Re: [PATCH] mm: add stack trace when bad rss-counter state is detected

[Kees Cook <kees@kernel.org>]

On Wed, Jul 23, 2025 at 03:23:49PM +0800, Xuanye Liu wrote:
> The check_mm() function verifies the correctness of rss counters in
> struct mm_struct. Currently, it only prints an alert when a bad
> rss-counter state is detected, but lacks sufficient context for
> debugging.
> 
> This patch adds a dump_stack() call to provide a stack trace when
> the rss-counter state is invalid. This helps developers identify
> where the corrupted mm_struct is being checked and trace the
> underlying cause of the inconsistency.

Why not just convert the pr_alert to a WARN?

-Kees

> 
> Signed-off-by: Xuanye Liu <liuqiye2025@163.com>
> ---
>  kernel/fork.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index cfe2f1df5f27..d38f1c5270ea 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -584,9 +584,11 @@ static void check_mm(struct mm_struct *mm)
>  	for (i = 0; i < NR_MM_COUNTERS; i++) {
>  		long x = percpu_counter_sum(&mm->rss_stat[i]);
>  
> -		if (unlikely(x))
> +		if (unlikely(x)) {
>  			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
>  				 mm, resident_page_types[i], x);
> +			dump_stack();
> +		}
>  	}
>  
>  	if (mm_pgtables_bytes(mm))
> -- 
> 2.43.0
> 

-- 
Kees Cook