linux-kernel - Re: [RFC PATCH 16/35] ovl: readd lsattr/chattr support

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Date:   Fri, 13 Apr 2018 17:48:56 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     overlayfs <linux-unionfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 16/35] ovl: readd lsattr/chattr support

On Thu, Apr 12, 2018 at 6:08 PM, Miklos Szeredi <mszeredi@...hat.com> wrote:
> Implement FS_IOC_GETFLAGS and FS_IOC_SETFLAGS.
>
> Needs vfs_ioctl() exported to modules.
>
> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> ---
>  fs/internal.h       |  1 -
>  fs/ioctl.c          |  1 +
>  fs/overlayfs/file.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/fs.h  |  2 ++
>  4 files changed, 62 insertions(+), 1 deletion(-)
>
> diff --git a/fs/internal.h b/fs/internal.h
> index 3319bf39e339..d5108d9c6a2f 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -176,7 +176,6 @@ extern const struct dentry_operations ns_dentry_operations;
>   */
>  extern int do_vfs_ioctl(struct file *file, unsigned int fd, unsigned int cmd,
>                     unsigned long arg);
> -extern long vfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
>
>  /*
>   * iomap support:
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 5ace7efb0d04..696f4c46a868 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -49,6 +49,7 @@ long vfs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>   out:
>         return error;
>  }
> +EXPORT_SYMBOL(vfs_ioctl);
>
>  static int ioctl_fibmap(struct file *filp, int __user *p)
>  {
> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> index 05e3e2f80b89..cc004ff1b05b 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -8,6 +8,7 @@
>
>  #include <linux/cred.h>
>  #include <linux/file.h>
> +#include <linux/mount.h>
>  #include <linux/xattr.h>
>  #include <linux/uio.h>
>  #include "overlayfs.h"
> @@ -291,6 +292,63 @@ long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
>         return ret;
>  }
>
> +static long ovl_real_ioctl(struct file *file, unsigned int cmd,
> +                          unsigned long arg)
> +{
> +       struct fd real;
> +       const struct cred *old_cred;
> +       long ret;
> +
> +       ret = ovl_real_file(file, &real);
> +       if (ret)
> +               return ret;
> +
> +       old_cred = ovl_override_creds(file_inode(file)->i_sb);
> +       ret = vfs_ioctl(real.file, cmd, arg);
> +       revert_creds(old_cred);
> +
> +       fdput(real);
> +
> +       return ret;
> +}
> +
> +long ovl_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> +{
> +       long ret;
> +       struct inode *inode = file_inode(file);
> +
> +       switch (cmd) {
> +       case FS_IOC_GETFLAGS:
> +               ret = ovl_real_ioctl(file, cmd, arg);
> +               break;
> +
> +       case FS_IOC_SETFLAGS:
> +               if (!inode_owner_or_capable(inode))
> +                       return -EACCES;
> +
> +               ret = mnt_want_write_file(file);
> +               if (ret)
> +                       return ret;
> +
> +               ret = ovl_copy_up(file_dentry(file));
> +               if (!ret) {
> +                       ret = ovl_real_ioctl(file, cmd, arg);
> +
> +                       inode_lock(inode);
> +                       ovl_copyflags(ovl_inode_real(inode), inode);
> +                       inode_unlock(inode);
> +               }
> +
> +               mnt_drop_write_file(file);
> +               break;
> +
> +       default:
> +               ret = -ENOTTY;

I am wondering out loud.
This is a change of behavior that fs specific ioctls cannot be executed
on overlay file - arguably a good change of behavior, but still a change
that applications may got dependent on.

Would it have been better to opt-in for this change by a more generic
config/mount options, for example "consistent_fd" , instead of
"copy_up_shared" and then we can choose whether or not to
pass though unknown ioctls to real file.

I know we removed the want_write_file() protection from VFS, but
still, pass through of ioctls was the legacy behavior. Thoughts?
I don't mind to wait and see if someone shouts.

Thanks,
Amir.