On Thu 2022-02-03 13:49:02, Waiman Long wrote:

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

quoted

On 2/2/22 21:30, Waiman Long wrote:

quoted

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@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>

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.

In theory, we might micro-optimize also the case when "size == 1".

Well, I am not sure if it is worth it. After all, the primary use-case
is to print the message into a big-enough buffer. Lost information is
a bigger problem than the speed ;-)

Best Regards,
Petr