Is There Any API For Determining the Physical Address from Virtual Address in Linux

How to find the physical address of a variable from user-space in Linux?

First, why would you want to do this? The purpose of modern VM systems is to remove the application programmer from the complexity of physocal memory layout. Gving them each their own uniform address space to make their life easyer.

If you did want to do this you would almost certanly need to use a kernel module. Get the virtual address of the variable in the normal way, use this to index into the processes page tables and read the value you find(the physical address of the frame). Then add the page offset to get the complete physical address. Note you wont be able to use this address while paging is enabled.

(If your lucky you may be able to get the frame address of a VM region from the /proc file system and thus wouldnt require to write a kernel module.)

Convert physical address to virtual in Linux and read its content

You need a kernel driver to export the phyisical address to user-level.

Have a look at this driver: https://github.com/claudioscordino/mmap_alloc/blob/master/mmap_alloc.c

How to access physical addresses from user space in Linux?

You can map a device file to a user process memory using mmap(2) system call. Usually, device files are mappings of physical memory to the file system.
Otherwise, you have to write a kernel module which creates such a file or provides a way to map the needed memory to a user process.

Another way is remapping parts of /dev/mem to a user memory.

Edit:
Example of mmaping /dev/mem (this program must have access to /dev/mem, e.g. have root rights):

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    if (argc < 3) {
        printf("Usage: %s <phys_addr> <offset>\n", argv[0]);
        return 0;
    }

    off_t offset = strtoul(argv[1], NULL, 0);
    size_t len = strtoul(argv[2], NULL, 0);

    // Truncate offset to a multiple of the page size, or mmap will fail.
    size_t pagesize = sysconf(_SC_PAGE_SIZE);
    off_t page_base = (offset / pagesize) * pagesize;
    off_t page_offset = offset - page_base;

    int fd = open("/dev/mem", O_SYNC);
    unsigned char *mem = mmap(NULL, page_offset + len, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, page_base);
    if (mem == MAP_FAILED) {
        perror("Can't map memory");
        return -1;
    }

    size_t i;
    for (i = 0; i < len; ++i)
        printf("%02x ", (int)mem[page_offset + i]);

    return 0;
}

linux kernel - how to get physical address (memory management)?

The Linux kernel uses a generic four-page paging model, which is not only suitable for 32-bit systems but also for 64-bit systems. The paging unit is part of the MMU (Memory Management Unit), which converts a linear address into a physical address.

I wrote a kernel module for you to simulate the process of virtual address conversion to physical address. I am assuming you know the principal of paging system.

static void get_pgtable_macro(void)
{
    printk("PAGE_OFFSET = 0x%lx\n", PAGE_OFFSET);
    printk("PGDIR_SHIFT = %d\n", PGDIR_SHIFT);
    printk("PUD_SHIFT = %d\n", PUD_SHIFT);
    printk("PMD_SHIFT = %d\n", PMD_SHIFT);
    printk("PAGE_SHIFT = %d\n", PAGE_SHIFT);

    printk("PTRS_PER_PGD = %d\n", PTRS_PER_PGD);
    printk("PTRS_PER_PUD = %d\n", PTRS_PER_PUD);
    printk("PTRS_PER_PMD = %d\n", PTRS_PER_PMD);
    printk("PTRS_PER_PTE = %d\n", PTRS_PER_PTE);

    printk("PAGE_MASK = 0x%lx\n", PAGE_MASK);
}

static unsigned long vaddr2paddr(unsigned long vaddr)
{
    pgd_t *pgd;
    pud_t *pud;
    pmd_t *pmd;
    pte_t *pte;
    unsigned long paddr = 0;
        unsigned long page_addr = 0;
    unsigned long page_offset = 0;

    pgd = pgd_offset(current->mm, vaddr);
    printk("pgd_val = 0x%lx\n", pgd_val(*pgd));
    printk("pgd_index = %lu\n", pgd_index(vaddr));
    if (pgd_none(*pgd)) {
        printk("not mapped in pgd\n");
        return -1;
    }

    pud = pud_offset(pgd, vaddr);
    printk("pud_val = 0x%lx\n", pud_val(*pud));
    if (pud_none(*pud)) {
        printk("not mapped in pud\n");
        return -1;
    }

    pmd = pmd_offset(pud, vaddr);
    printk("pmd_val = 0x%lx\n", pmd_val(*pmd));
    printk("pmd_index = %lu\n", pmd_index(vaddr));
    if (pmd_none(*pmd)) {
        printk("not mapped in pmd\n");
        return -1;
    }

    pte = pte_offset_kernel(pmd, vaddr);
    printk("pte_val = 0x%lx\n", pte_val(*pte));
    printk("pte_index = %lu\n", pte_index(vaddr));
    if (pte_none(*pte)) {
        printk("not mapped in pte\n");
        return -1;
    }

    /* Page frame physical address mechanism | offset */
    page_addr = pte_val(*pte) & PAGE_MASK;
    page_offset = vaddr & ~PAGE_MASK;
    paddr = page_addr | page_offset;
    printk("page_addr = %lx, page_offset = %lx\n", page_addr, page_offset);
        printk("vaddr = %lx, paddr = %lx\n", vaddr, paddr);

    return paddr;
}

static int __init v2p_init(void)
{
    unsigned long vaddr = 0;

    printk("vaddr to paddr module is running..\n");
    get_pgtable_macro();
    printk("\n");

    vaddr = (unsigned long)vmalloc(1000 * sizeof(char));
    if (vaddr == 0) {
        printk("vmalloc failed..\n");
        return 0;
    }
    printk("vmalloc_vaddr=0x%lx\n", vaddr);
    vaddr2paddr(vaddr);

    printk("\n\n");
    vaddr = __get_free_page(GFP_KERNEL);
    if (vaddr == 0) {
        printk("__get_free_page failed..\n");
        return 0;
    }
    printk("get_page_vaddr=0x%lx\n", vaddr);
    vaddr2paddr(vaddr);

    return 0;
}

static void __exit v2p_exit(void)
{
    printk("vaddr to paddr module is leaving..\n");
        vfree((void *)vaddr);
        free_page(vaddr);
}

1. Get_pgtable_macro () Prints some macros in the current system paging mechanism.

2. Through vmalloc () in the allocation of memory space in kernel space, calling vaddr2paddr () will be converted into a virtual address physical address.

3. Use vaddr2paddr () to translate the virtual address into a physical address by allocating the frame in kernel space with __get_free_pages ().
4. Release the requested memory space through vfree () and free_page (), respectively.

Vaddr2paddr () is executed as follows:

1. Calculate the linear address pgd of the page global catalog entry by pgd_offset, passing in the memory descriptor mm and the linear address vaddr. Next, print the page global catalog entry pointed to by pgd.

2. Calculate the linear address pud of the page parent directory entry by pud_offset, passing the parameters to the linear address pgd of the page global directory entry and the linear address vaddr. Then print the pud referred to the parent directory entry.

3. Calculate the linear address pmd of the page middle directory entry through pmd_offset, passing the parameters to the linear address pud and the linear address vaddr of the parent directory entry. Then print the middle of the page referred to pmd directory entries.

4. Pte_offset_kernel pte_offset_kernel calculated by the linear address pte, the parameters for the middle of the directory entry of the linear address pmd linear address and address vaddr. Then print the page table item pointed to by pte.

5. pte_val (* pte) to remove the page table entries, and PAGE_MASK phase and the result is to access the page's physical address; vaddr & ~ PAGE_MASK used to get linear address offset field; the two or the final physical address calculation.

6. Print the physical address

Virtual address to physical address and reverse in android linux kernel

I found fix.
ioremap have a check logic for validation of address.
This function is for reserved address but it tring to map address that's already mapped to a process.
So, I modify the check logic in ioreamp and it works well.

---

Related Topics