On Sat, 11 Jul 2020 01:17:28 -0400 Changming <charley.ashbrin...@gmail.com>
wrote:
> From: Changming Liu <charley.ashbrin...@gmail.com>
>
> Since panic_timeout is an integer passed-in through sysctl,
> the loop boundary panic_timeout * 1000 could overflow and
> result in a zero-delay panic when panic_timeout is greater
> than INT_MAX/1000.
>
> Fix this by moving 1000 to the left, also in case i/1000
> might never be greater than panic_timeout, change i to
> long long so that it strictly has more bits.
>
> ...
>
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -178,7 +178,8 @@ void panic(const char *fmt, ...)
> {
> static char buf[1024];
> va_list args;
> - long i, i_next = 0, len;
> + long long i;
> + long i_next = 0, len;
> int state = 0;
> int old_cpu, this_cpu;
> bool _crash_kexec_post_notifiers = crash_kexec_post_notifiers;
> @@ -315,7 +316,7 @@ void panic(const char *fmt, ...)
> */
> pr_emerg("Rebooting in %d seconds..\n", panic_timeout);
>
> - for (i = 0; i < panic_timeout * 1000; i += PANIC_TIMER_STEP) {
> + for (i = 0; i / 1000 < panic_timeout; i += PANIC_TIMER_STEP) {
Problem is, 32-bit machines generally cannot perform 64-bit divides.
So a call is emitted to the library function __divsi64() (I forget the exact
name) which Linux doesn't implement (because it's so slow, and we don't
want to be calling it by accident).
So a fix would be to call do_div() or something from
include/linux/div64.h but it's all a great mess.
However we can do native 64-bit multiplication on 32-bit! So how about
something like
--- a/kernel/panic.c~a
+++ a/kernel/panic.c
@@ -313,13 +313,16 @@ void panic(const char *fmt, ...)
* Delay timeout seconds before rebooting the machine.
* We can't use the "normal" timers since we just panicked.
*/
+ u64 timeout = panic_timeout * 1000; /* avoid overflow */
+ u64 timer;
+
pr_emerg("Rebooting in %d seconds..\n", panic_timeout);
- for (i = 0; i < panic_timeout * 1000; i += PANIC_TIMER_STEP) {
+ for (timer = 0; timer < timeout; timer += PANIC_TIMER_STEP) {
touch_nmi_watchdog();
- if (i >= i_next) {
- i += panic_blink(state ^= 1);
- i_next = i + 3600 / PANIC_BLINK_SPD;
+ if (timer >= i_next) {
+ timer += panic_blink(state ^= 1);
+ i_next = timer + 3600 / PANIC_BLINK_SPD;
}
mdelay(PANIC_TIMER_STEP);
}
(untested)
There's still the 3600/PANIC_BLINK_SPD in there, but a) that will be
done at compile-time and b) the 64-bit promotion should be done after
the division.
And... oh crap, i_next needs to be 64-bit as well.
