linux-kernel - Re: [PATCH 07/14] lib/vsprintf.c: slightly refactor vscnprintf()

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Date:	Thu, 26 Nov 2015 22:23:45 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/14] lib/vsprintf.c: slightly refactor vscnprintf()

On Mon, Nov 23 2015, Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> On Mon, Nov 23, 2015 at 11:29 PM, Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
>> If we're given a size of 0, the vsnprintf() won't have any side
>> effects, and neither "i < size" or "size != 0" will trigger. So we
>> might as well return 0 immediately.
>>
>> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
>> ---
>>  lib/vsprintf.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 8af5535fd738..e22a6189548f 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -2036,13 +2036,14 @@ int vscnprintf(char *buf, size_t size, const char *fmt, va_list args)
>>  {
>>         int i;
>>
>> +       if (unlikely(!size))
>> +               return 0;
>> +
>
> Might it potentially shadow any issue when run vsnprintf(buf, 0, fmt,
> args); with certain arguments?

Only if we ever come up with a %p extension with side effects, but then
people couldn't rely on them happening exactly once anyway (kasprintf
would make them happen twice). printf-like calls are also often compiled
out or disabled (dyndebug, ratelimit, ...) without it being obvious at
the call site whether they'll run or not, so I think such a hypothetical
%p extension would meet some resistance.

> I can imagine something like %pV with unstable pointer.

I don't see how %pV is different than any other current %p
extensions. 

Rasmus
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