Patch "dax: fix PMD faults on zero-length files" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

dax: fix PMD faults on zero-length files

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dax-fix-pmd-faults-on-zero-length-files.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jeff Moyer <[email protected]>
Date: Tue, 14 Nov 2017 20:37:27 -0500
Subject: dax: fix PMD faults on zero-length files

From: Jeff Moyer <[email protected]>

commit 957ac8c421ad8b5eef9b17fe98e146d8311a541e upstream.

PMD faults on a zero length file on a file system mounted with -o dax will not generate SIGBUS as expected.

	fd = open(...O_TRUNC);
	addr = mmap(NULL, 2*1024*1024, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
	*addr = 'a';
        <expect SIGBUS>

The problem is this code in dax_iomap_pmd_fault:

	max_pgoff = (i_size_read(inode) - 1) >> PAGE_SHIFT;

If the inode size is zero, we end up with a max_pgoff that is way larger than 0. :) Fix it by using DIV_ROUND_UP, as is done elsewhere in the kernel.

I tested this with some simple test code that ensured that SIGBUS was received where expected.

---
 fs/dax.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

@@ -1327,7 +1327,7 @@ static int dax_iomap_pmd_fault(struct vm
 	 * this is a reliable test.
 	 */
 	pgoff = linear_page_index(vma, pmd_addr);
-	max_pgoff = (i_size_read(inode) - 1) >> PAGE_SHIFT;
+	max_pgoff = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
 
 	trace_dax_pmd_fault(inode, vmf, max_pgoff, 0);
 

@@ -1351,13 +1351,13 @@ static int dax_iomap_pmd_fault(struct vm
 	if ((pmd_addr + PMD_SIZE) > vma->vm_end)
 		goto fallback;
 
-	if (pgoff > max_pgoff) {
+	if (pgoff >= max_pgoff) {
 		result = VM_FAULT_SIGBUS;
 		goto out;
 	}
 
 	/* If the PMD would extend beyond the file size */
-	if ((pgoff | PG_PMD_COLOUR) > max_pgoff)
+	if ((pgoff | PG_PMD_COLOUR) >= max_pgoff)
 		goto fallback;
 
 	/*

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/dax-fix-pmd-faults-on-zero-length-files.patch