Re: [PATCH v1] mm: fix div by zero in bdi_ratio_from_pages

[David Hildenbrand <david@redhat.com>]

On 04.01.25 02:20, 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_ration.
> 
> 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>
> ---
>   mm/page-writeback.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index d213ead95675..91aa7a5c0078 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;

bdi_set_min_bytes() calls bdi_ratio_from_pages() and passes the result 
to __bdi_set_min_ratio().

__bdi_set_min_ratio() expects an "unsigned int min_ratio". I assume this 
will work because "max_ratio > 100 * BDI_RATIO_SCALE", but it is rather 
confusing ...

Maybe we want something like:

/* Use 101% to indicate "invalid" */
#define BDI_RATIO_INVALID (101 * BDI_RATIO_SCALE)

Or alternatively, just handle it in the callers of 
bdi_ratio_from_pages(), checking for -EINVAL manually.

-- 
Cheers,

David / dhildenb