Linux Kernel Architecture

Mauerer app04.tex V1 - 09/04/2008 6:12pm Page 1239

Appendix D: System Startup

is held inexecute_commandwhen the command line is parsed). If none of the options functions, a kernel

panic is triggered because the system is unusable, as shown here:

init/main.c

static int noinline init_post(void)

{

if (execute_command) {

run_init_process(execute_command);

printk(KERN_WARNING "Failed to execute %s. Attempting "

"defaults...\n", execute_command);

}

run_init_process("/sbin/init");

run_init_process("/etc/init");

run_init_process("/bin/init");

run_init_process("/bin/sh");

panic("No init found. Try passing init= option to kernel.");

}

run_init_postsets up a minimal environment for theinitprocessasfollows:

init/main.c

static char * argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };

char * envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };

static void run_init_process(char *init_filename)

{

argv_init[0] = init_filename;

kernel_execve(init_filename, argv_init, envp_init);

}

kernel_execveis a wrapper for thesys_execvesystem call, which must be provided by each

architecture.

D.3 Summary

Booting the Linux kernel is a highly architecture-specific process, at least for the initial stages. This

chapter introduced you to some of the intricacies toget a kernel up and running on IA-32 systems. Addi-

tionally, this chapter discussed the higher-level startup process in which the kernel sets up the hardware

step-by-step until it can finally invoke the first userland process (usually/sbin/init) and can commence

its regular execution.