linux-kernel - Re: Does vm_operations_struct require a .owner field?

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Message-ID: <20160105205812.GA24738@node.shutemov.name>
Date:	Tue, 5 Jan 2016 22:58:12 +0200
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	linux-mm@...ck.org,
	Kernel development list <linux-kernel@...r.kernel.org>,
	David Laight <David.Laight@...LAB.COM>,
	"'Steinar H. Gunderson'" <sesse@...gle.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: Does vm_operations_struct require a .owner field?

On Tue, Jan 05, 2016 at 11:27:45AM -0500, Alan Stern wrote:
> Hello:
> 
> Question: The vm_operations_struct structure contains lots of callback
> pointers.  Is there any mechanism to prevent the callback routines and
> the structure itself being unloaded from memory (if they are built into
> modules) while the relevant VMAs are still in use?
> 
> Consider a simple example: A user program calls mmap(2) on a device
> file.  Later on, the file is closed and the device driver's module is
> unloaded.  But until munmap(2) is called or the user program exits, the
> memory mapping and the corresponding VMA will remain in existence.  
> (The man page for munmap specifically says "closing the file descriptor
> does not unmap the region".)  Thus when the user program does do an
> munmap(), the callback to vma->vm_ops->close will reference nonexistent
> memory and cause an oops.
> 
> Normally this sort of thing is prevented by try_module_get(...->owner).  
> But vm_operations_struct doesn't include a .owner field.
> 
> Am I missing something here?

mmap(2) takes reference of the file, therefore the file is not closed from
kernel POV until vma is gone and you cannot unload relevant module.
See get_file() in mmap_region().

-- 
 Kirill A. Shutemov
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