Re: [PATCH] lib: Fix bug in int_sqrt() for 64 bit longs

From: Peter Williams
Date: Fri Jan 06 2006 - 18:19:56 EST


Eric Dumazet wrote:
Peter Williams a écrit :

The implementation of int_sqrt() assumes that longs have 32 bits. On systems that have 64 bit longs this will result in gross errors when the argument to the function is greater than 2^32 - 1 on such systems. I doubt whether any such use is currently made of int_sqrt() but the attached patch fixes the problem anyway.

Signed-off-by: Peter Williams <pwil3058@xxxxxxxxxxxxxx>


------------------------------------------------------------------------

Index: GIT-warnings/lib/int_sqrt.c
===================================================================
--- GIT-warnings.orig/lib/int_sqrt.c 2005-10-25 13:55:22.000000000 +1000
+++ GIT-warnings/lib/int_sqrt.c 2006-01-06 14:29:19.000000000 +1100
@@ -15,7 +15,7 @@ unsigned long int_sqrt(unsigned long x)
op = x;
res = 0;
- one = 1 << 30;
+ one = 1 << (BITS_PER_LONG - 2);
while (one > op)
one >>= 2;


Are you sure it works ?

I would have writen :

one = 1L << (BITS_PER_LONG - 2);

I think you're right as just using 1 would make the expression an integer variable and the compiler would complain about shifting too far unless integers were the same size as longs. (I don't have a 64 bit system to confirm that on which is why it snuck by me. :-()

Thanks
Peter
--
Peter Williams pwil3058@xxxxxxxxxxxxxx

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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