linux-kernel - Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM

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Date:	Thu, 29 Jan 2009 12:48:54 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Lee Schermerhorn <Lee.Schermerhorn@...com>
cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Maksim Yevmenkin <maksim.yevmenkin@...il.com>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, will@...wder-design.com,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED
 file segments



On Thu, 29 Jan 2009, Linus Torvalds wrote:
> 
> THIS PATCH IS TOTALLY UNTESTED!

Well, it boots. FWIW. I've not really tested anything interesting with it, 
but any potential breakage is at least not catastrophic and immediate.

> diff --git a/mm/mmap.c b/mm/mmap.c
> index 8d95902..3f78ead 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -769,6 +769,10 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
>  	if (vm_flags & VM_SPECIAL)
>  		return NULL;
>  
> +	/* Anonymous shared mappings are unsharable */
> +	if ((vm_flags & VM_SHARED) && !file)
> +		return NULL;
> +

.. and I think this part of it is actually unnecessary, because what 
happens is that a shared anon mapping is turned into a shmem mapping when 
it is inserted, and that actually ends up allocating a file for it. So the 
vma->vm_file for anon mappings will not match a NULL file pointer 
_anyway_, so there's no way it would end up merging.

So my patch can be further simplified, I think, to just the following. 
Even more total lines removed.

I still want somebody else to look at and think about it, though.

		Linus

---
 mm/mmap.c |   26 ++++++--------------------
 1 files changed, 6 insertions(+), 20 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 8d95902..d3fa10a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1134,16 +1134,11 @@ munmap_back:
 	}
 
 	/*
-	 * Can we just expand an old private anonymous mapping?
-	 * The VM_SHARED test is necessary because shmem_zero_setup
-	 * will create the file object for a shared anonymous map below.
+	 * Can we just expand an old mapping?
 	 */
-	if (!file && !(vm_flags & VM_SHARED)) {
-		vma = vma_merge(mm, prev, addr, addr + len, vm_flags,
-					NULL, NULL, pgoff, NULL);
-		if (vma)
-			goto out;
-	}
+	vma = vma_merge(mm, prev, addr, addr + len, vm_flags, NULL, file, pgoff, NULL);
+	if (vma)
+		goto out;
 
 	/*
 	 * Determine the object being mapped and call the appropriate
@@ -1206,17 +1201,8 @@ munmap_back:
 	if (vma_wants_writenotify(vma))
 		vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
 
-	if (file && vma_merge(mm, prev, addr, vma->vm_end,
-			vma->vm_flags, NULL, file, pgoff, vma_policy(vma))) {
-		mpol_put(vma_policy(vma));
-		kmem_cache_free(vm_area_cachep, vma);
-		fput(file);
-		if (vm_flags & VM_EXECUTABLE)
-			removed_exe_file_vma(mm);
-	} else {
-		vma_link(mm, vma, prev, rb_link, rb_parent);
-		file = vma->vm_file;
-	}
+	vma_link(mm, vma, prev, rb_link, rb_parent);
+	file = vma->vm_file;
 
 	/* Once vma denies write, undo our temporary denial count */
 	if (correct_wcount)
--
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