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

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Message-ID: <YgJLPpUqkTaA298Y@alley>
Date:   Tue, 8 Feb 2022 11:51:42 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <guro@...com>, Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v4 2/4] mm/page_owner: Use scnprintf() to avoid excessive
 buffer overrun check

On Thu 2022-02-03 13:49:02, Waiman Long wrote:
> 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@...hat.com>
> > > Acked-by: David Rientjes <rientjes@...gle.com>
> > > Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.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