linux-kernel - [PATCH v2] bitmap: remove explicit newline handling using scnprintf format string

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Date:	Tue, 28 Apr 2015 16:36:41 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>
Cc:	Sudeep Holla <sudeep.holla@....com>,
	Pawel Moll <Pawel.Moll@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: [PATCH v2] bitmap: remove explicit newline handling using scnprintf format string

bitmap_print_to_pagebuf uses scnprintf to copy the cpumask/list to page
buffer. It handles the newline and trailing null character explicitly.

It's unnecessary and also partially duplicated as scnprintf already adds
trailing null character. The newline can be passed through format string
to scnprintf. This patch does that simplification.

However theoritically there's one behavior difference: when the buffer
is too small, the original code would still output '\n' at the end while
the new code(with this patch) would just continue to print the formatted
string. Since this function is dealing with only page buffers, it's
highly unlikely to hit that corner case.

This patch will help in auditing the users of bitmap_print_to_pagebuf
to verify that the buffer passed is large enough and get rid of it
completely by replacing them with direct scnprintf()

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Tejun Heo <tj@...nel.org>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Suggested-by: Pawel Moll <Pawel.Moll@....com>
Signed-off-by: Sudeep Holla <sudeep.holla@....com>
---
 lib/bitmap.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

v1->v2:
 - Updated the doxygen comment and the commit log as suggested by
   Tejun.

diff --git a/lib/bitmap.c b/lib/bitmap.c
index 64c0926f5dd8..10088473e3ba 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -462,19 +462,18 @@ EXPORT_SYMBOL(bitmap_parse_user);
  * Output format is a comma-separated list of decimal numbers and
  * ranges if list is specified or hex digits grouped into comma-separated
  * sets of 8 digits/set. Returns the number of characters written to buf.
+ * It is assumed that the buffer passed is atleast PAGE_SIZE and easily
+ * accommodate nmaskbits.
  */
 int bitmap_print_to_pagebuf(bool list, char *buf, const unsigned long *maskp,
 			    int nmaskbits)
 {
-	ptrdiff_t len = PTR_ALIGN(buf + PAGE_SIZE - 1, PAGE_SIZE) - buf - 2;
+	ptrdiff_t len = PTR_ALIGN(buf + PAGE_SIZE - 1, PAGE_SIZE) - buf;
 	int n = 0;

-	if (len > 1) {
-		n = list ? scnprintf(buf, len, "%*pbl", nmaskbits, maskp) :
-			   scnprintf(buf, len, "%*pb", nmaskbits, maskp);
-		buf[n++] = '\n';
-		buf[n] = '\0';
-	}
+	if (len > 1)
+		n = list ? scnprintf(buf, len, "%*pbl\n", nmaskbits, maskp) :
+			   scnprintf(buf, len, "%*pb\n", nmaskbits, maskp);
 	return n;
 }
 EXPORT_SYMBOL(bitmap_print_to_pagebuf);
--
1.9.1

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