Re: [PATCH 30/38] vfs: syscall: Add fsmount() to create a mount for a superblock [ver #10]

[Andy Lutomirski <luto@amacapital.net>]

> On Jul 27, 2018, at 10:34 AM, David Howells <dhowells@redhat.com> wrote:
> 
> Provide a system call by which a filesystem opened with fsopen() and
> configured by a series of writes can be mounted:
> 
>    int ret = fsmount(int fsfd, unsigned int flags,
>              unsigned int ms_flags);
> 
> where fsfd is the file descriptor returned by fsopen().  flags can be 0 or
> FSMOUNT_CLOEXEC.  ms_flags is a bitwise-OR of the following flags:

I have a potentially silly objection. For the old timers, “mount” means to stick a reel of tape or some similar object onto a reader, which seems to imply that “mount” means to start up the filesystem. For younguns, this meaning is probably lost, and the more obvious meaning is to “mount” it into some location in the VFS hierarchy a la vfsmount. The patch description doesn’t disambiguate it, and obviously people used to mount(2)/mount(8) are just likely to be confused.

At the very least, your description should make it absolutely clear what you mean. Even better IMO would be to drop the use of the word “mount” entirely and maybe rename the syscall.

From a very brief reading, I think you are giving it the meaning that would be implied by fsstart(2).

> 
>    MS_RDONLY
>    MS_NOSUID
>    MS_NODEV
>    MS_NOEXEC
>    MS_NOATIME
>    MS_NODIRATIME
>    MS_RELATIME
>    MS_STRICTATIME
> 
>    MS_UNBINDABLE
>    MS_PRIVATE
>    MS_SLAVE
>    MS_SHARED
> 
> In the event that fsmount() fails, it may be possible to get an error
> message by calling read() on fsfd.  If no message is available, ENODATA
> will be reported.
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: linux-api@vger.kernel.org
> ---
> 
> arch/x86/entry/syscalls/syscall_32.tbl |    1 
> arch/x86/entry/syscalls/syscall_64.tbl |    1 
> fs/namespace.c                         |  140 +++++++++++++++++++++++++++++++-
> include/linux/syscalls.h               |    1 
> include/uapi/linux/fs.h                |    2 
> 5 files changed, 141 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index f9970310c126..c78b68256f8a 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -402,3 +402,4 @@
> 388    i386    move_mount        sys_move_mount            __ia32_sys_move_mount
> 389    i386    fsopen            sys_fsopen            __ia32_sys_fsopen
> 390    i386    fsconfig        sys_fsconfig            __ia32_sys_fsconfig
> +391    i386    fsmount            sys_fsmount            __ia32_sys_fsmount
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index 4185d36e03bb..d44ead5d4368 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -347,6 +347,7 @@
> 336    common    move_mount        __x64_sys_move_mount
> 337    common    fsopen            __x64_sys_fsopen
> 338    common    fsconfig        __x64_sys_fsconfig
> +339    common    fsmount            __x64_sys_fsmount
> 
> #
> # x32-specific system call numbers start at 512 to avoid cache impact
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ea07066a2731..b1661b90256d 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2503,7 +2503,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
> 
>    attached = mnt_has_parent(old);
>    /*
> -     * We need to allow open_tree(OPEN_TREE_CLONE) followed by
> +     * We need to allow open_tree(OPEN_TREE_CLONE) or fsmount() followed by
>     * move_mount(), but mustn't allow "/" to be moved.
>     */
>    if (old->mnt_ns && !attached)
> @@ -3348,9 +3348,141 @@ struct vfsmount *kern_mount(struct file_system_type *type)
> EXPORT_SYMBOL_GPL(kern_mount);
> 
> /*
> - * Move a mount from one place to another.
> - * In combination with open_tree(OPEN_TREE_CLONE [| AT_RECURSIVE]) it can be
> - * used to copy a mount subtree.
> + * Create a kernel mount representation for a new, prepared superblock
> + * (specified by fs_fd) and attach to an open_tree-like file descriptor.
> + */
> +SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags, unsigned int, ms_flags)
> +{
> +    struct fs_context *fc;
> +    struct file *file;
> +    struct path newmount;
> +    struct fd f;
> +    unsigned int mnt_flags = 0;
> +    long ret;
> +
> +    if (!may_mount())
> +        return -EPERM;
> +
> +    if ((flags & ~(FSMOUNT_CLOEXEC)) != 0)
> +        return -EINVAL;
> +
> +    if (ms_flags & ~(MS_RDONLY | MS_NOSUID | MS_NODEV | MS_NOEXEC |
> +             MS_NOATIME | MS_NODIRATIME | MS_RELATIME |
> +             MS_STRICTATIME))
> +        return -EINVAL;
> +
> +    if (ms_flags & MS_RDONLY)
> +        mnt_flags |= MNT_READONLY;
> +    if (ms_flags & MS_NOSUID)
> +        mnt_flags |= MNT_NOSUID;
> +    if (ms_flags & MS_NODEV)
> +        mnt_flags |= MNT_NODEV;
> +    if (ms_flags & MS_NOEXEC)
> +        mnt_flags |= MNT_NOEXEC;
> +    if (ms_flags & MS_NODIRATIME)
> +        mnt_flags |= MNT_NODIRATIME;
> +
> +    if (ms_flags & MS_STRICTATIME) {
> +        if (ms_flags & MS_NOATIME)
> +            return -EINVAL;
> +    } else if (ms_flags & MS_NOATIME) {
> +        mnt_flags |= MNT_NOATIME;
> +    } else {
> +        mnt_flags |= MNT_RELATIME;
> +    }
> +
> +    f = fdget(fs_fd);
> +    if (!f.file)
> +        return -EBADF;
> +
> +    ret = -EINVAL;
> +    if (f.file->f_op != &fscontext_fops)
> +        goto err_fsfd;
> +
> +    fc = f.file->private_data;
> +
> +    /* There must be a valid superblock or we can't mount it */
> +    ret = -EINVAL;
> +    if (!fc->root)
> +        goto err_fsfd;
> +
> +    ret = -EPERM;
> +    if (mount_too_revealing(fc->root->d_sb, &mnt_flags)) {
> +        pr_warn("VFS: Mount too revealing\n");
> +        goto err_fsfd;
> +    }
> +
> +    ret = mutex_lock_interruptible(&fc->uapi_mutex);
> +    if (ret < 0)
> +        goto err_fsfd;
> +
> +    ret = -EBUSY;
> +    if (fc->phase != FS_CONTEXT_AWAITING_MOUNT)
> +        goto err_unlock;
> +
> +    ret = -EPERM;
> +    if ((fc->sb_flags & SB_MANDLOCK) && !may_mandlock())
> +        goto err_unlock;
> +
> +    newmount.mnt = vfs_create_mount(fc, mnt_flags);
> +    if (IS_ERR(newmount.mnt)) {
> +        ret = PTR_ERR(newmount.mnt);
> +        goto err_unlock;
> +    }
> +    newmount.dentry = dget(fc->root);
> +
> +    /* We've done the mount bit - now move the file context into more or
> +     * less the same state as if we'd done an fspick().  We don't want to
> +     * do any memory allocation or anything like that at this point as we
> +     * don't want to have to handle any errors incurred.
> +     */
> +    if (fc->ops && fc->ops->free)
> +        fc->ops->free(fc);
> +    fc->fs_private = NULL;
> +    fc->s_fs_info = NULL;
> +    fc->sb_flags = 0;
> +    fc->sloppy = false;
> +    fc->silent = false;
> +    security_fs_context_free(fc);
> +    fc->security = NULL;
> +    kfree(fc->subtype);
> +    fc->subtype = NULL;
> +    kfree(fc->source);
> +    fc->source = NULL;
> +
> +    fc->purpose = FS_CONTEXT_FOR_RECONFIGURE;
> +    fc->phase = FS_CONTEXT_AWAITING_RECONF;
> +
> +    /* Attach to an apparent O_PATH fd with a note that we need to unmount
> +     * it, not just simply put it.
> +     */
> +    file = dentry_open(&newmount, O_PATH, fc->cred);
> +    if (IS_ERR(file)) {
> +        ret = PTR_ERR(file);
> +        goto err_path;
> +    }
> +    file->f_mode |= FMODE_NEED_UNMOUNT;
> +
> +    ret = get_unused_fd_flags((flags & FSMOUNT_CLOEXEC) ? O_CLOEXEC : 0);
> +    if (ret >= 0)
> +        fd_install(ret, file);
> +    else
> +        fput(file);
> +
> +err_path:
> +    path_put(&newmount);
> +err_unlock:
> +    mutex_unlock(&fc->uapi_mutex);
> +err_fsfd:
> +    fdput(f);
> +    return ret;
> +}
> +
> +/*
> + * Move a mount from one place to another.  In combination with
> + * fsopen()/fsmount() this is used to install a new mount and in combination
> + * with open_tree(OPEN_TREE_CLONE [| AT_RECURSIVE]) it can be used to copy
> + * a mount subtree.
>  *
>  * Note the flags value is a combination of MOVE_MOUNT_* flags.
>  */
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 9628d14a7ede..65db661cc2da 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -907,6 +907,7 @@ asmlinkage long sys_move_mount(int from_dfd, const char __user *from_path,
> asmlinkage long sys_fsopen(const char __user *fs_name, unsigned int flags);
> asmlinkage long sys_fsconfig(int fs_fd, unsigned int cmd, const char __user *key,
>                 const void __user *value, int aux);
> +asmlinkage long sys_fsmount(int fs_fd, unsigned int flags, unsigned int ms_flags);
> 
> /*
>  * Architecture-specific system calls
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 7c9e165e8689..297362908d01 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -349,6 +349,8 @@ typedef int __bitwise __kernel_rwf_t;
>  */
> #define FSOPEN_CLOEXEC        0x00000001
> 
> +#define FSMOUNT_CLOEXEC        0x00000001
> +
> /*
>  * The type of fsconfig() call made.
>  */
> 
> --
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