int get_write_access(struct inode * inode)
{spin_lock(&inode->i_lock);if (atomic_read(&inode->i_writecount) < 0) {spin_unlock(&inode->i_lock);return -ETXTBSY;}atomic_inc(&inode->i_writecount);spin_unlock(&inode->i_lock);return 0;
}int deny_write_access(struct file * file)
{struct inode *inode = file->f_path.dentry->d_inode;spin_lock(&inode->i_lock);if (atomic_read(&inode->i_writecount) > 0) {//如果文件被打开了，返回失败spin_unlock(&inode->i_lock);return -ETXTBSY;}atomic_dec(&inode->i_writecount); spin_unlock(&inode->i_lock);
}

struct file *open_exec(const char *name)
{struct file *file;int err;file = do_filp_open(AT_FDCWD, name,O_LARGEFILE | O_RDONLY | FMODE_EXEC, 0,MAY_EXEC | MAY_OPEN);if (IS_ERR(file))goto out;err = -EACCES;if (!S_ISREG(file->f_path.dentry->d_inode->i_mode))goto exit;if (file->f_path.mnt->mnt_flags & MNT_NOEXEC)goto exit;fsnotify_open(file->f_path.dentry);err = deny_write_access(file);//调用if (err)goto exit;out:return file;exit:fput(file);return ERR_PTR(err);
}

if (f->f_mode & FMODE_WRITE) {error = __get_file_write_access(inode, mnt);if (error)goto cleanup_file;if (!special_file(inode->i_mode))file_take_write(f);
}

if (f->f_mode & FMODE_WRITE) { error = get_write_access(inode); if (error) goto cleanup_file;
}

if (file->f_mode & FMODE_WRITE) put_write_access(inode);

static inline void put_write_access(struct inode * inode)
{ atomic_dec(&inode->i_writecount);
}

if (vm_flags & VM_DENYWRITE) {          error = deny_write_access(file);if (error)goto free_vma;correct_wcount = 1;
}

#include <stdio.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
int main()
{int fd;void *src = NULL;fd = open("test.txt",O_RDONLY);if (fd != 0){if ((src = mmap(0,5,PROT_READ|PROT_EXEC  ,MAP_PRIVATE|        MAP_DENYWRITE,fd,0))== MAP_FAILED){printf("MMAP error\n");printf("%s\n",strerror(errno));}else{printf("%x\n",src);}}FILE * fd_t = fopen("test.txt","w");if( !fd_t){printf("open for write error\n");printf("%s\n",strerror(errno));return 0;}if (fwrite("0000",sizeof(char),4,fd_t) != 4){printf("fwrite error \n");}fclose(fd_t);close(fd);return 1;
}