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Don't compute ((uint64_t)1)<<64 in round_to_power_of_2

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This would be undefined behavior if it happened. (It can't actually
happen as we're using round_to_power_of_2, since we would have to
be trying to allocate exabytes of data.)

While we're at it, fix the behavior of round_to_power_of_2(0),
and document the function better.

Fix for bug 6831.
This commit is contained in:
Nick Mathewson 2012-09-13 10:34:49 -04:00
parent 582f2187a7
commit 37953497d8
3 changed files with 31 additions and 4 deletions
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4
changes/bug6831 Normal file
View File

@ -0,0 +1,4 @@
o Minor bugfixes:
- Fix round_to_power_of_2 so it doesn't invoke undefined behavior
with large values. This was untriggered, but nevertheless incorrect.
Fixes bug 6831; bugfix on 0.2.0.1-alpha.

18
src/common/util.c
View File

@ -394,12 +394,24 @@ tor_log2(uint64_t u64)
return r;
}
/** Return the power of 2 closest to <b>u64</b>. */
/** Return the power of 2 in range [1,UINT64_MAX] closest to <b>u64</b>. If
* there are two powers of 2 equally close, round down. */
uint64_t
round_to_power_of_2(uint64_t u64)
{
int lg2 = tor_log2(u64);
uint64_t low = U64_LITERAL(1) << lg2, high = U64_LITERAL(1) << (lg2+1);
int lg2;
uint64_t low;
uint64_t high;
if (u64 == 0)
return 1;
lg2 = tor_log2(u64);
low = U64_LITERAL(1) << lg2;
if (lg2 == 63)
return low;
high = U64_LITERAL(1) << (lg2+1);
if (high - u64 < u64 - low)
return high;
else

13
src/test/test_util.c
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@ -1109,6 +1109,7 @@ test_util_pow2(void)
test_eq(tor_log2(64), 6);
test_eq(tor_log2(65), 6);
test_eq(tor_log2(63), 5);
test_eq(tor_log2(0), 0); /* incorrect mathematically, but as specified */
test_eq(tor_log2(1), 0);
test_eq(tor_log2(2), 1);
test_eq(tor_log2(3), 1);
@ -1123,7 +1124,17 @@ test_util_pow2(void)
test_eq(round_to_power_of_2(130), 128);
test_eq(round_to_power_of_2(U64_LITERAL(40000000000000000)),
U64_LITERAL(1)<<55);
test_eq(round_to_power_of_2(0), 2);
test_eq(round_to_power_of_2(U64_LITERAL(0xffffffffffffffff)),
U64_LITERAL(1)<<63);
test_eq(round_to_power_of_2(0), 1);
test_eq(round_to_power_of_2(1), 1);
test_eq(round_to_power_of_2(2), 2);
test_eq(round_to_power_of_2(3), 2);
test_eq(round_to_power_of_2(4), 4);
test_eq(round_to_power_of_2(4), 4);
test_eq(round_to_power_of_2(5), 4);
test_eq(round_to_power_of_2(6), 4);
test_eq(round_to_power_of_2(7), 8);
done:
;