Re: [PATCH v2] mm: fix div by zero in bdi_ratio_from_pages

[David Hildenbrand <david@redhat.com>]

On 08.01.25 02:47, Stefan Roesch wrote:
> During testing it has been detected, that it is possible to get div by
> zero error in bdi_set_min_bytes. The error is caused by the function
> bdi_ratio_from_pages(). bdi_ratio_from_pages() calls
> global_dirty_limits. If the dirty threshold is 0, the div by zero is
> raised. This can happen if the root user is setting:
> 
> echo 0 > /proc/sys/vm/dirty_ratio
> 
> The following is a test case:
> 
> echo 0 > /proc/sys/vm/dirty_ratio
> cd /sys/class/bdi/<device>
> echo 1 > strict_limit
> echo 8192 > min_bytes
> 
> ==> error is raised.
> 
> The problem is addressed by returning -EINVAL if dirty_ratio or
> dirty_bytes is set to 0.
> 
> Reported-by: cheung wall <zzqq0103.hey@gmail.com>
> Closes: https://lore.kernel.org/linux-mm/87pll35yd0.fsf@devkernel.io/T/#t
> Signed-off-by: Stefan Roesch <shr@devkernel.io>
> 
> ---
> Changes in V2:
> - check for -EINVAL in bdi_set_min_bytes()
> - check for -EINVAL in bdi_set_max_bytes()
> ---
>   mm/page-writeback.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index d213ead95675..fcc486e0d5c2 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -692,6 +692,8 @@ static unsigned long bdi_ratio_from_pages(unsigned long pages)
>   	unsigned long ratio;
>   
>   	global_dirty_limits(&background_thresh, &dirty_thresh);
> +	if (!dirty_thresh)
> +		return -EINVAL;
>   	ratio = div64_u64(pages * 100ULL * BDI_RATIO_SCALE, dirty_thresh);
>   
>   	return ratio;
> @@ -797,6 +799,8 @@ int bdi_set_min_bytes(struct backing_dev_info *bdi, u64 min_bytes)
>   		return ret;
>   
>   	min_ratio = bdi_ratio_from_pages(pages);
> +	if (min_ratio == -EINVAL)
> +		return -EINVAL;
>   	return __bdi_set_min_ratio(bdi, min_ratio);
>   }
>   
> @@ -816,6 +820,8 @@ int bdi_set_max_bytes(struct backing_dev_info *bdi, u64 max_bytes)
>   		return ret;
>   
>   	max_ratio = bdi_ratio_from_pages(pages);
> +	if (max_ratio == -EINVAL)
> +		return -EINVAL;

I would have done it slightly differently, something like:


diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index d213ead956750..4b02f18f7d01f 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -685,13 +685,15 @@ static int bdi_check_pages_limit(unsigned long pages)
         return 0;
  }
  
-static unsigned long bdi_ratio_from_pages(unsigned long pages)
+static long bdi_ratio_from_pages(unsigned long pages)
  {
         unsigned long background_thresh;
         unsigned long dirty_thresh;
         unsigned long ratio;
  
         global_dirty_limits(&background_thresh, &dirty_thresh);
+       if (!dirty_thresh)
+               return -EINVAL;
         ratio = div64_u64(pages * 100ULL * BDI_RATIO_SCALE, dirty_thresh);
  
         return ratio;
@@ -790,13 +792,15 @@ int bdi_set_min_bytes(struct backing_dev_info *bdi, u64 min_bytes)
  {
         int ret;
         unsigned long pages = min_bytes >> PAGE_SHIFT;
-       unsigned long min_ratio;
+       long min_ratio;
  
         ret = bdi_check_pages_limit(pages);
         if (ret)
                 return ret;
  
         min_ratio = bdi_ratio_from_pages(pages);
+       if (min_ratio < 0)
+               return min_ratio;
         return __bdi_set_min_ratio(bdi, min_ratio);
  }
  
@@ -809,13 +813,15 @@ int bdi_set_max_bytes(struct backing_dev_info *bdi, u64 max_bytes)
  {
         int ret;
         unsigned long pages = max_bytes >> PAGE_SHIFT;
-       unsigned long max_ratio;
+       long max_ratio;
  
         ret = bdi_check_pages_limit(pages);
         if (ret)
                 return ret;
  
         max_ratio = bdi_ratio_from_pages(pages);
+       if (min_ratio < 0)
+               return min_ratio;
         return __bdi_set_max_ratio(bdi, max_ratio);
  }
  


-- 
Cheers,

David / dhildenb