Double swapon on a device causes a crash:
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
PGD 1dc0b067 PUD 1dc09067 PMD 0
Oops: 0000 [#1] SMP
last sysfs file:
CPU 1
Modules linked in:
Pid: 562, comm: swapon Tainted: G W 2.6.32-rc5-mm1_64 #867
RIP: 0010:[<ffffffff810af160>] [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
...
It is due to swap_info_struct->first_swap_extent.list not being
initialized. ->next is NULL in such a situation and
destroy_swap_extents fails to iterate over the list with the BUG
above.
Introduced by swap_info-include-first_swap_extent.patch. Revert the
INIT_LIST_HEAD move.
Signed-off-by: Jiri Slaby <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Rik van Riel <[email protected]>
---
mm/swapfile.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 93e71cf..26ef6a2 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1313,7 +1313,6 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
if (start_page == 0) {
se = &sis->first_swap_extent;
sis->curr_swap_extent = se;
- INIT_LIST_HEAD(&se->list);
se->start_page = 0;
se->nr_pages = nr_pages;
se->start_block = start_block;
@@ -1769,6 +1768,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
kfree(p);
goto out;
}
+ INIT_LIST_HEAD(&p->first_swap_extent.list);
if (type >= nr_swapfiles) {
p->type = type;
swap_info[type] = p;
--
1.6.4.2
On Mon, 2 Nov 2009, Jiri Slaby wrote:
> Double swapon on a device causes a crash:
> BUG: unable to handle kernel NULL pointer dereference at (null)
Thanks a lot for finding that: it doesn't just happen with a double
swapon of the same device, it happens with most kinds of error in
the swapon sequence. I thought I was being nice and tidy moving
that initialization, but actually I was just being careless.
> IP: [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
> PGD 1dc0b067 PUD 1dc09067 PMD 0
> Oops: 0000 [#1] SMP
> last sysfs file:
> CPU 1
> Modules linked in:
> Pid: 562, comm: swapon Tainted: G W 2.6.32-rc5-mm1_64 #867
> RIP: 0010:[<ffffffff810af160>] [<ffffffff810af160>] sys_swapon+0x1f0/0xc60
> ...
>
> It is due to swap_info_struct->first_swap_extent.list not being
> initialized. ->next is NULL in such a situation and
> destroy_swap_extents fails to iterate over the list with the BUG
> above.
>
> Introduced by swap_info-include-first_swap_extent.patch. Revert the
> INIT_LIST_HEAD move.
>
> Signed-off-by: Jiri Slaby <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: KAMEZAWA Hiroyuki <[email protected]>
> Cc: Rik van Riel <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
> ---
> mm/swapfile.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 93e71cf..26ef6a2 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1313,7 +1313,6 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
> if (start_page == 0) {
> se = &sis->first_swap_extent;
> sis->curr_swap_extent = se;
> - INIT_LIST_HEAD(&se->list);
> se->start_page = 0;
> se->nr_pages = nr_pages;
> se->start_block = start_block;
> @@ -1769,6 +1768,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> kfree(p);
> goto out;
> }
> + INIT_LIST_HEAD(&p->first_swap_extent.list);
> if (type >= nr_swapfiles) {
> p->type = type;
> swap_info[type] = p;
> --
> 1.6.4.2
Sorry, just noticed what the diff contexts don't show: Jiri's patch
is initializing p->first_swap_extent.list at a point before p has
been decided - we may kfree that newly allocated p and go on to
reuse an existing free entry for p.
Now, the patch is not actually wrong: an existing free entry will have
a good empty first_swap_extent.list; but it looks suspicious, it seems
strange to initialize a field in something we're about to kfree, and
I'd rather we put that initialization back to where it was in 2.6.32.
Signed-off-by: Hugh Dickins <[email protected]>
---
mm/swapfile.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- mmotm/mm/swapfile.c 2009-11-03 18:05:36.000000000 +0000
+++ linux/mm/swapfile.c 2009-11-03 18:08:26.000000000 +0000
@@ -1768,7 +1768,6 @@ SYSCALL_DEFINE2(swapon, const char __use
kfree(p);
goto out;
}
- INIT_LIST_HEAD(&p->first_swap_extent.list);
if (type >= nr_swapfiles) {
p->type = type;
swap_info[type] = p;
@@ -1787,6 +1786,7 @@ SYSCALL_DEFINE2(swapon, const char __use
* would be relying on p->type to remain valid.
*/
}
+ INIT_LIST_HEAD(&p->first_swap_extent.list);
p->flags = SWP_USED;
p->next = -1;
spin_unlock(&swap_lock);