linux-kernel - [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)

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Message-ID: <202210111756.pMpxXUwp-lkp@intel.com>
Date:   Tue, 11 Oct 2022 17:59:30 +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 24/26]
 include/linux/random.h:64:77: sparse: sparse: cast truncates bits from
 constant value (e24d4d50 becomes 50)

tree:   git://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git jd/get_random_u32_below
head:   64e577cf130af4c0a0cb0dcc9cd64de6525ab80f
commit: 5c94e468bcbf3081935eb45d6ce9137b2dc78b77 [24/26] ext4: use get_random_u32_below() for more efficient selection
config: i386-randconfig-s001-20221010
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=5c94e468bcbf3081935eb45d6ce9137b2dc78b77
        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 5c94e468bcbf3081935eb45d6ce9137b2dc78b77
        # 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

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 >>)
   fs/ext4/mmp.c: note: in included file (through include/linux/nodemask.h, include/linux/list_lru.h, include/linux/fs.h):
>> include/linux/random.h:64:77: sparse: sparse: cast truncates bits from constant value (e24d4d50 becomes 50)
>> include/linux/random.h:68:79: sparse: sparse: cast truncates bits from constant value (e24d4d50 becomes 4d50)

vim +64 include/linux/random.h

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;
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  66  		} else if (ceil <= 1U << 16) {
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  67  			u32 mult = ceil * get_random_u16();
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08 @68  			if (is_power_of_2(ceil) || (u16)mult >= -(__force u16)ceil % ceil)
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  69  				return mult >> 16;
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  70  		} else {
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  71  			u64 mult = (u64)ceil * get_random_u32();
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  72  			if (is_power_of_2(ceil) || (u32)mult >= -ceil % ceil)
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  73  				return mult >> 32;
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  74  		}
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  75  	}
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  76  }
dbc0933c6b1d8c Jason A. Donenfeld 2022-10-08  77  

:::::: The code at line 64 was first introduced by commit
:::::: dbc0933c6b1d8c21eb938d44038ae0ffaf930c0c random: use rejection sampling for uniform bounded random integers

:::::: TO: Jason A. Donenfeld <Jason@...c4.com>
:::::: CC: Jason A. Donenfeld <Jason@...c4.com>

-- 
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