linux-kernel - Re: [PATCH] [media] v4l2 core: return -ENOIOCTLCMD if an ioctl doesn't exist

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Date:	Sun, 26 Jun 2011 19:13:05 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Mauro Carvalho Chehab <mchehab@...hat.com>
Cc:	Sakari Ailus <sakari.ailus@....fi>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] [media] v4l2 core: return -ENOIOCTLCMD if an ioctl doesn't exist

On Sunday 26 June 2011 18:20:21 Mauro Carvalho Chehab wrote:

> > The V4L2 core probably should return -ENOIOCTLCMD when an IOCTL isn't implemented, but as long as vfs_ioctl() would stay as it is, the user space would still get -EINVAL. Or is vfs_ioctl() about to change?
> > 
> > fs/ioctl.c:
> > ----8<-----------
> > static long vfs_ioctl(struct file *filp, unsigned int cmd,
> >                       unsigned long arg)
> > {
> >         int error = -ENOTTY;
> > 
> >         if (!filp->f_op || !filp->f_op->unlocked_ioctl)
> >                 goto out;
> > 
> >         error = filp->f_op->unlocked_ioctl(filp, cmd, arg);
> >         if (error == -ENOIOCTLCMD)
> >                 error = -EINVAL;
> >  out:
> >         return error;
> > }
> > ----8<-----------

One of the differences between the old ->ioctl() and the ->unlocked_ioctl()
function is that unlocked_ioctl could point to the same function as
->compat_ioctl(), so we have to catch functions returning -ENOIOCTLCMD.

> Good catch!
> 
> At the recent git history, the return for -ENOIOCTLCMD were modified
> by this changeset:
> 
> commit b19dd42faf413b4705d4adb38521e82d73fa4249
> Author: Arnd Bergmann <arnd@...db.de>
> Date:   Sun Jul 4 00:15:10 2010 +0200
> 
>     bkl: Remove locked .ioctl file operation
> ...
> @@ -39,21 +38,12 @@ static long vfs_ioctl(struct file *filp, unsigned int cmd,
>  {
>         int error = -ENOTTY;
>  
> -   if (!filp->f_op)
> + if (!filp->f_op || !filp->f_op->unlocked_ioctl)
>                 goto out;
>  
> -   if (filp->f_op->unlocked_ioctl) {
> -           error = filp->f_op->unlocked_ioctl(filp, cmd, arg);
> -           if (error == -ENOIOCTLCMD)
> -                   error = -EINVAL;
> -           goto out;
> -   } else if (filp->f_op->ioctl) {
> -           lock_kernel();
> -           error = filp->f_op->ioctl(filp->f_path.dentry->d_inode,
> -                                     filp, cmd, arg);
> -           unlock_kernel();
> ...
> 
> Before Arnd's patch, locked ioctl's were returning -ENOIOCTLCMD, and
> unlocked ones were returning -EINVAL. Now, the return of -ENOIOCTLCMD
> doesn't go to userspace anymore. IMO, that's wrong and can cause
> regressions, as some subsystems like DVB were returning -ENOIOCTLCMD
> to userspace.

ENOIOCTLCMD should never be returned to user space, see the comment
in include/linux/errno.h:

/*
 * These should never be seen by user programs.  To return one of ERESTART*
 * codes, signal_pending() MUST be set.  Note that ptrace can observe these
 * at syscall exit tracing, but they will never be left for the debugged user
 * process to see.
 */

There was a lot of debate whether undefined ioctls on non-ttys should
return -EINVAL or -ENOTTY, including mass-conversions from -ENOTTY to
-EINVAL at some point in the pre-git era, IIRC.

Inside of v4l2, I believe this is handled by video_usercopy(), which
turns the driver's -ENOIOCTLCMD into -ENOTTY. What cases do you observe
where this is not done correctly and we do return ENOIOCTLCMD to
vfs_ioctl?

> The right fix would be to remove this from fs:
> 
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 1d9b9fc..802fbbd 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -41,8 +41,6 @@ static long vfs_ioctl(struct file *filp, unsigned int cmd,
>  		goto out;
>  
>  	error = filp->f_op->unlocked_ioctl(filp, cmd, arg);
> -	if (error == -ENOIOCTLCMD)
> -		error = -EINVAL;
>   out:
>  	return error;
>  }
> 
> However, the replacement from -EINVAL to -ENOIOCTLCMD is there since 2.6.12 for
> unlocked_ioctl:
> 
> $ git blame b19dd42f^1 fs/ioctl.c 
> ...
> ^1da177e (Linus Torvalds    2005-04-16 15:20:36 -0700  46)              error = filp->f_op->unlocked_ioctl(filp, cmd, arg);
> ^1da177e (Linus Torvalds    2005-04-16 15:20:36 -0700  47)              if (error == -ENOIOCTLCMD)
> ^1da177e (Linus Torvalds    2005-04-16 15:20:36 -0700  48)                      error = -EINVAL;
> 
> Linus,
> 
> what would be the expected behaviour?

Note that 1da177e is the initial commit to git, Linus did not write that
code, although he might have an opinion.

	Arnd
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