[patch] Broken CLEAR_BITMAP() macro

[patch] Broken CLEAR_BITMAP() macro

[Matthew Dobson]

[-- Attachment #1: Type: text/plain, Size: 1293 bytes --]

Hello all,
	It appears that the CLEAR_BITMAP() macro in include/linux/types.h is 
broken.

(Examples done with BITS_PER_LONG == 32, but would work with == 64, or 
anything but 8 for that matter! :)

 >#define DECLARE_BITMAP(name,bits) \
 >	unsigned long name[((bits)+BITS_PER_LONG-1)/BITS_PER_LONG]
 >#define CLEAR_BITMAP(name,bits) \
 >	memset(name, 0, ((bits)+BITS_PER_LONG-1)/8)

If, for example, we DECLARE_BITMAP(foo, 64), we're going to get:
	unsigned long foo[((64)+32-1)/32] =>
	unsigned long foo[95/32] =>
	unsigned long foo[2]

Now, that's all well and good (and correct! ;)  But, look at what 
happens if we do a CLEAR_BITMAP(foo, 64):
	memset(foo, 0, ((64)+32-1)/8) =>
	memset(foo, 0, 95/8) =>
	memset(foo, 0, 11)

So the memset is going to try and clear 11 bytes starting at foo, which 
will overflow the foo array by 3 bytes.  This is bad.

What CLEAR_BITMAP wants to be doing is this:
  #define CLEAR_BITMAP(name,bits) \
  	memset(name, 0, 
(((bits)+BITS_PER_LONG-1)/BITS_PER_LONG)*sizeof(unsigned long))

Attatched is a patch that creates a macro BITS_TO_LONGS that just rounds 
a number of bits up to the closest number of unsigned longs.  This makes 
the DECLARE & CLEAR _BITMAP macros more readable.  I also modify the 
CLEAR_BITMAP macro to work correctly.

Cheers!

-Matt

[-- Attachment #2: clear_bitmap_fix-2.5.59.patch --]
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diff -Nur --exclude-from=/usr/src/.dontdiff linux-2.5.59-vanilla/include/linux/types.h linux-2.5.59-bitmap_fix/include/linux/types.h
--- linux-2.5.59-vanilla/include/linux/types.h	Thu Jan 16 18:22:41 2003
+++ linux-2.5.59-bitmap_fix/include/linux/types.h	Thu Feb  6 13:51:23 2003
@@ -4,10 +4,12 @@
 #ifdef	__KERNEL__
 #include <linux/config.h>
 
+#define BITS_TO_LONGS(bits) \
+	(((bits)+BITS_PER_LONG-1)/BITS_PER_LONG)
 #define DECLARE_BITMAP(name,bits) \
-	unsigned long name[((bits)+BITS_PER_LONG-1)/BITS_PER_LONG]
+	unsigned long name[BITS_TO_LONGS(bits)]
 #define CLEAR_BITMAP(name,bits) \
-	memset(name, 0, ((bits)+BITS_PER_LONG-1)/8)
+	memset(name, 0, BITS_TO_LONGS(bits)*sizeof(unsigned long))
 #endif
 
 #include <linux/posix_types.h>