Messages in this thread | | | Date | Tue, 11 Oct 2022 15:21:44 +0200 | Subject | Re: [crng-random:jd/get_random_u32_below 24/26] include/linux/random.h:64:77: sparse: sparse: cast truncates bits from constant value (e24d4d50 becomes 50) | From | Rasmus Villemoes <> |
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On 11/10/2022 11.59, kernel test robot wrote:
> c440408cf6901e Jason A. Donenfeld 2017-01-22 53 > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 54 u32 __get_random_u32_below(u32 ceil); > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 55 /* Returns a random integer in the interval [0, ceil), with uniform distribution. */ > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 56 static inline u32 get_random_u32_below(u32 ceil) > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 57 { > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 58 if (!__builtin_constant_p(ceil)) > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 59 return __get_random_u32_below(ceil); > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 60 > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 61 for (;;) { > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 62 if (ceil <= 1U << 8) { > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 63 u32 mult = ceil * get_random_u8(); > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 @64 if (is_power_of_2(ceil) || (u8)mult >= -(__force u8)ceil % ceil) > dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 65 return mult >> 8;
I don't have a good suggestion for how to silence sparse, but I think the cast and unary minus here needs to be interchanged. I.e., the condition should be
if (is_power_of_2(ceil) || (u8)mult >= ((__force u8)-ceil) % ceil)
Otherwise it fails to provide uniform distribution for ceil=11, 19, 22, 23, ... [these are the numbers that are not divisors of 2^32-2^8].
Rasmus
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