal the PID contained in the PID file will instead
signal a root process chosen by the non-root user (a vulnerability).

This is commonly exploitable by init scripts that are run as root and
which blindly trust the contents of their PID files. If one daemon
flushes its cache in response to SIGUSR2 and another daemon drops all
connections in response to SIGUSR2, it is not hard to imagine a
denial-of-service by the user of the first daemon against the second.

Exploitation

There is only a risk of exploitation when some other user relies on
the data in the PID file. But you have to wonder, what's the point of
the PID file if not to provide the PID to other people? Any situation
where the PID file is used is therefore suspicious.