mmap of /dev/mem fails with invalid argument for virt_to_phys address, but address is page aligned

Question

For some reason my mmap failed with an Invalid argument message even though my offset is page aligned. Page size is 4096 bytes. Also CONFIG_S

Answer 1

nopat kernel command line argument

Just add that and it works, as mentioned at: https://stackoverflow.com/a/36634422/895245

Here is my test setup:

kernel module:

#include  /* virt_to_phys */
#include 
#include  /* usleep_range */
#include 
#include 
#include 
#include  /* single_open, single_release */
#include  /* kmalloc, kfree */

static volatile u32 *i;

static struct dentry *debugfs_file;

static int show(struct seq_file *m, void *v)
{
    seq_printf(m,
        "*i 0x%llx\n"
        "i %p\n"
        "virt_to_phys 0x%llx\n",
        (unsigned long long)*i,
        i,
        (unsigned long long)virt_to_phys((void *)i)
    );
    return 0;
}

static int open(struct inode *inode, struct  file *file)
{
    return single_open(file, show, NULL);
}

static const struct file_operations fops = {
    .llseek = seq_lseek,
    .open = open,
    .owner = THIS_MODULE,
    .read = seq_read,
    .release = single_release,
};

static int myinit(void)
{
    i = kmalloc(sizeof(i), GFP_KERNEL);
    *i = 0x12345678;
    debugfs_file = debugfs_create_file(
        "lkmc_virt_to_phys", S_IRUSR, NULL, NULL, &fops);
    return 0;
}

static void myexit(void)
{
    debugfs_remove(debugfs_file);
    kfree((void *)i);
}

module_init(myinit)
module_exit(myexit)
MODULE_LICENSE("GPL");

/dev/mem userland:

#!/bin/sh
set -ex
insmod /virt_to_phys.ko
cd /sys/kernel/debug
cat lkmc_virt_to_phys
# *i = 0x12345678
addr=$(grep virt_to_phys lkmc_virt_to_phys | cut -d ' ' -f 2)
devmem2 "$addr"
devmem2 "$addr" w 0x9ABCDEF0
cat lkmc_virt_to_phys
# *i = 0x9ABCDEF0
rmmod virt_to_phys

nopat being passed at: https://github.com/cirosantilli/linux-kernel-module-cheat/blob/2eca9280e12dbab79ccb67d0640b2a0edc2c9ffc/runqemu#L65

Also try xp on QEMU monitor.

And devmem2 is upstreamed by Buildroot itself: http://free-electrons.com/pub/mirror/devmem2.c See also: Accessing physical address from user space