linux内核mount系统调用源码分析

@Author:  Gordon Wu

@Time : 2014/12

0.摘要

mount是Linux很常见的命令，本文将从用户空间的命令行开始，一步一步切入到内核的源代码，解释一个文件系统是如果挂载的。本文基于linux 2.6.32

1.SYSCALL_DEFINE5, 系统调用

Linux kernel通过系统调用的方式为用户提供陷入到内核，mount的系统调用是SYSCALL_DEFINE5，位于fs/namespace.c:
主要完成把一些安装信息从用户空间拷贝到内核空间, 见代码的中文注释。

SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,char __user *, type, unsigned long, flags, void __user *, data){int ret;char *kernel_type;char *kernel_dir;char *kernel_dev;unsigned long data_page;ret = copy_mount_string(type, &kernel_type);       // 把用户空间的挂载类型复制到内核if (ret < 0)goto out_type;kernel_dir = getname(dir_name);                    //通过kmem_cache_alloc从names_cachep一个PATH_MAX大小的内核空间，把用户空间的dir_name复制过去if (IS_ERR(kernel_dir)) {ret = PTR_ERR(kernel_dir);goto out_dir;}ret = copy_mount_string(dev_name, &kernel_dev);     //复制挂载的设备过去if (ret < 0)goto out_dev;ret = copy_mount_options(data, &data_page);if (ret < 0)goto out_data;ret = do_mount(kernel_dev, kernel_dir, kernel_type, flags,   (void *) data_page);                           //数据准备好，开始do_mountfree_page(data_page);out_data:kfree(kernel_dev);out_dev:putname(kernel_dir);out_dir:kfree(kernel_type);out_type:return ret;}

2. do_mount

验证挂载的选项Flags，以及获取并填充相应挂载目录dir_name的路径Path结构。 再根据挂载选项Flags来判断，挂载的动作。

/* * Flags is a 32-bit value that allows up to 31 non-fs dependent flags to * be given to the mount() call (ie: read-only, no-dev, no-suid etc). * * data is a (void *) that can point to any structure up to * PAGE_SIZE-1 bytes, which can contain arbitrary fs-dependent * information (or be NULL). * * Pre-0.97 versions of mount() didn't have a flags word. * When the flags word was introduced its top half was required * to have the magic value 0xC0ED, and this remained so until 2.4.0-test9. * Therefore, if this magic number is present, it carries no information * and must be discarded. */long do_mount(char *dev_name, char *dir_name, char *type_page,  unsigned long flags, void *data_page){struct path path;int retval = 0;int mnt_flags = 0;/* Discard magic */if ((flags & MS_MGC_MSK) == MS_MGC_VAL)flags &= ~MS_MGC_MSK;/* Basic sanity checks */if (!dir_name || !*dir_name || !memchr(dir_name, 0, PAGE_SIZE))           //挂载目录的有效性， memchr是判断dir_name是否在0~PAGE_SIZE（用户空间）中return -EINVAL;                                                  if (data_page)((char *)data_page)[PAGE_SIZE - 1] = 0;/* Default to relatime unless overriden */if (!(flags & MS_NOATIME))mnt_flags |= MNT_RELATIME;/* Separate the per-mountpoint flags */if (flags & MS_NOSUID)mnt_flags |= MNT_NOSUID;                                           //忽略suid和sgid位的影响if (flags & MS_NODEV)mnt_flags |= MNT_NODEV;                                         //不允许访问设备专用文件if (flags & MS_NOEXEC)mnt_flags |= MNT_NOEXEC;                                    //不允许执行程序if (flags & MS_NOATIME)mnt_flags |= MNT_NOATIME;                                  //下面三个标志是关于是否更新文件或目录的atimeif (flags & MS_NODIRATIME)mnt_flags |= MNT_NODIRATIME;if (flags & MS_STRICTATIME)mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);if (flags & MS_RDONLY)                                            //只读标志mnt_flags |= MNT_READONLY;                                                                            //相应的一些标志已经备份到mnt_flags了，flags去除相应位flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |   MS_STRICTATIME); /* ... and get the mountpoint */retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);                  //！！根据dir_name，获取挂载目录的路径信息pathif (retval)return retval;retval = security_sb_mount(dev_name, &path,   type_page, flags, data_page);if (retval)goto dput_out;        <span style="color:#FF0000;">/*根据不同选项，进行下面五种不同的挂载*/</span>if (flags & MS_REMOUNT)retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,    data_page);else if (flags & MS_BIND)retval = do_loopback(&path, dev_name, flags & MS_REC);else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))retval = do_change_type(&path, flags);else if (flags & MS_MOVE)retval = do_move_mount(&path, dev_name);elseretval = do_new_mount(&path, type_page, flags, mnt_flags,                //do_new_mount是最常用的挂载,path: 挂载目录，dev_name：挂载设备      dev_name, data_page);dput_out:path_put(&path);return retval;}

3. do_new_mount

利用do_kern_mount为用户空间生成一个新的挂载，并do_add_mount把新安装加入到命名空间树上

/* * create a new mount for userspace and request it to be added into the * namespace's tree */  // *path: 挂载目录， *name:挂载设备，*type:挂载文件系统类型static int do_new_mount(struct path *path, char *type, int flags,           int mnt_flags, char *name, void *data){struct vfsmount *mnt;                  //包含已挂载文件系统的信息。if (!type)return -EINVAL;/* we need capabilities... */           // root权限，才能挂载if (!capable(CAP_SYS_ADMIN))return -EPERM;        //内核锁lock_kernel();mnt = do_kern_mount(type, flags, name, data);        //完成mnt信息的填充。unlock_kernel();if (IS_ERR(mnt))return PTR_ERR(mnt);return do_add_mount(mnt, path, mnt_flags, NULL);     //添加到命名空间树上}

4. do_kern_mount

4.1 do_kern_mount函数首先会调用get_fs_type来查看内核是否注册了参数type所指的文件系统，对于内核源码下fs目录下的所有文件系统都会通过调用register_filesystem来注册这个文件系统，其实就是添加到内核文件系统链表中，get_fs_type会将参数type字符串跟内核链表中所有已经注册的文件系统结构体file_system_type的name成员向比较，如果找到，则说明内核已经注册了相应文件系统，并且返回相应文件系统注册的file_system_type结构体。后面的挂载过程需要使用到这个结构体中的成员。[1]

struct vfsmount *do_kern_mount(const char *fstype, int flags, const char *name, void *data){struct file_system_type *type = get_fs_type(fstype);    //获取挂载文件系统的类型结构，见4.2,4.3简介struct vfsmount *mnt;if (!type)return ERR_PTR(-ENODEV);mnt = vfs_kern_mount(type, flags, name, data);           if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&    !mnt->mnt_sb->s_subtype)mnt = fs_set_subtype(mnt, fstype);put_filesystem(type);return mnt;}

4.2 struct file_system_type结构体，下面的get_sb函数会在下面的挂载过程中用到，用来完成具体文件系统挂载的操作。

struct file_system_type {const char *name;int fs_flags;int (*get_sb) (struct file_system_type *, int,       const char *, void *, struct vfsmount *);void (*kill_sb) (struct super_block *);struct module *owner;struct file_system_type * next;struct list_head fs_supers;struct lock_class_key s_lock_key;struct lock_class_key s_umount_key;struct lock_class_key i_lock_key;struct lock_class_key i_mutex_key;struct lock_class_key i_mutex_dir_key;struct lock_class_key i_alloc_sem_key;};

4.3 get_fs_type用到的最主要的函数: 就是从已挂载文件系统队列file_systems中查找，返回相应name类型的file_system_type.

static struct file_system_type **find_filesystem(const char *name, unsigned len){struct file_system_type **p;for (p=&file_systems; *p; p=&(*p)->next)if (strlen((*p)->name) == len &&    strncmp((*p)->name, name, len) == 0)break;return p;}

5. vfs_kern_mount

具体文件系统file_system_type的get_sb成员函数是关键，它填充vfsmnt结构体的super_block结构体，是写文件系统的第一步。

struct vfsmount *vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data){struct vfsmount *mnt;char *secdata = NULL;int error;if (!type)return ERR_PTR(-ENODEV);error = -ENOMEM;mnt = alloc_vfsmnt(name);      //alloc_vfsmnt以设备名为参数，为mnt函数分配一个空间，初始化mnt的基本信息，包括mnt->mnt_devname，以及一些listif (!mnt)goto out;if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {secdata = alloc_secdata();if (!secdata)goto out_mnt;error = security_sb_copy_data(data, secdata);if (error)goto out_free_secdata;}error = type->get_sb(type, flags, name, data, mnt);      //调用具体文件系统file_system_type的get_sb函数，填充vfsmnt结构体的super_block结构体if (error < 0)goto out_free_secdata;BUG_ON(!mnt->mnt_sb); error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata); if (error) goto out_sb;/* * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE * but s_maxbytes was an unsigned long long for many releases. Throw * this warning for a little while to try and catch filesystems that * violate this rule. This warning should be either removed or * converted to a BUG() in 2.6.34. */WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to ""negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);mnt->mnt_mountpoint = mnt->mnt_root;                   //mnt->mnt_parent = mnt;up_write(&mnt->mnt_sb->s_umount);free_secdata(secdata);return mnt;out_sb:dput(mnt->mnt_root);deactivate_locked_super(mnt->mnt_sb);out_free_secdata:free_secdata(secdata);out_mnt:free_vfsmnt(mnt);out:return ERR_PTR(error);}

5.2 ext2_get_sb, 以ext2文件系统为例，通过ext2_fill_super来填充super_block的信息。

static int ext2_get_sb(struct file_system_type *fs_type,int flags, const char *dev_name, void *data, struct vfsmount *mnt){return get_sb_bdev(fs_type, flags, dev_name, data, ext2_fill_super, mnt);}

5.3  get_sb_bdev

获取相应块设备的super_block，通过ext2_fill_super来完成super_block信息的填充。

int get_sb_bdev(struct file_system_type *fs_type,int flags, const char *dev_name, void *data,int (*fill_super)(struct super_block *, void *, int),struct vfsmount *mnt){struct block_device *bdev;          //块设备信息struct super_block *s;fmode_t mode = FMODE_READ;int error = 0;if (!(flags & MS_RDONLY))mode |= FMODE_WRITE;bdev = open_bdev_exclusive(dev_name, mode, fs_type);    //根据dev_name，去读取块设备信息。if (IS_ERR(bdev))return PTR_ERR(bdev);/* * once the super is inserted into the list by sget, s_umount * will protect the lockfs code from trying to start a snapshot * while we are mounting */mutex_lock(&bdev->bd_fsfreeze_mutex);if (bdev->bd_fsfreeze_count > 0) {mutex_unlock(&bdev->bd_fsfreeze_mutex);error = -EBUSY;goto error_bdev;}s = sget(fs_type, test_bdev_super, set_bdev_super, bdev);     //根据bdev，去读取相应的super_blockmutex_unlock(&bdev->bd_fsfreeze_mutex);if (IS_ERR(s))goto error_s;if (s->s_root) {if ((flags ^ s->s_flags) & MS_RDONLY) {deactivate_locked_super(s);error = -EBUSY;goto error_bdev;}close_bdev_exclusive(bdev, mode);} else {char b[BDEVNAME_SIZE];s->s_flags = flags;s->s_mode = mode;strlcpy(s->s_id, bdevname(bdev, b), sizeof(s->s_id));sb_set_blocksize(s, block_size(bdev));error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);        //具体文件系统的fill_super，这里是ext2_fill_super，完成对super_block各个域的初始化if (error) {deactivate_locked_super(s);goto error;}s->s_flags |= MS_ACTIVE;bdev->bd_super = s;}simple_set_mnt(mnt, s);return 0;error_s:error = PTR_ERR(s);error_bdev:close_bdev_exclusive(bdev, mode);error:return error;}

6.do_new_mount->do_add_mount

回到do_new_mount的do_add_mount函数，把mnt挂载到命名空间上。do_add_mount将新挂载的文件系统（由vfsmnt表示）添加到系统的命名空间结构体的已挂载文件系统链表中，命名空间是指系统中以挂载文件系统树，每个进程的PCB中都有namespace成员来表示该进程的命名空间，大多数的进程共享同一个命名空间，所以如果在一个进程中将磁盘挂载到系统中，在另一个进程也是可以看到的，这就是由命名空间来实现的[1]。

* * add a mount into a namespace's mount tree * - provide the option of adding the new mount to an expiration list */int do_add_mount(struct vfsmount *newmnt, struct path *path, int mnt_flags, struct list_head *fslist){int err;down_write(&namespace_sem);/* Something was mounted here while we slept */             /*挂载目录可能是已挂载的，follow_down可以把path->mnt和path->dentry指向上一层挂载*/while (d_mountpoint(path->dentry) &&       follow_down(path));err = -EINVAL;if (!(mnt_flags & MNT_SHRINKABLE) && !check_mnt(path->mnt))goto unlock;/* Refuse the same filesystem on the same mount point */err = -EBUSY;if (path->mnt->mnt_sb == newmnt->mnt_sb &&    path->mnt->mnt_root == path->dentry)goto unlock;err = -EINVAL;if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))goto unlock;newmnt->mnt_flags = mnt_flags;if ((err = graft_tree(newmnt, path)))goto unlock;if (fslist) /* add to the specified expiration list */                  /*添加*/list_add_tail(&newmnt->mnt_expire, fslist);up_write(&namespace_sem);return 0;unlock:up_write(&namespace_sem);mntput(newmnt);return err;}

参考博文: [1]:  http://blog.csdn.net/skyflying2012/article/details/9748133