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Subject[PATCH 4.14 136/193] dax: fix PMD faults on zero-length files
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jeff Moyer <jmoyer@redhat.com>

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.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/fs/dax.c
+++ b/fs/dax.c
@@ -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;

/*

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