failed mmap fop attempt

kernel_module/README.md

@@ -11,6 +11,7 @@
     1.  [fops](fops.c)
         1.  [ioctl](ioctl.c)
         1.  [poll](poll.c)
+        1.  [mmap](mmap.c)
         1.  [anonymous_inode](anonymous_inode.c)
         1.  [seq_file](seq_file.c)
             1. [seq_file_single](seq_file_inode.c)

kernel_module/mmap.c

@@ -0,0 +1,165 @@
+/* TODO neither works, mmap never gets called. */
+
+#if 0
+
+/*
+Adapted from:
+https://coherentmusings.wordpress.com/2014/06/10/implementing-mmap-for-transferring-data-from-user-space-to-kernel-space/
+*/
+
+#include <linux/debugfs.h>
+#include <linux/fs.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+
+#ifndef VM_RESERVED
+# define  VM_RESERVED (VM_DONTEXPAND | VM_DONTDUMP)
+#endif
+
+static struct dentry *debugfs_file;
+
+struct mmap_info
+{
+	char *data;
+	int reference;
+};
+
+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--;
+}
+
+static int vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	struct page *page;
+	struct mmap_info *info;
+
+	pr_info("vm_fault\n");
+	info = (struct mmap_info *)vma->vm_private_data;
+	if (info->data) {
+		page = virt_to_page(info->data);
+		get_page(page);
+		vmf->page = page;
+	}
+	return 0;
+}
+
+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 =
+{
+	.close = vm_close,
+	.fault = vm_fault,
+	.open = vm_open,
+};
+
+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_private_data = filp->private_data;
+	vm_open(vma);
+	return 0;
+}
+
+static int open(struct inode *inode, struct file *filp)
+{
+	struct mmap_info *info;
+
+	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);
+	filp->private_data = info;
+	return 0;
+}
+
+static int release(struct inode *inode, struct file *filp)
+{
+	struct mmap_info *info;
+
+	pr_info("release\n");
+	info = filp->private_data;
+	free_page((unsigned long)info->data);
+	kfree(info);
+	filp->private_data = NULL;
+	return 0;
+}
+
+static const struct file_operations fops = {
+	.mmap = mmap,
+	.open = open,
+	.release = release,
+};
+
+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");
+
+#endif
+
+#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 map_mmap(struct file *filp, struct vm_area_struct *vma)
+{
+	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    = map_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");

kernel_module/user/mmap.c

@@ -0,0 +1,35 @@
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#define PAGE_SIZE 4096
+
+int main(int argc, char **argv)
+{
+	int fd;
+	char *address = NULL;
+
+	if (argc < 2) {
+		printf("Usage: %s <mmap_file>\n", argv[0]);
+		return EXIT_FAILURE;
+	}
+	fd = open(argv[1], O_RDWR | O_SYNC);
+	if (fd < 0) {
+		perror("open");
+		return EXIT_FAILURE;
+	}
+	printf("fd = %d\n", fd);
+	address = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (address == MAP_FAILED) {
+		perror("mmap");
+		return EXIT_FAILURE;
+	}
+	printf("%s\n", address);
+	memcpy(address + 11 , "qwer", 6);
+	printf("%s\n", address);
+	close(fd);
+	return EXIT_SUCCESS;
+}

rootfs_overlay/mmap.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+insmod /mmap.ko
+/mmap.out /sys/kernel/debug/lkmc_mmap
+rmmod /mmap.ko