Re: [PATCH v4 2/4] mm/page_owner: Use scnprintf() to avoid excessive buffer overrun check

[Vlastimil Babka]

On 2/2/22 21:30, Waiman Long wrote:
> The snprintf() function can return a length greater than the given
> input size. That will require a check for buffer overrun after each
> invocation of snprintf(). scnprintf(), on the other hand, will never
> return a greater length. By using scnprintf() in selected places, we
> can avoid some buffer overrun checks except after stack_depot_snprint()
> and after the last snprintf().
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>

Looks like this will work, but note that if the purpose of patch 1/4 was
that after the first scnprintf() that overflows the following calls will be
short-cut thanks to passing the size as 0, AFAICS that won't work. Because
scnprintf() returns the number without trailing zero, 'ret' will be 'count -
1' after the overflow, so 'count - ret' will be 1, never 0.

> ---
>  mm/page_owner.c | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 99e360df9465..28dac73e0542 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -338,19 +338,16 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
>  	if (!kbuf)
>  		return -ENOMEM;
>  
> -	ret = snprintf(kbuf, count,
> +	ret = scnprintf(kbuf, count,
>  			"Page allocated via order %u, mask %#x(%pGg), pid %d, ts %llu ns, free_ts %llu ns\n",
>  			page_owner->order, page_owner->gfp_mask,
>  			&page_owner->gfp_mask, page_owner->pid,
>  			page_owner->ts_nsec, page_owner->free_ts_nsec);
>  
> -	if (ret >= count)
> -		goto err;
> -
>  	/* Print information relevant to grouping pages by mobility */
>  	pageblock_mt = get_pageblock_migratetype(page);
>  	page_mt  = gfp_migratetype(page_owner->gfp_mask);
> -	ret += snprintf(kbuf + ret, count - ret,
> +	ret += scnprintf(kbuf + ret, count - ret,
>  			"PFN %lu type %s Block %lu type %s Flags %pGp\n",
>  			pfn,
>  			migratetype_names[page_mt],
> @@ -358,19 +355,14 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
>  			migratetype_names[pageblock_mt],
>  			&page->flags);
>  
> -	if (ret >= count)
> -		goto err;
> -
>  	ret += stack_depot_snprint(handle, kbuf + ret, count - ret, 0);
>  	if (ret >= count)
>  		goto err;
>  
>  	if (page_owner->last_migrate_reason != -1) {
> -		ret += snprintf(kbuf + ret, count - ret,
> +		ret += scnprintf(kbuf + ret, count - ret,
>  			"Page has been migrated, last migrate reason: %s\n",
>  			migrate_reason_names[page_owner->last_migrate_reason]);
> -		if (ret >= count)
> -			goto err;
>  	}
>  
>  	ret += snprintf(kbuf + ret, count - ret, "\n");