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Subject: Re: [PATCH v2 1/3] lib/vsprintf: Avoid redundant work with 0 size
To:     Waiman Long <longman@redhat.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        David Rientjes <rientjes@google.com>
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        Rafael Aquini <aquini@redhat.com>
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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
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Date:   Tue, 1 Feb 2022 08:12:41 +0100
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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