linux-kernel - Re: [PATCH 3/4] lib: vsprintf: Optimize put_dec

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Date:	24 Sep 2012 10:33:05 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	akpm@...ux-foundation.org, linux@...izon.com, rabin@....in
Cc:	hughd@...gle.com, linux-kernel@...r.kernel.org, mina86@...a86.com,
	vda.linux@...glemail.com
Subject: Re: [PATCH 3/4] lib: vsprintf: Optimize put_dec_trunc8

Rabin Vincent <rabin@....in> wrote:
> This patch breaks IP address printing with "%pI4" (and by extension,
> nfsroot).  Example:
>
>  - Before: 10.0.0.1
>  - After: 10...1

Mea culpa, and thank you for catching it!  As I said in my earlier
comment, I tested this most extensively wrapped by some sprintf code
that liked 0 converted to a 0-length string, as that works naturally
with the ANSI spec for %.0u.  And it turns out not to matter for the
usual printf code, as num_to_str special-cases that anyway.

The fix is straightforward:

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index e755083..9872855 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -180,8 +180,6 @@ char *put_dec_trunc8(char *buf, unsigned r)
 		*buf++ = q - 10*r;
 	}

-	if (r == 0)
-		return buf;
 	q      = (r * 0x199a) >> 16;	/* r <= 9999 */
 	*buf++ = (r - 10 * q)  + '0';
 	if (q == 0)

Inspired by Michal Nazarewicz, I have some ideas for more tweaking to
that code.

AKPM: How should I submit this to you?  Would you like it as a fixup
patch, or would you like a revised patch from baseline?  You're free to
do either manually and add my Signed-off-by: to the result if you want
the fix faster.

I'm also working on addressing Denys Vlasenko's comments, but I figure
the bugfix is more urgent.
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