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Use a time-invariant comparison in choose_array_element_by_weight

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This commit is contained in:
Nick Mathewson 2012-08-28 12:42:21 -04:00
parent 5c3199cda7
commit 9982122f34
1 changed files with 19 additions and 3 deletions
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22
src/or/routerlist.c
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@ -1681,6 +1681,20 @@ scale_array_elements_to_u64(u64_dbl_t *entries, int n_entries,
#undef SCALE_TO_U64_MAX
}
/** Time-invariant 64-bit greater-than; works on two integers in the range
* (0,INT64_MAX). */
#if SIZEOF_VOID_P == 8
#define gt_i64_timei(a,b) ((a) > (b))
#else
static INLINE int
gt_i64_timei(uint64_t a, uint64_t b)
{
int64_t diff = (int64_t) (b - a);
int res = diff >> 63;
return res & 1;
}
#endif
/** Pick a random element of <b>n_entries</b>-element array <b>entries</b>,
* choosing each element with a probability proportional to its (uint64_t)
* value, and return the index of that element. If all elements are 0, choose
@ -1703,16 +1717,18 @@ choose_array_element_by_weight(const u64_dbl_t *entries, int n_entries)
if (total == 0)
return crypto_rand_int(n_entries);
tor_assert(total < INT64_MAX);
rand_val = crypto_rand_uint64(total);
for (i = 0; i < n_entries; ++i) {
total_so_far += entries[i].u64;
if (total_so_far > rand_val) {
if (gt_i64_timei(total_so_far, rand_val)) {
i_chosen = i;
n_chosen++;
/* Set rand_val to UINT_MAX rather than stopping the loop. This way,
/* Set rand_val to INT64_MAX rather than stopping the loop. This way,
* the time we spend in the loop does not leak which element we chose. */
rand_val = UINT64_MAX;
rand_val = INT64_MAX;
}
}
tor_assert(total_so_far == total);