From: Szabolcs Nagy
> Sent: 05 June 2020 15:56
...
> currently there is no libc interface contract in place that
> says which calls may use libc internal fds e.g. i've seen
>
> openlog(...) // opens libc internal syslog fd
> ...
> fork()
> closefrom(...) // close syslog fd
> open(...) // something that reuses the closed fd
> syslog(...) // unsafe: uses the wrong fd
> execve(...)
>
> syslog uses a libc internal fd that the user trampled on and
> this can go bad in many ways depending on what libc apis are
> used between closefrom (or equivalent) and exec.
It is, of course, traditional that daemons only call
close(0); close(1); close(2);
Took us ages to discover that a misspelt fprintf()
was adding data to the stdout buffer and eventually
flushing 10k of ascii text into an inter-process pipe
that had a 32bit field for 'message extension length'.
FWIW isn't syslog() going to go badly wrong after fork()
anyway?
Unless libc's fork() calls closelog().
David
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