Re: [PATCH] zram: fix idle age_sec underflow in idle_store()

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (26/08/28 13:23), Sergey Senozhatsky wrote:
> Can we instead just reject such cases?   It should be an error to
> request writeback of pages that are 2y old on a system that has
> uptime of 2d.
> 
> Maybe something like this:
> 
> +             time64_t uptime;
> 
> -             if (IS_ENABLED(CONFIG_ZRAM_TRACK_ENTRY_ACTIME) &&
> -                 !kstrtouint(buf, 0, &age_sec))
> -                     cutoff = ktime_sub((u32)ktime_get_boottime_seconds(),
> -                                        age_sec);
> -             else
> +             if (!IS_ENABLED(CONFIG_ZRAM_TRACK_ENTRY_ACTIME) ||
> +                 kstrtouint(buf, 0, &age_sec))
>                       return -EINVAL;
> +
> +             /* No slot can be older than the system uptime */
> +             uptime = ktime_get_boottime_seconds();
> +             if (age_sec >= uptime)
> +                     return -ERANGE;
> +
> +             cutoff = uptime - age_sec;
>       }
> 
> We are probably fine doing just basic arithmetics operations on those two.
> One is u32 the other one is s64: we should be fine just doing >= and sub.
> I guess we also can use plain comparison
> 	cutoff > zram->table[index].attr.ac_time
> in mark_idle().  Or am I hallucinating?

That implies that cutoff becomes time64_t both in idle_store() and in
mark_idle().