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

[Waiman Long]

On 2/3/22 10:46, Vlastimil Babka wrote:
> 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.

Yes, I am aware of that. Patch 1 is just a micro-optimization for the 
very rare case.

Cheers,
Longman