Commit 957ac8c4 authored Nov 14, 2017 by Jeff Moyer Committed by Dan Williams Nov 14, 2017

dax: fix PMD faults on zero-length files



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.

Cc: <stable@vger.kernel.org>
Fixes: 642261ac ("dax: add struct iomap based DAX PMD support")
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

parent 6a21586a

fs/dax.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -1333,7 +1333,7 @@ static int dax_iomap_pmd_fault(struct vm_fault vmf, pfn_t pfnp,
		* 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);

		@@ -1357,13 +1357,13 @@ static int dax_iomap_pmd_fault(struct vm_fault vmf, pfn_t pfnp,
		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;

		/*

Admin message