The patch titled
Subject: swap: divide-by-zero when zero length swap file on ssd
has been added to the -mm tree. Its filename is
swap-divide-by-zero-when-zero-length-swap-file-on-ssd.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/swap-divide-by-zero-when-zero-length-swap-file-on-ssd.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/swap-divide-by-zero-when-zero-length-swap-file-on-ssd.patch
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------------------------------------------------------
From: "Tom Abraham" <tabraham@xxxxxxxx>
Subject: swap: divide-by-zero when zero length swap file on ssd
Calling swapon() on a zero length swap file on SSD can lead to a
divide-by-zero.
Although creating such files isn't possible with mkswap and they woud be
considered invalid, it would be better for the swapon code to be more
robust and handle this condition gracefully (return -EINVAL). Especially
since the fix is small and straight-forward.
To help with wear leveling on SSD, the swapon syscall calculates a random
position in the swap file using modulo p->highest_bit, which is set to
maxpages - 1 in read_swap_header.
If the swap file is zero length, read_swap_header sets maxpages=1 and
last_page=0, resulting in p->highest_bit=0 and we divide-by-zero when we
modulo p->highest_bit in swapon syscall.
This can be prevented by having read_swap_header return zero if last_page is
zero.
Link: http://lkml.kernel.org/r/5AC747C1020000A7001FA82C@xxxxxxxxxxxxxxxxxxxxxxx
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/swapfile.c | 4 ++++
1 file changed, 4 insertions(+)
diff -puN mm/swapfile.c~swap-divide-by-zero-when-zero-length-swap-file-on-ssd mm/swapfile.c
--- a/mm/swapfile.c~swap-divide-by-zero-when-zero-length-swap-file-on-ssd
+++ a/mm/swapfile.c
@@ -2961,6 +2961,10 @@ static unsigned long read_swap_header(st
maxpages = swp_offset(pte_to_swp_entry(
swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
last_page = swap_header->info.last_page;
+ if (!last_page) {
+ pr_warn("Empty swap-file\n");
+ return 0;
+ }
if (last_page > maxpages) {
pr_warn("Truncating oversized swap area, only using %luk out of %luk\n",
maxpages << (PAGE_SHIFT - 10),
_
Patches currently in -mm which might be from tabraham@xxxxxxxx are
swap-divide-by-zero-when-zero-length-swap-file-on-ssd.patch
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