Create Post “2023-11-26-kernel-creation-of-new-process”

content/posts/2023-11-26-kernel-creation-of-new-process.md

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+---
+title: Kernel, creation of new process
+date: 2023-11-26T07:44:51.449Z
+slug: kernel-new-process-creation
+description: "Deep dive Linux Kernel #3"
+---
+```c
+static int init(void * unused)
+{
+	lock_kernel();
+	do_basic_setup();
+
+	/*
+	 * Ok, we have completed the initial bootup, and
+	 * we're essentially up and running. Get rid of the
+	 * initmem segments and start the user-mode stuff..
+	 */
+	free_initmem();
+	unlock_kernel();
+
+	if (open("/dev/console", O_RDWR, 0) < 0)
+		printk("Warning: unable to open an initial console.\n");
+
+	(void) dup(0);
+	(void) dup(0);
+	
+	/*
+	 * We try each of these until one succeeds.
+	 *
+	 * The Bourne shell can be used instead of init if we are 
+	 * trying to recover a really broken machine.
+	 */
+
+	if (execute_command)
+		execve(execute_command,argv_init,envp_init);
+	execve("/sbin/init",argv_init,envp_init);
+	execve("/etc/init",argv_init,envp_init);
+	execve("/bin/init",argv_init,envp_init);
+	execve("/bin/sh",argv_init,envp_init);
+	panic("No init found.  Try passing init= option to kernel.");
+}
+```
+
+At `open("/dev/console") is opens fd 0, and with two `dup(0)`, fd 0, 1, 2 as stdin, stdout, stderr opens. When `execve()` or `fork()` happens, it simply copies calling process's `task_struct` as process context, so all file descriptors are also passed down.
+
+All processes' file descriptor 0, 1, 2 are all originates to `init`'s file descriptor, set up like above.