clinuxmemory-managementlinux-kernellinux-device-driver

Is kmalloc allocation not virtually contiguous?


I found that kmalloc returns physically and virtually contiguous memory.

I wrote some code to observe the behavior, but only the physical memory seems to be contiguous and not the virtual. Am I making any mistake?

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/moduleparam.h>

MODULE_LICENSE("GPL");

static char *ptr;
int alloc_size = 1024;

module_param(alloc_size, int, 0);

static int test_hello_init(void)
{
    ptr = kmalloc(alloc_size,GFP_ATOMIC);
    if(!ptr) {
        /* handle error */
        pr_err("memory allocation failed\n");
        return -ENOMEM;
    } else {
        pr_info("Memory allocated successfully:%p\t%p\n", ptr, ptr+100);
        pr_info("Physical address:%llx\t %llx\n", virt_to_phys(ptr), virt_to_phys(ptr+100));
    }

    return 0;
}

static void test_hello_exit(void)
{
    kfree(ptr);
    pr_info("Memory freed\n");

}

module_init(test_hello_init);
module_exit(test_hello_exit);

dmesg output:

Memory allocated successfully:0000000083318b28  000000001fba1614
Physical address:1d5d09c00   1d5d09c64

Solution

  • Printing kernel pointers is in general a bad idea, because it basically means leaking kernel addresses to user space, so when using %p in printk() (or similar macros like pr_info() etc.), the kernel tries to protect itself and does not print the real address. Instead, it prints a different hashed unique identifier for that address.

    If you really want to print that address, you can use %px.


    From Documentation/core-api/printk-formats.rst (web version, git):

    Pointer Types

    Pointers printed without a specifier extension (i.e unadorned %p) are hashed to give a unique identifier without leaking kernel addresses to user space. On 64 bit machines the first 32 bits are zeroed. If you really want the address see %px below.

    %p    abcdef12 or 00000000abcdef12
    

    Then, later below:

    Unmodified Addresses

    %px   01234567 or 0123456789abcdef
    

    For printing pointers when you really want to print the address. Please consider whether or not you are leaking sensitive information about the Kernel layout in memory before printing pointers with %px. %px is functionally equivalent to %lx. %px is preferred to %lx because it is more uniquely grep'able. If, in the future, we need to modify the way the Kernel handles printing pointers it will be nice to be able to find the call sites.


    For what it's worth, the code that converts plain pointers (formatted with %p) before printing can be found here. The call chain in Linux 6.6 is: _printk()vprintk()vprintk_default()vprintk_emit()vprintk_store()printk_sprint()vscnprintf()vsnprintf()pointer()default_pointer()ptr_to_id().