linux-kernel - [crng-random:jd/get_random_u32_below 23/23] include/linux/random.h:64:69: sparse: sparse: cast truncates bits from constant value (fffffc00 becomes 0)

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Date:   Mon, 10 Oct 2022 08:54:04 +0800
From:   kernel test robot <lkp@...el.com>
To:     "Jason A. Donenfeld" <zx2c4@...nel.org>
Cc:     kbuild-all@...ts.01.org, zx2c4@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [crng-random:jd/get_random_u32_below 23/23]
 include/linux/random.h:64:69: sparse: sparse: cast truncates bits from
 constant value (fffffc00 becomes 0)

tree:   git://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git jd/get_random_u32_below
head:   7e1db96a81c31621b3dfd6247d6e6dbad2af6893
commit: 7e1db96a81c31621b3dfd6247d6e6dbad2af6893 [23/23] random: use rejection sampling for uniform bounded integers
config: i386-randconfig-s002
compiler: gcc-11 (Debian 11.3.0-5) 11.3.0
reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-39-gce1a6720-dirty
        # https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git/commit/?id=7e1db96a81c31621b3dfd6247d6e6dbad2af6893
        git remote add crng-random git://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
        git fetch --no-tags crng-random jd/get_random_u32_below
        git checkout 7e1db96a81c31621b3dfd6247d6e6dbad2af6893
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=i386 SHELL=/bin/bash net/vmw_vsock/

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@...el.com>

sparse warnings: (new ones prefixed by >>)
   net/vmw_vsock/af_vsock.c: note: in included file (through include/linux/nodemask.h, include/linux/list_lru.h, include/linux/fs.h, ...):
>> include/linux/random.h:64:69: sparse: sparse: cast truncates bits from constant value (fffffc00 becomes 0)

vim +64 include/linux/random.h

    53	
    54	u32 __get_random_u32_below(u32 ceil);
    55	/* Returns a random integer in the interval [0, ceil), with uniform distribution. */
    56	static inline u32 get_random_u32_below(u32 ceil)
    57	{
    58		if (!__builtin_constant_p(ceil))
    59			return __get_random_u32_below(ceil);
    60	
    61		for (;;) {
    62			if (ceil <= 1U << 8) {
    63				u32 mult = ceil * get_random_u8();
  > 64				if (is_power_of_2(ceil) || (u8)mult >= -(u8)ceil % ceil)
    65					return mult >> 8;
    66			} else if (ceil <= 1U << 16) {
    67				u32 mult = ceil * get_random_u16();
    68				if (is_power_of_2(ceil) || (u16)mult >= -(u16)ceil % ceil)
    69					return mult >> 16;
    70			} else {
    71				u64 mult = (u64)ceil * get_random_u32();
    72				if (is_power_of_2(ceil) || (u32)mult >= -ceil % ceil)
    73					return mult >> 32;
    74			}
    75		}
    76	}
    77	

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