linux-kernel - Re: [PATCH] fs: print warning when mount flags was ignored

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Date:	Thu, 19 Apr 2012 12:28:57 +0200
From:	Karel Zak <kzak@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mbroz@...hat.com,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] fs: print warning when mount flags was ignored

On Thu, Apr 19, 2012 at 11:07:55AM +0200, Lukas Czerner wrote:
> Some mount flags can conflict with each other so they can not be
> handled together. Currently when conflicting flags are specified,
> some of them are silently ignored putting user in believe that
> they was handled correctly.

Unfortunately, it's not so simple ;-)

> -	if (flags & MS_REMOUNT)
> +	if (flags & MS_REMOUNT) {
>  		retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
>  				    data_page);
> -	else if (flags & MS_BIND)
> +		flags &= ~MS_REMOUNT;

This is incorrect, the flags may also include many others flags. For
example MS_REMOUNT|MS_BIND|MS_RDONLY is valid (see do_remoun() code).

And it's normal that for "mount -o remount" the mount command reads
flags from mtab/fstab so it includes for example MS_RELATIME, ...

> +	} 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))
> +		flags &= ~MS_BIND;

 what about MS_REC ?

> +	} else if (flags & (MS_SHARED | MS_PRIVATE |
> +			    MS_SLAVE | MS_UNBINDABLE)) {
>  		retval = do_change_type(&path, flags);
> -	else if (flags & MS_MOVE)
> +		flags &= ~(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE);

 what about MS_REC ?

 Note that do_change_type() already checks for unexpected flags and
 returns -EINVAL if more flags are specified.

> +	} else if (flags & MS_MOVE) {
>  		retval = do_move_mount(&path, dev_name);
> -	else
> +		flags &= ~MS_MOVE;
> +	} else
>  		retval = do_new_mount(&path, type_page, flags, mnt_flags,
>  				      dev_name, data_page);
> +
> +	flags &= (MS_REMOUNT | MS_BIND | MS_SHARED | MS_PRIVATE |
> +		  MS_SLAVE | MS_UNBINDABLE | MS_MOVE);
> +
> +	if (!retval && flags)
> +		printk(KERN_WARNING "%s(%u): (%s -> %s) Conflicting mount flags"
> +				    " specified. These flags has been "
> +				    "ignored: %#.8lx\n", __func__, current->pid,
> +				    dev_name, dir_name, flags);


    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com
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