mmap example works

kernel_module/mmap.c

@@ -1,43 +1,35 @@
 /*
-Remember: mmap does not work with debugfs as of 4.9!
+Remember: mmap, like most fops, does not work with debugfs as of 4.9! https://patchwork.kernel.org/patch/9252557/
-*/
 
-#if 0
-
-/*
 Adapted from:
 https://coherentmusings.wordpress.com/2014/06/10/implementing-mmap-for-transferring-data-from-user-space-to-kernel-space/
 */
 
+#include <asm/uaccess.h> /* copy_from_user */
 #include <linux/debugfs.h>
 #include <linux/fs.h>
 #include <linux/init.h>
-#include <linux/kernel.h>
+#include <linux/kernel.h> /* min */
 #include <linux/mm.h>
 #include <linux/module.h>
+#include <linux/proc_fs.h>
 #include <linux/slab.h>
 
-#ifndef VM_RESERVED
+static const char *filename = "lkmc_mmap";
-# define  VM_RESERVED (VM_DONTEXPAND | VM_DONTDUMP)
-#endif
 
-static struct dentry *debugfs_file;
+enum { BUFFER_SIZE = 4 };
 
-struct mmap_info
+struct mmap_info {
-{
 	char *data;
-	int reference;
 };
 
+/* After unmap. */
 static void vm_close(struct vm_area_struct *vma)
 {
-	struct mmap_info *info;
-
 	pr_info("vm_close\n");
-	info = (struct mmap_info *)vma->vm_private_data;
-	info->reference--;
 }
 
+/* First page access. */
 static int vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 {
 	struct page *page;
@@ -53,13 +45,10 @@ static int vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	return 0;
 }
 
+/* Aftr mmap. TODO vs mmap, when can this happen at a different time than mmap? */
 static void vm_open(struct vm_area_struct *vma)
 {
-	struct mmap_info *info;
-
 	pr_info("vm_open\n");
-	info = (struct mmap_info *)vma->vm_private_data;
-	info->reference++;
 }
 
 static struct vm_operations_struct vm_ops =
@@ -73,7 +62,7 @@ static int mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	pr_info("mmap\n");
 	vma->vm_ops = &vm_ops;
-	vma->vm_flags |= VM_RESERVED;
+	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_private_data = filp->private_data;
 	vm_open(vma);
 	return 0;
@@ -86,11 +75,38 @@ static int open(struct inode *inode, struct file *filp)
 	pr_info("open\n");
 	info = kmalloc(sizeof(struct mmap_info), GFP_KERNEL);
 	info->data = (char *)get_zeroed_page(GFP_KERNEL);
-	memcpy(info->data, "abc", 4);
+	memcpy(info->data, "asdf", BUFFER_SIZE);
 	filp->private_data = info;
 	return 0;
 }
 
+static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
+{
+	struct mmap_info *info;
+    int ret;
+
+	pr_info("read\n");
+	info = filp->private_data;
+    ret = min(len, (size_t)BUFFER_SIZE);
+    if (copy_to_user(buf, info->data, ret)) {
+        ret = -EFAULT;
+	}
+	return ret;
+}
+
+static ssize_t write(struct file *filp, const char __user *buf, size_t len, loff_t *off)
+{
+	struct mmap_info *info;
+
+	pr_info("write\n");
+	info = filp->private_data;
+    if (copy_from_user(info->data, buf, min(len, (size_t)BUFFER_SIZE))) {
+        return -EFAULT;
+    } else {
+        return len;
+    }
+}
+
 static int release(struct inode *inode, struct file *filp)
 {
 	struct mmap_info *info;
@@ -107,138 +123,8 @@ static const struct file_operations fops = {
 	.mmap = mmap,
 	.open = open,
 	.release = release,
-};
+	.read = read,
-
+	.write = write,
-static int myinit(void)
-{
-	debugfs_file = debugfs_create_file("lkmc_mmap", S_IRWXU, NULL, NULL, &fops);
-	return 0;
-}
-
-static void myexit(void)
-{
-	debugfs_remove(debugfs_file);
-}
-
-module_init(myinit);
-module_exit(myexit);
-MODULE_LICENSE("GPL");
-
-
-/*minimized debugfs*/
-
-
-#include <linux/debugfs.h>
-#include <linux/kernel.h>
-#include <linux/module.h>
-#include <linux/init.h>
-#include <linux/fs.h>
-#include <linux/mm.h>
-
-static struct dentry *debugfs_file;
-
-static int mmap(struct file *filp, struct vm_area_struct *vma)
-{
-	pr_info("mmap\n");
-	if (remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
-				vma->vm_end - vma->vm_start, vma->vm_page_prot)) {
-		return -EAGAIN;
-	}
-	return 0;
-}
-
-struct file_operations fops =
-{
-	.owner = THIS_MODULE,
-	.open    = nonseekable_open,
-	.mmap    = mmap
-};
-
-static int myinit(void)
-{
-	debugfs_file = debugfs_create_file("lkmc_mmap", S_IRWXU, NULL, NULL, &fops);
-	return 0;
-}
-
-static void myexit(void)
-{
-	debugfs_remove(debugfs_file);
-}
-
-module_init(myinit);
-module_exit(myexit);
-MODULE_LICENSE("GPL");
-
-/*working chardev*/
-
-#include <linux/debugfs.h>
-#include <linux/kernel.h>
-#include <linux/module.h>
-#include <linux/init.h>
-#include <linux/fs.h>
-#include <linux/mm.h>
-
-#define NAME "lkmc_mmap"
-
-static int major;
-
-static int mmap(struct file *filp, struct vm_area_struct *vma)
-{
-	pr_info("mmap\n");
-	if (remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
-				vma->vm_end - vma->vm_start, vma->vm_page_prot)) {
-		return -EAGAIN;
-	}
-	return 0;
-}
-
-struct file_operations fops =
-{
-	.owner = THIS_MODULE,
-	.open    = nonseekable_open,
-	.mmap    = mmap
-};
-
-static int myinit(void)
-{
-	major = register_chrdev(0, NAME, &fops);
-	return 0;
-}
-
-static void myexit(void)
-{
-	unregister_chrdev(major, NAME);
-}
-
-module_init(myinit)
-module_exit(myexit)
-MODULE_LICENSE("GPL");
-
-#endif
-
-/* proc attempt */
-
-#include <linux/debugfs.h>
-#include <linux/kernel.h>
-#include <linux/module.h>
-#include <linux/init.h>
-#include <linux/fs.h>
-#include <linux/mm.h>
-#include <linux/proc_fs.h>
-
-static const char *filename = "lkmc_procfs";
-
-static int mmap(struct file *filp, struct vm_area_struct *vma)
-{
-	pr_info("mmap\n");
-	return 0;
-}
-
-struct file_operations fops =
-{
-	.owner = THIS_MODULE,
-	.open = nonseekable_open,
-	.mmap = mmap
 };
 
 static int myinit(void)

kernel_module/user/Makefile

@@ -1,6 +1,6 @@
 .PHONY: clean
 
-CC ?= gcc
+CC ?= gcc -ggdb3 -O0 -std=c99 -Wall -Werror -Wextra
 IN_EXT ?= .c
 OUT_EXT ?= .out
 

kernel_module/user/mmap.c

@@ -1,36 +1,80 @@
+#include <assert.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
-#include <unistd.h>
+#include <unistd.h> /* sysconf */
 
-#define PAGE_SIZE 4096
+enum { BUFFER_SIZE = 4 };
 
 int main(int argc, char **argv)
 {
 	int fd;
-	char *address = NULL;
+	long page_size;
+	char *address1, *address2;
+	char buf[BUFFER_SIZE];
 
 	if (argc < 2) {
 		printf("Usage: %s <mmap_file>\n", argv[0]);
 		return EXIT_FAILURE;
 	}
+	page_size = sysconf(_SC_PAGE_SIZE);
 	printf("open pathname = %s\n", argv[1]);
 	fd = open(argv[1], O_RDWR | O_SYNC);
 	if (fd < 0) {
 		perror("open");
-		return EXIT_FAILURE;
+		assert(0);
 	}
 	printf("fd = %d\n", fd);
-	address = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+
-	if (address == MAP_FAILED) {
+    /* mmap twice for double fun. */
+	puts("mmap 1");
+	address1 = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (address1 == MAP_FAILED) {
+		perror("mmap");
+		assert(0);
+	}
+	puts("mmap 2");
+	address2 = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (address2 == MAP_FAILED) {
 		perror("mmap");
 		return EXIT_FAILURE;
 	}
-	printf("%s\n", address);
+	assert(address1 != address2);
-	memcpy(address + 11 , "qwer", 6);
+
-	printf("%s\n", address);
+    /* Read and modify memory. */
+	puts("access 1");
+	assert(!strcmp(address1, "asdf"));
+	/* vm_fault */
+	puts("access 2");
+	assert(!strcmp(address2, "asdf"));
+	/* vm_fault */
+	strcpy(address1, "qwer");
+	/* Also modified. So both virtual addresses point to the same physical address. */
+	assert(!strcmp(address2, "qwer"));
+
+    /* Check that modifications made from userland are also visible from the kernel. */
+	read(fd, buf, BUFFER_SIZE);
+	assert(!memcmp(buf, "qwer", BUFFER_SIZE));
+
+	/* Modify the data from the kernel, and check that the change is visible from userland. */
+	write(fd, "zxcv", 4);
+	assert(!strcmp(address1, "zxcv"));
+	assert(!strcmp(address2, "zxcv"));
+
+    /* Cleanup. */
+    puts("munmap 1");
+	if (munmap(address1, page_size)) {
+		perror("munmap");
+		assert(0);
+	}
+    puts("munmap 2");
+	if (munmap(address2, page_size)) {
+		perror("munmap");
+		assert(0);
+	}
+    puts("close");
 	close(fd);
 	return EXIT_SUCCESS;
 }

rootfs_overlay/mmap.sh

@@ -1,4 +1,5 @@
 #!/bin/sh
+set -ex
 insmod /mmap.ko
-/mmap.out /sys/kernel/debug/lkmc_mmap
+/mmap.out /proc/lkmc_mmap
 rmmod /mmap.ko