Problem - Kevin Tafuro

1.6 Creating a Child Process Securely
Problem
Your program needs to create a child process either to perform work within the same
program or, more frequently, to execute another program.
Solution
On Unix, creating a child process is done by calling fork( ). When fork( ) completes
successfully, a nearly identical copy of the calling process is created as a new process.
Most frequently, a new program is immediately executed using one of the
exec*( ) family of functions (see Recipe 1.7). However, especially in the days before
threading, it was common to use fork( ) to create separate “threads” of execution
within a program. *
If the newly created process is going to continue running the same program, any
pseudo-random number generators (PRNGs) must be reseeded so that the two processes
will each yield different random data as they continue to execute. In addition,
any inherited file descriptors that are not needed should be closed; they remain open
in the other process because the new process only has a copy of them.
Finally, if the original process had extra privileges from being executed as setuid or
setgid, those privileges will be inherited by the new process, and they should be
dropped immediately if they are not needed. In particular, if the new process is going
to be used to execute a new program, privileges should always be dropped so that
the new program does not inherit privileges that it should not have.
Discussion
When fork( ) is used to create a new process, the new process is a nearly identical
copy of the original process. The only differences in the processes are the process ID,
the parent process ID, and the resource utilization counters, which are reset to zero
in the new process. Execution in both processes continues immediately after the
return from fork( ). Each process can determine whether it is the parent or the child
by checking the return value from fork( ). In the parent or original process, fork( )
returns the process ID of the new process, while 0 will be returned in the child process.
* Note that we say “program” here rather than “process.” When fork( ) completes, the same program is running,
but there are now two processes. The newly created process has a nearly identical copy of the original
process, but it is a copy; any action performed in one process does not affect the other. In a threaded environment,
each thread shares the same process, so all memory, file descriptors, signals, and so on are shared.
26 | Chapter 1: Safe Initialization
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