Bug in math.random() range checking
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- Subject: Bug in math.random() range checking
- From: Bruce Hill <bruce@...>
- Date: 2018年2月28日 19:32:47 -0800
I was doing some testing that involved generating very large random
integers and I noticed a bug in Lua 5.3.4's math.random(). If you pass
in a range larger than 2^31-1 (L_RANDMAX), math.random() will generate
random integers that only have 31 bits of randomness. For example, this
code will run without errors:
for _=1,1000000 do
assert(math.random(0, 0x7fffffffffffffff) & 0xffffffff == 0)
end
print("The lowest 32 bits were always zero!")
The fix for this is pretty simple:
** $Id: lmathlib.c,v 1.119 2016年12月22日 13:08:50 roberto Exp $
269,270c269
< luaL_argcheck(L, low >= 0 || up <= LUA_MAXINTEGER + low, 1,
< "interval too large");
---
luaL_argcheck(L, up - low + 1 <= L_RANDMAX, 1, "interval too large");
which makes math.random(0, 0x7fffffffffffffff) produce an
interval-too-large error. Also, the documentation would need to be
updated to reflect this constraint (it currently just says: "The value
n-m cannot be negative and must fit in a Lua integer.").
Or, alternatively, the C code could be changed to use multiple calls to
l_random() to produce fully random integers from larger ranges, which
would introduce some additional complexity, but might be a good idea
anyway.