anonymous inode: move doc to readme

Author: cirosantilli

README.adoc

@@ -3101,6 +3101,37 @@ Sources:
 
 Bibliography: https://stackoverflow.com/questions/5970595/how-to-create-a-device-node-from-the-init-module-code-of-a-linux-kernel-module/45531867#45531867
 
+==== Anonymous inode
+
+In guest:
+
+....
+/anonymous_inode.sh
+echo $?
+....
+
+Outcome: the test passes:
+
+....
+0
+....
+
+Sources:
+
+* link:kernel_module/anonymous_inode.c[]
+* link:kernel_module/anonymous_inode.h[]
+* link:kernel_module/user/anonymous_inode.c[]
+* link:rootfs_overlay/anonymous_inode.sh[]
+
+This example:
+
+* gets an anonymous inode via `ioctl` from a debugfs entry `anon_inode_getfd` from a debugfs file
+* read jiffies from that inode
+
+Anonymous inodes allow getting multiple file descriptors from a single filesystem entry, which reduces namespace pollution compared to creating multiple device files.
+
+Bibliography: https://stackoverflow.com/questions/4508998/what-is-an-anonymous-inode-in-linux
+
 === Kernel panic and oops
 
 To test out kernel panics and oops in controlled circumstances, try out the modules:
@@ -7734,7 +7765,7 @@ We are slowly automating testable guest tests with:
 ./run -F '/test_all.sh;/poweroff.out' | grep lkmc_test
 ....
 
-which outputs to stdout `lkmc_test_pass` or `lkmc_test_fail` to stdout, which we can grep from host to automate testing.
+which outputs `lkmc_test_pass` or `lkmc_test_fail`.
 
 Source: link:rootfs_overlay/test_all.sh[].
 

kernel_module/README.adoc

@@ -19,7 +19,6 @@
 ... link:dep.c[]
 ... link:dep2.c[]
 . Pseudo filesystems
-.. link:anonymous_inode.c[]
 .. link:ioctl.c[]
 .. link:mmap.c[]
 .. link:poll.c[]

kernel_module/anonymous_inode.c

@@ -1,20 +1,9 @@
-/*
-https://stackoverflow.com/questions/4508998/what-is-anonymous-inode
-
-anon_inode_getfd example:
-
-- get an anonymous inode via ioctl from a debugfs entry
-- read jiffies from that inode
-
-This method allows getting multiple file descriptors from a single filesystem,
-which reduces namespace pollution.
-*/
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#anonymous-inode */
 
 #include <linux/anon_inodes.h>
 #include <linux/debugfs.h>
 #include <linux/errno.h> /* EFAULT */
 #include <linux/fs.h>
-#include <linux/jiffies.h>
 #include <linux/kernel.h> /* min */
 #include <linux/module.h>
 #include <linux/printk.h> /* printk */
@@ -23,16 +12,18 @@ which reduces namespace pollution.
 #include "anonymous_inode.h"
 
 static struct dentry *debugfs_file;
+static u32 myval = 1;
 
 static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
 {
-	char kbuf[1024];
+	char kbuf[8];
 	size_t ret;
 
-	ret = snprintf(kbuf, sizeof(kbuf), "%llu", (unsigned long long)jiffies);
+	ret = snprintf(kbuf, sizeof(kbuf), "%x", myval);
 	if (copy_to_user(buf, kbuf, ret)) {
 		ret = -EFAULT;
 	}
+	myval <<= 4;
 	return ret;
 }
 
@@ -47,7 +38,7 @@ static long unlocked_ioctl(struct file *filp, unsigned int cmd, unsigned long ar
 	switch (cmd) {
 		case LKMC_ANONYMOUS_INODE_GET_FD:
 			fd = anon_inode_getfd(
-				"random",
+				"todo_what_is_this_for",
 				&fops_anon,
 				NULL,
 				O_RDONLY | O_CLOEXEC

kernel_module/user/anonymous_inode.c

@@ -14,10 +14,15 @@ int main(int argc, char **argv)
 {
 	char buf[1024];
 	int fd_ioctl, fd_ioctl_anon, ret;
+	size_t i, nreads;
 
 	if (argc < 2) {
-		puts("Usage: ./prog <ioctl-file>");
+		puts("Usage: ./prog <ioctl-file> [<nreads>]");
 		return EXIT_FAILURE;
+	} else if (argc > 2) {
+		nreads = strtol(argv[2], NULL, 10);
+	} else {
+		nreads = 3;
 	}
 	fd_ioctl = open(argv[1], O_RDONLY);
 	if (fd_ioctl == -1) {
@@ -29,11 +34,10 @@ int main(int argc, char **argv)
 		perror("ioctl");
 		return EXIT_FAILURE;
 	}
-	ret = read(fd_ioctl_anon, buf, sizeof(buf));
-	printf("%.*s\n", ret, buf);
-	sleep(1);
-	ret = read(fd_ioctl_anon, buf, sizeof(buf));
-	printf("%.*s\n", ret, buf);
+	for (i = 0; i < nreads; ++i) {
+		ret = read(fd_ioctl_anon, buf, sizeof(buf));
+		printf("%.*s\n", ret, buf);
+	}
 	close(fd_ioctl_anon);
 	close(fd_ioctl);
 	return EXIT_SUCCESS;

rootfs_overlay/anonymous_inode.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
 set -e
 insmod /anonymous_inode.ko
-/anonymous_inode.out /sys/kernel/debug/lkmc_anonymous_inode
+[ "$(/anonymous_inode.out /sys/kernel/debug/lkmc_anonymous_inode 3)" = "$(printf '1\n10\n100')" ]
 rmmod anonymous_inode

rootfs_overlay/test_all.sh

@@ -1,5 +1,6 @@
 #!/bin/sh
 for test in \
+  /anonymous_inode.sh \
   /character_device.sh \
   /character_device_create.sh \
   /debugfs.sh \