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: 642261ac995e ("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>

diff --git a/fs/dax.c b/fs/dax.c
index 27ba300660ff51d6307ce83566c53d1ad7bdd5c0..f757cd0e2d077a13a251faf68b67566e6c2281f6 100644 (file)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -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;
 
        /*