目录

• 1.写在前面
• 2.devmem使用
• 3.应用层
• 4.内核层

1.写在前面

最近在调试时需要在用户层访问物理内存，发现应用层可以使用devmem工具访问物理地址。查看源码，实际上是对/dev/mem操作，通过mmap可以将物理地址映射到用户空间的虚拟地址上，在用户空间完成对设备寄存器的读写。藉由此原因，想深入理解下mmap的具体实现。

2.devmem使用

devmem的配置，可以在busybox的杂项中找到。

CONFIG_USER_BUSYBOX_DEVMEM:                                       

devmem is a small program that reads and writes from physical
memory using /dev/mem.                                           

Symbol: USER_BUSYBOX_DEVMEM [=y]
Prompt: devmem
  Defined at ../user/busybox/busybox-1.23.2/miscutils/Kconfig:216
  Depends on: USER_BUSYBOX_BUSYBOX
  Location:
    -> BusyBox (USER_BUSYBOX_BUSYBOX [=y])
      -> Miscellaneous Utilities

# busybox devmem
BusyBox v1.23.2 (2018-08-02 11:08:33 CST) multi-call binary.

Usage: devmem ADDRESS [WIDTH [VALUE]]

Read/write from physical address

    ADDRESS Address to act upon
    WIDTH   Width (8/16/...)
    VALUE   Data to be written

基本测试用法

# devmem 0x44e07134 16
0xFFEF
# devmem 0x44e07134 32
0xFFFFFFEF
# devmem 0x44e07134 8
0xEF

3.应用层

接口定义如下：

#include <sys/mman.h>

void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);
int munmap(void *addr, size_t length);

详细参数如下:

以devmem的实现为例，

如果argv[3]存在，需要映射读写权限；如果不存在，只需要映射读权限。

    map_base = mmap(NULL,
            mapped_size,
            argv[3] ? (PROT_READ | PROT_WRITE) : PROT_READ,
            MAP_SHARED,
            fd,
            target & ~(off_t)(page_size - 1));

4.内核层

因篇幅有限，这里不在表述glibc、系统调用的关系，直接查找系统调用的代码实现。

arch/arm/include/uapi/asm/unistd.h

#define __NR_OABI_SYSCALL_BASE  0x900000

#if defined(__thumb__) || defined(__ARM_EABI__)
#define __NR_SYSCALL_BASE   0
#else
#define __NR_SYSCALL_BASE   __NR_OABI_SYSCALL_BASE
#endif

#define __NR_mmap           (__NR_SYSCALL_BASE+ 90)
#define __NR_munmap         (__NR_SYSCALL_BASE+ 91)

#define __NR_mmap2          (__NR_SYSCALL_BASE+192)

arch/arm/kernel/entry-common.S

/*=============================================================================
 * SWI handler
 *-----------------------------------------------------------------------------
 */

    .align  5
ENTRY(vector_swi)
#ifdef CONFIG_CPU_V7M
    v7m_exception_entry
#else
    sub sp, sp, #S_FRAME_SIZE
    stmia   sp, {r0 - r12}          @ Calling r0 - r12
 ARM(   add r8, sp, #S_PC       )
 ARM(   stmdb   r8, {sp, lr}^       )   @ Calling sp, lr
 THUMB( mov r8, sp          )
 THUMB( store_user_sp_lr r8, r10, S_SP  )   @ calling sp, lr
    mrs r8, spsr            @ called from non-FIQ mode, so ok.
    str lr, [sp, #S_PC]         @ Save calling PC
    str r8, [sp, #S_PSR]        @ Save CPSR
    str r0, [sp, #S_OLD_R0]     @ Save OLD_R0
#endif
    zero_fp

#ifdef CONFIG_ALIGNMENT_TRAP
    ldr ip, __cr_alignment
    ldr ip, [ip]
    mcr p15, 0, ip, c1, c0      @ update control register
#endif

    enable_irq
    ...
    

/*
 * Note: off_4k (r5) is always units of 4K.  If we can‘t do the requested
 * offset, we return EINVAL.
 */
sys_mmap2:
#if PAGE_SHIFT > 12
        tst r5, #PGOFF_MASK
        moveq   r5, r5, lsr #PAGE_SHIFT - 12
        streq   r5, [sp, #4]
        beq sys_mmap_pgoff
        mov r0, #-EINVAL
        mov pc, lr
#else
        str r5, [sp, #4]
        b   sys_mmap_pgoff
#endif
ENDPROC(sys_mmap2)

arch/arm/kernel/calls.S

/* 90 */    CALL(OBSOLETE(sys_old_mmap))    /* used by libc4 */
            CALL(sys_munmap)
            ...
/* 190 */   CALL(sys_vfork)
            CALL(sys_getrlimit)
            CALL(sys_mmap2)

include/linux/syscalls.h

asmlinkage long sys_mmap_pgoff(unsigned long addr, unsigned long len,
            unsigned long prot, unsigned long flags,
            unsigned long fd, unsigned long pgoff);

搜索mmap_pgoff函数定义，位于mm/mmap.c，省略一些我们不太关心的代码。

SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
        unsigned long, prot, unsigned long, flags,
        unsigned long, fd, unsigned long, pgoff)
{
    struct file *file = NULL;
    unsigned long retval = -EBADF;

    if (!(flags & MAP_ANONYMOUS)) {
        audit_mmap_fd(fd, flags);
        file = fget(fd);
        if (!file)
            goto out;
        if (is_file_hugepages(file))
            len = ALIGN(len, huge_page_size(hstate_file(file)));
        retval = -EINVAL;
        if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
            goto out_fput;
    }
    ...

    flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);

    retval = vm_mmap_pgoff(file, addr, len, prot, flags, pgoff);
out_fput:
    if (file)
        fput(file);
out:
    return retval;
}

mm/util.c

unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
    unsigned long len, unsigned long prot,
    unsigned long flag, unsigned long pgoff)
{
    unsigned long ret;
    struct mm_struct *mm = current->mm;
    unsigned long populate;

    ret = security_mmap_file(file, prot, flag);
    if (!ret) {
        down_write(&mm->mmap_sem);
        ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
                    &populate);
        up_write(&mm->mmap_sem);
        if (populate)
            mm_populate(ret, populate);
    }
    return ret;
}

vm_area_struct结构用来描述进程的虚拟内存区域，和进程的内存描述符mm_struct关联，通过链表和红黑树进行管理。

unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
            unsigned long len, unsigned long prot,
            unsigned long flags, unsigned long pgoff,
            unsigned long *populate)
{

    struct mm_struct * mm = current->mm;
    vm_flags_t vm_flags;

    *populate = 0;   

    //搜索进程地址空间，查找一个可以使用的线性地址区间，len指定区间的长度，非空addr参数指定从哪个地址开始进行查找
    addr = get_unmapped_area(file, addr, len, pgoff, flags);

    vm_flags = calc_vm_prot_bits(prot) | calc_vm_flag_bits(flags) |
            mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC; 

    //file指针不为空，建立从文件到虚拟空间的映射，根据flags标志设定访问权限。
    if (file) {
        struct inode *inode = file_inode(file);

        switch (flags & MAP_TYPE) {
        case MAP_SHARED:
            vm_flags |= VM_SHARED | VM_MAYSHARE;
            break;
        ...
    } else {    //file指针为空，仅创建虚拟空间，不做映射。
        switch (flags & MAP_TYPE) {
        case MAP_SHARED:
            pgoff = 0;
            vm_flags |= VM_SHARED | VM_MAYSHARE;
            break;
        case MAP_PRIVATE:
            pgoff = addr >> PAGE_SHIFT;
            break;
    }     

    //创建虚拟空间，并进行映射。
    addr = mmap_region(file, addr, len, vm_flags, pgoff);

    return addr;
}

unsigned long mmap_region(struct file *file, unsigned long addr,
        unsigned long len, vm_flags_t vm_flags, unsigned long pgoff)
{
    ...
    //检查是否需要对该虚拟空间进行扩容
    if (!may_expand_vm(mm, len >> PAGE_SHIFT)) {
        unsigned long nr_pages;

        /*
         * MAP_FIXED may remove pages of mappings that intersects with
         * requested mapping. Account for the pages it would unmap.
         */
        if (!(vm_flags & MAP_FIXED))
            return -ENOMEM;

        nr_pages = count_vma_pages_range(mm, addr, addr + len);

        if (!may_expand_vm(mm, (len >> PAGE_SHIFT) - nr_pages))
            return -ENOMEM;
    }

    //扫描当前进程地址空间的vm_area_struct结构相关的红黑树，确定线性区域的位置，如果找到一个区域，说明addr所在的虚拟区间已经被使用，表示已经被映射；因此需要调用do_munmap把这个区域从进程地址空间中撤销。
munmap_back:
    if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent)) {
        if (do_munmap(mm, addr, len))
            return -ENOMEM;
        goto munmap_back;
    }    

    vma = vma_merge(mm, prev, addr, addr + len, vm_flags, NULL, file, pgoff, NULL);
    if (vma)
        goto out;  

    //分配映射虚拟空间
    vma = kmem_cache_zalloc(vm_area_cachep, GFP_KERNEL);
    if (!vma) {
        error = -ENOMEM;
        goto unacct_error;
    }

    vma->vm_mm = mm;
    vma->vm_start = addr;
    vma->vm_end = addr + len;
    vma->vm_flags = vm_flags;
    vma->vm_page_prot = vm_get_page_prot(vm_flags);
    vma->vm_pgoff = pgoff;
    INIT_LIST_HEAD(&vma->anon_vma_chain); 

    if (file) {
        if (vm_flags & VM_DENYWRITE) {
            error = deny_write_access(file);
            if (error)
                goto free_vma;
        }
        vma->vm_file = get_file(file);
        error = file->f_op->mmap(file, vma);
        if (error)
            goto unmap_and_free_vma;

        /* Can addr have changed??
         *
         * Answer: Yes, several device drivers can do it in their
         *         f_op->mmap method. -DaveM
         * Bug: If addr is changed, prev, rb_link, rb_parent should
         *      be updated for vma_link()
         */
        WARN_ON_ONCE(addr != vma->vm_start);

        addr = vma->vm_start;
        vm_flags = vma->vm_flags;
    } else if (vm_flags & VM_SHARED) {
        error = shmem_zero_setup(vma);
        if (error)
            goto free_vma;
    }    

    ...
}

mmap_region函数实现中的file->f_op->mmap(file, vma)，对应mmap_mem，位于/drivers/char/mem.c，代码如下：

static const struct file_operations mem_fops = {
    .llseek     = memory_lseek,
    .read       = read_mem,
    .write      = write_mem,
    .mmap       = mmap_mem,
    .open       = open_mem,
    .get_unmapped_area = get_unmapped_area_mem,
};

static int mmap_mem(struct file *file, struct vm_area_struct *vma)
{
    size_t size = vma->vm_end - vma->vm_start;

    if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))
        return -EINVAL;

    if (!private_mapping_ok(vma))
        return -ENOSYS;

    if (!range_is_allowed(vma->vm_pgoff, size))
        return -EPERM;

    if (!phys_mem_access_prot_allowed(file, vma->vm_pgoff, size,
                        &vma->vm_page_prot))
        return -EINVAL;

    vma->vm_page_prot = phys_mem_access_prot(file, vma->vm_pgoff,
                         size,
                         vma->vm_page_prot);

    vma->vm_ops = &mmap_mem_ops;

    /* Remap-pfn-range will mark the range VM_IO */
    if (remap_pfn_range(vma,
                vma->vm_start,
                vma->vm_pgoff,
                size,
                vma->vm_page_prot)) {
        return -EAGAIN;
    }
    return 0;
}

remap_pfn_range函数建立物理地址与虚拟地址页表。其中vm_pgoff代表要映射的物理地址，vm_page_prot代表该页的权限。这些参数和mmap的参数相互对应，现在就可以通过应用层访问物理地址了。

原文地址：https://www.cnblogs.com/tinylaker/p/9823517.html