linux-kernel - Re: [PATCH v2 1/3] lib/vsprintf: Avoid redundant work with 0 size

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Message-ID: <f3bcf541-e77b-ca93-ef5c-862f4de99366@rasmusvillemoes.dk>
Date:   Tue, 1 Feb 2022 08:12:41 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Waiman Long <longman@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v2 1/3] lib/vsprintf: Avoid redundant work with 0 size

On 31/01/2022 19.48, Waiman Long wrote:
> On 1/31/22 05:34, Andy Shevchenko wrote:

>> Also it seems currently the kernel documentation is not aligned with
>> the code
>>
>>    "If @size is == 0 the function returns 0."
>>
>> It should mention the (theoretical?) possibility of getting negative
>> value,
>> if vsnprintf() returns negative value.
> 
> AFAICS, the kernel's vsnprintf() function will not return -1.

Even if it did, the "i < size" comparison in vscnprintf() is "int v
size_t", so integer promotion says that even if i were negative, that
comparison would be false, so we wouldn't forward that negative value
anyway.

> So in that
> sense it is not fully POSIX compliant. 

Of course it's not, but not because it doesn't return -1. POSIX just
says to return that in case of an error, and as a matter of QoI, the
kernel's implementation simply can't (and must not) fail. There are
other cases where we don't follow POSIX/C, e.g. in some corner cases
around field length and precision (documented in test_printf.c), and the
non-support of %n (and floating point and handling of wchar_t*), and the
whole %p<> extension etc.

Rasmus