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BUG/MEDIUM: sample: fix random number upper-bound

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random() will generate a number between 0 and RAND_MAX. POSIX mandates
RAND_MAX to be at least 32767. GNU libc uses (1<<31 - 1) as
RAND_MAX.

In smp_fetch_rand(), a reduction is done with a multiply and shift to
avoid skewing the results. However, the shift was always 32 and hence
the numbers were not distributed uniformly in the specified range. We
fix that by dividing by RAND_MAX+1. gcc is smart enough to turn that
into a shift:

    0x000000000046ecc8 <+40>:    shr    $0x1f,%rax
This commit is contained in:
Vincent Bernat 2014-12-10 10:31:37 +01:00 committed by Willy Tarreau
parent f2dd68d0e0
commit 1228dc0e7a
1 changed files with 1 additions and 1 deletions
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src/sample.c
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@ -1824,7 +1824,7 @@ smp_fetch_rand(struct proxy *px, struct session *s, void *l7, unsigned int opt,
/* reduce if needed. Don't do a modulo, use all bits! */ /* reduce if needed. Don't do a modulo, use all bits! */
if (args && args[0].type == ARGT_UINT) if (args && args[0].type == ARGT_UINT)
smp->data.uint = ((uint64_t)smp->data.uint * args[0].data.uint) >> 32; smp->data.uint = ((uint64_t)smp->data.uint * args[0].data.uint) / ((u64)RAND_MAX+1);
smp->type = SMP_T_UINT; smp->type = SMP_T_UINT;
smp->flags |= SMP_F_VOL_TEST | SMP_F_MAY_CHANGE; smp->flags |= SMP_F_VOL_TEST | SMP_F_MAY_CHANGE;