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Linux Device Drivers, Second Edition by Alessandro Rubini, Jonathan Corbet

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The init Process

When start_kernel forks out the init thread (implemented by the init function in init/main.c), it is still running in kernel mode, and so is the init thread. When all initializations described earlier are complete, the thread drops the kernel lock and prepares to execute the user-space init process. The file being executed resides in /sbin/init, /etc/init, or /bin/init. If none of those are found, /bin/sh is run as a recovery measure in case the real init got lost or corrupted. As an alternative, the user can specify on the kernel command line which file the init thread should execute.

The procedure to enter user space is simple. The code opens /dev/console as standard input by calling the open system call and connects the console to stdout and stderr by calling dup; it finally calls execve to execute the user-space program.

The thread is able to invoke system calls while running in kernel mode because init/main.c has declared __KERNEL_SYSCALLS__ before including <asm/unistd.h>. The header defines special code that allows kernel code to invoke a limited number of system calls just as if it were running in user space. More information about kernel system calls can be found in http://www.linux.it/kerneldocs/ksys.

The final call to execve finalizes the transition to user space. There is no magic involved in this transition. As with any execve call in Unix, this one replaces the memory maps of the current process with new memory maps defined by the binary file being executed (you should remember how executing a file means mapping it to the virtual address space of the current process). It doesn’t matter that, in this case, the calling process is running in kernel space. That’s transparent to the implementation of execve, which just finds that there are no previous memory maps to release before activating the new ones.

Whatever the system setup or command line, the init process is now executing in user space and any further kernel operation takes place in response to system calls coming from init itself or from the processes it forks out.

More information about how the init process brings up the whole system can be found in http://www.linux.it/kerneldocs/init. We’ll now proceed on our tour by looking at the system calls implemented in each source directory, and then at how device drivers are laid out and organized in the source tree.

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