hello,
e2p_percent() doesn't work for zero percent (``mke2fs -m0'' gets killed with
SIGFPE).
is there a reason for not simply using 64 bit arithmetic? perhaps to avoid
overflows for 7e+9TB file-systems :-), or just for correctness...
So, here is the patch that fixes this. In order to avoid integer overflows,
we pick 16 more bits to store ``base'' and do the exact division on 48 bits.
Since ``100 * base'' always fits in 48 bits, there's never overflow. This
still work if we remplace 'unsigned int' by 'unsigned long long'.
cheers,
-- Alexandre
Signed-off-by: Alexandre Ratchov <alexandre.ratchov@xxxxxxxx>
Index: e2fsprogs-1.39/lib/e2p/percent.c
===================================================================
--- e2fsprogs-1.39.orig/lib/e2p/percent.c 2006-09-20 12:07:05.000000000 +0200
+++ e2fsprogs-1.39/lib/e2p/percent.c 2006-09-20 12:33:20.000000000 +0200
@@ -14,18 +14,41 @@
/*
* We work really hard to calculate this accurately, while avoiding
* an overflow. "Is there a hyphen in anal-retentive?" :-)
+ *
+ * -- "Yes there is, as in hair-splitting and nit-picking"
*/
unsigned int e2p_percent(int percent, unsigned int base)
{
- unsigned int mask = ~((1 << (sizeof(unsigned int) - 1) * 8) - 1);
+ unsigned hi, lo, q, r;
- if (100 % percent == 0)
- return base / (100 / percent);
- if (mask & base)
- return (base / 100) * percent;
- return base * percent / 100;
+ /*
+ * in order to avoid overflow we write 'base' as:
+ *
+ * base = hi * 2^16 + lo
+ *
+ * then we do all computations separately on 'hi' and 'lo'.
+ * By using the definition of division:
+ *
+ * precent * base = result * 100 + reminder
+ *
+ * (reminder < 100), we obtain the exact value of 'result'
+ * as follows:
+ */
+#define BITS 16
+#define MASK ((1 << BITS) - 1)
+
+ hi = percent * (base >> BITS);
+ lo = percent * (base & MASK);
+
+ q = ((hi / 100) << BITS) + lo / 100;
+ r = ((hi % 100) << BITS) + lo % 100;
+
+ return q + r / 100;
+#undef BITS
+#undef MASK
}
+
#ifdef DEBUG
#include <unistd.h>
#include <stdio.h>
-
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