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mm/swapfile.c: fix swapon size off-by-one 

There's an off-by-one disagreement between mkswap and swapon about the
meaning of swap_header last_page: mkswap (in all versions I've looked at:
util-linux-ng and BusyBox and old util-linux; probably as far back as
1999) consistently means the offset (in page units) of the last page of
the swap area, whereas kernel sys_swapon (as far back as 2.2 and 2.3)
strangely takes it to mean the size (in page units) of the swap area.

This disagreement is the safe way round; but it's worrying people, and
loses us one page of swap.

The fix is not just to add one to nr_good_pages: we need to get maxpages
(the size of the swap_map array) right before that; and though that is an
unsigned long, be careful not to overflow the unsigned int p->max which
later holds it (probably why header uses __u32 last_page instead of size).

Why did we subtract one from the maximum swp_offset to calculate maxpages?
 Though it was probably me who made that change in 2.4.10, I don't get it:
and now we should be adding one (without risk of overflow in this case).

Fix the handling of swap_header badpages: it could have overrun the
swap_map when very large swap area used on a more limited architecture.

Remove pre-initializations of swap_header, nr_good_pages and maxpages:
those date from when sys_swapon was supporting other versions of header.

Reported-by: Nitin Gupta <ngupta@vflare.org>
Reported-by: Jarkko Lavinen <jarkko.lavinen@nokia.com>
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2010-03-05 13:42:12 -08:00 committed by Linus Torvalds
parent fc148a5f7e
commit ad2bd7e0e9
1 changed files with 18 additions and 13 deletions
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31
mm/swapfile.c
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@ -1760,11 +1760,11 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
unsigned int type;
int i, prev;
int error;
union swap_header *swap_header = NULL;
unsigned int nr_good_pages = 0;
union swap_header *swap_header;
unsigned int nr_good_pages;
int nr_extents = 0;
sector_t span;
unsigned long maxpages = 1;
unsigned long maxpages;
unsigned long swapfilepages;
unsigned char *swap_map = NULL;
struct page *page = NULL;
@ -1923,9 +1923,13 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
* swap pte.
*/
maxpages = swp_offset(pte_to_swp_entry(
swp_entry_to_pte(swp_entry(0, ~0UL)))) - 1;
if (maxpages > swap_header->info.last_page)
maxpages = swap_header->info.last_page;
swp_entry_to_pte(swp_entry(0, ~0UL)))) + 1;
if (maxpages > swap_header->info.last_page) {
maxpages = swap_header->info.last_page + 1;
/* p->max is an unsigned int: don't overflow it */
if ((unsigned int)maxpages == 0)
maxpages = UINT_MAX;
}
p->highest_bit = maxpages - 1;
error = -EINVAL;
@ -1949,23 +1953,24 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
}
memset(swap_map, 0, maxpages);
nr_good_pages = maxpages - 1; /* omit header page */
for (i = 0; i < swap_header->info.nr_badpages; i++) {
int page_nr = swap_header->info.badpages[i];
if (page_nr <= 0 || page_nr >= swap_header->info.last_page) {
unsigned int page_nr = swap_header->info.badpages[i];
if (page_nr == 0 || page_nr > swap_header->info.last_page) {
error = -EINVAL;
goto bad_swap;
}
swap_map[page_nr] = SWAP_MAP_BAD;
if (page_nr < maxpages) {
swap_map[page_nr] = SWAP_MAP_BAD;
nr_good_pages--;
}
}
error = swap_cgroup_swapon(type, maxpages);
if (error)
goto bad_swap;
nr_good_pages = swap_header->info.last_page -
swap_header->info.nr_badpages -
1 /* header page */;
if (nr_good_pages) {
swap_map[0] = SWAP_MAP_BAD;
p->max = maxpages;