Linux Kernel 及 binder mmap實現

1. 簡介

對於mmap在用戶態通過函數以下函數進行調用:

1. void* mmap( void* addr, size_t size, int prot, int flags, int fd, long offset )

然後進入系統調用。

2. Kernel mmap實現

1）然後進入系統調用，其系統調用號為:

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

#define __NR_mmap2 (__NR_SYSCALL_BASE+192)

2）觸發軟中斷

其ISR 代碼位於kernel/arch/arm/kernel/entry-common.S的ENTRY(vector_swi), __NR_mmap2對應的函數為:sys_mmap2(位於linux/arch/arm/kernel/calls.S)

3）sys_mmap2的實現

位於kernel/arch/arm/kernel/entry-common.S,實現代碼如下：

1. /*
2. * Note: off_4k (r5) is always units of 4K. If we can't do the requested
3. * offset, we return EINVAL.
4. */
5. sys_mmap2:
6. #if PAGE_SHIFT > 12
7. tst r5, #PGOFF_MASK
8. moveq r5, r5, lsr #PAGE_SHIFT - 12
9. streq r5, [sp, #4]
10. beq sys_mmap_pgoff
11. mov r0, #-EINVAL
12. mov pc, lr
13. #else
14. str r5, [sp, #4]
15. b sys_mmap_pgoff
16. #endif

4） 調用sys_mmap_pgoff

在kernel/include/linux/syscalls.h中定義如下：

1. asmlinkage long sys_mmap_pgoff(unsigned long addr, unsigned long len,
2. unsigned long prot, unsigned long flags,
3. unsigned long fd, unsigned long pgoff);

6）sys_mmap_pgoff實現
在kernel/mm/mmap.c中實現如下：

1. SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
2. unsigned long, prot, unsigned long, flags,
3. unsigned long, fd, unsigned long, pgoff)
4. {
5. struct file *file = NULL;
6. unsigned long retval = -EBADF;
8. if (!(flags & MAP_ANONYMOUS)) {
9. audit_mmap_fd(fd, flags);
10. if (unlikely(flags & MAP_HUGETLB))
11. return -EINVAL;
12. file = fget(fd);
13. if (!file)
14. goto out;
15. } else if (flags & MAP_HUGETLB) {
16. struct user_struct *user = NULL;
17. /*
18. * VM_NORESERVE is used because the reservations will be
19. * taken when vm_ops->mmap() is called
20. * A dummy user value is used because we are not locking
21. * memory so no accounting is necessary
22. */
23. len = ALIGN(len, huge_page_size(&default_hstate));
24. file = hugetlb_file_setup(HUGETLB_ANON_FILE, len, VM_NORESERVE,
25. &user, HUGETLB_ANONHUGE_INODE);
26. if (IS_ERR(file))
27. return PTR_ERR(file);
28. }
30. flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);
32. down_write(¤t->mm->mmap_sem);
33. retval = do_mmap_pgoff(file, addr, len, prot, flags, pgoff);
34. up_write(¤t->mm->mmap_sem);
36. if (file)
37. fput(file);
38. out:
39. return retval;
40. }

其功能為：從當前進程中獲取用戶態可用的虛擬地址空間（vm_area_struct *vma），在mmap_region中真正獲取vma，然後調用file->f_op->mmap(file, vma)，調用具體的支持mmap的驅動來處理。

下面以binder驅動為例。