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Diffstat (limited to 'kernel/crypto/jitterentropy.c')
| -rw-r--r-- | kernel/crypto/jitterentropy.c | 787 |
1 files changed, 787 insertions, 0 deletions
diff --git a/kernel/crypto/jitterentropy.c b/kernel/crypto/jitterentropy.c new file mode 100644 index 000000000..acf44b2d2 --- /dev/null +++ b/kernel/crypto/jitterentropy.c @@ -0,0 +1,787 @@ +/* + * Non-physical true random number generator based on timing jitter -- + * Jitter RNG standalone code. + * + * Copyright Stephan Mueller <smueller@chronox.de>, 2015 + * + * Design + * ====== + * + * See http://www.chronox.de/jent.html + * + * License + * ======= + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions + * are met: + * 1. Redistributions of source code must retain the above copyright + * notice, and the entire permission notice in its entirety, + * including the disclaimer of warranties. + * 2. Redistributions in binary form must reproduce the above copyright + * notice, this list of conditions and the following disclaimer in the + * documentation and/or other materials provided with the distribution. + * 3. The name of the author may not be used to endorse or promote + * products derived from this software without specific prior + * written permission. + * + * ALTERNATIVELY, this product may be distributed under the terms of + * the GNU General Public License, in which case the provisions of the GPL2 are + * required INSTEAD OF the above restrictions. (This clause is + * necessary due to a potential bad interaction between the GPL and + * the restrictions contained in a BSD-style copyright.) + * + * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED + * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES + * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF + * WHICH ARE HEREBY DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE + * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR + * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT + * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR + * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF + * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT + * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE + * USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH + * DAMAGE. + */ + +/* + * This Jitterentropy RNG is based on the jitterentropy library + * version 1.1.0 provided at http://www.chronox.de/jent.html + */ + +#ifdef __OPTIMIZE__ + #error "The CPU Jitter random number generator must not be compiled with optimizations. See documentation. Use the compiler switch -O0 for compiling jitterentropy.c." +#endif + +typedef unsigned long long __u64; +typedef long long __s64; +typedef unsigned int __u32; +#define NULL ((void *) 0) + +/* The entropy pool */ +struct rand_data { + /* all data values that are vital to maintain the security + * of the RNG are marked as SENSITIVE. A user must not + * access that information while the RNG executes its loops to + * calculate the next random value. */ + __u64 data; /* SENSITIVE Actual random number */ + __u64 old_data; /* SENSITIVE Previous random number */ + __u64 prev_time; /* SENSITIVE Previous time stamp */ +#define DATA_SIZE_BITS ((sizeof(__u64)) * 8) + __u64 last_delta; /* SENSITIVE stuck test */ + __s64 last_delta2; /* SENSITIVE stuck test */ + unsigned int stuck:1; /* Time measurement stuck */ + unsigned int osr; /* Oversample rate */ + unsigned int stir:1; /* Post-processing stirring */ + unsigned int disable_unbias:1; /* Deactivate Von-Neuman unbias */ +#define JENT_MEMORY_BLOCKS 64 +#define JENT_MEMORY_BLOCKSIZE 32 +#define JENT_MEMORY_ACCESSLOOPS 128 +#define JENT_MEMORY_SIZE (JENT_MEMORY_BLOCKS*JENT_MEMORY_BLOCKSIZE) + unsigned char *mem; /* Memory access location with size of + * memblocks * memblocksize */ + unsigned int memlocation; /* Pointer to byte in *mem */ + unsigned int memblocks; /* Number of memory blocks in *mem */ + unsigned int memblocksize; /* Size of one memory block in bytes */ + unsigned int memaccessloops; /* Number of memory accesses per random + * bit generation */ +}; + +/* Flags that can be used to initialize the RNG */ +#define JENT_DISABLE_STIR (1<<0) /* Disable stirring the entropy pool */ +#define JENT_DISABLE_UNBIAS (1<<1) /* Disable the Von-Neuman Unbiaser */ +#define JENT_DISABLE_MEMORY_ACCESS (1<<2) /* Disable memory access for more + * entropy, saves MEMORY_SIZE RAM for + * entropy collector */ + +/* -- error codes for init function -- */ +#define JENT_ENOTIME 1 /* Timer service not available */ +#define JENT_ECOARSETIME 2 /* Timer too coarse for RNG */ +#define JENT_ENOMONOTONIC 3 /* Timer is not monotonic increasing */ +#define JENT_EMINVARIATION 4 /* Timer variations too small for RNG */ +#define JENT_EVARVAR 5 /* Timer does not produce variations of + * variations (2nd derivation of time is + * zero). */ +#define JENT_EMINVARVAR 6 /* Timer variations of variations is tooi + * small. */ + +/*************************************************************************** + * Helper functions + ***************************************************************************/ + +void jent_get_nstime(__u64 *out); +__u64 jent_rol64(__u64 word, unsigned int shift); +void *jent_zalloc(unsigned int len); +void jent_zfree(void *ptr); +int jent_fips_enabled(void); +void jent_panic(char *s); +void jent_memcpy(void *dest, const void *src, unsigned int n); + +/** + * Update of the loop count used for the next round of + * an entropy collection. + * + * Input: + * @ec entropy collector struct -- may be NULL + * @bits is the number of low bits of the timer to consider + * @min is the number of bits we shift the timer value to the right at + * the end to make sure we have a guaranteed minimum value + * + * @return Newly calculated loop counter + */ +static __u64 jent_loop_shuffle(struct rand_data *ec, + unsigned int bits, unsigned int min) +{ + __u64 time = 0; + __u64 shuffle = 0; + unsigned int i = 0; + unsigned int mask = (1<<bits) - 1; + + jent_get_nstime(&time); + /* + * mix the current state of the random number into the shuffle + * calculation to balance that shuffle a bit more + */ + if (ec) + time ^= ec->data; + /* + * we fold the time value as much as possible to ensure that as many + * bits of the time stamp are included as possible + */ + for (i = 0; (DATA_SIZE_BITS / bits) > i; i++) { + shuffle ^= time & mask; + time = time >> bits; + } + + /* + * We add a lower boundary value to ensure we have a minimum + * RNG loop count. + */ + return (shuffle + (1<<min)); +} + +/*************************************************************************** + * Noise sources + ***************************************************************************/ + +/** + * CPU Jitter noise source -- this is the noise source based on the CPU + * execution time jitter + * + * This function folds the time into one bit units by iterating + * through the DATA_SIZE_BITS bit time value as follows: assume our time value + * is 0xabcd + * 1st loop, 1st shift generates 0xd000 + * 1st loop, 2nd shift generates 0x000d + * 2nd loop, 1st shift generates 0xcd00 + * 2nd loop, 2nd shift generates 0x000c + * 3rd loop, 1st shift generates 0xbcd0 + * 3rd loop, 2nd shift generates 0x000b + * 4th loop, 1st shift generates 0xabcd + * 4th loop, 2nd shift generates 0x000a + * Now, the values at the end of the 2nd shifts are XORed together. + * + * The code is deliberately inefficient and shall stay that way. This function + * is the root cause why the code shall be compiled without optimization. This + * function not only acts as folding operation, but this function's execution + * is used to measure the CPU execution time jitter. Any change to the loop in + * this function implies that careful retesting must be done. + * + * Input: + * @ec entropy collector struct -- may be NULL + * @time time stamp to be folded + * @loop_cnt if a value not equal to 0 is set, use the given value as number of + * loops to perform the folding + * + * Output: + * @folded result of folding operation + * + * @return Number of loops the folding operation is performed + */ +static __u64 jent_fold_time(struct rand_data *ec, __u64 time, + __u64 *folded, __u64 loop_cnt) +{ + unsigned int i; + __u64 j = 0; + __u64 new = 0; +#define MAX_FOLD_LOOP_BIT 4 +#define MIN_FOLD_LOOP_BIT 0 + __u64 fold_loop_cnt = + jent_loop_shuffle(ec, MAX_FOLD_LOOP_BIT, MIN_FOLD_LOOP_BIT); + + /* + * testing purposes -- allow test app to set the counter, not + * needed during runtime + */ + if (loop_cnt) + fold_loop_cnt = loop_cnt; + for (j = 0; j < fold_loop_cnt; j++) { + new = 0; + for (i = 1; (DATA_SIZE_BITS) >= i; i++) { + __u64 tmp = time << (DATA_SIZE_BITS - i); + + tmp = tmp >> (DATA_SIZE_BITS - 1); + new ^= tmp; + } + } + *folded = new; + return fold_loop_cnt; +} + +/** + * Memory Access noise source -- this is a noise source based on variations in + * memory access times + * + * This function performs memory accesses which will add to the timing + * variations due to an unknown amount of CPU wait states that need to be + * added when accessing memory. The memory size should be larger than the L1 + * caches as outlined in the documentation and the associated testing. + * + * The L1 cache has a very high bandwidth, albeit its access rate is usually + * slower than accessing CPU registers. Therefore, L1 accesses only add minimal + * variations as the CPU has hardly to wait. Starting with L2, significant + * variations are added because L2 typically does not belong to the CPU any more + * and therefore a wider range of CPU wait states is necessary for accesses. + * L3 and real memory accesses have even a wider range of wait states. However, + * to reliably access either L3 or memory, the ec->mem memory must be quite + * large which is usually not desirable. + * + * Input: + * @ec Reference to the entropy collector with the memory access data -- if + * the reference to the memory block to be accessed is NULL, this noise + * source is disabled + * @loop_cnt if a value not equal to 0 is set, use the given value as number of + * loops to perform the folding + * + * @return Number of memory access operations + */ +static unsigned int jent_memaccess(struct rand_data *ec, __u64 loop_cnt) +{ + unsigned char *tmpval = NULL; + unsigned int wrap = 0; + __u64 i = 0; +#define MAX_ACC_LOOP_BIT 7 +#define MIN_ACC_LOOP_BIT 0 + __u64 acc_loop_cnt = + jent_loop_shuffle(ec, MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT); + + if (NULL == ec || NULL == ec->mem) + return 0; + wrap = ec->memblocksize * ec->memblocks; + + /* + * testing purposes -- allow test app to set the counter, not + * needed during runtime + */ + if (loop_cnt) + acc_loop_cnt = loop_cnt; + + for (i = 0; i < (ec->memaccessloops + acc_loop_cnt); i++) { + tmpval = ec->mem + ec->memlocation; + /* + * memory access: just add 1 to one byte, + * wrap at 255 -- memory access implies read + * from and write to memory location + */ + *tmpval = (*tmpval + 1) & 0xff; + /* + * Addition of memblocksize - 1 to pointer + * with wrap around logic to ensure that every + * memory location is hit evenly + */ + ec->memlocation = ec->memlocation + ec->memblocksize - 1; + ec->memlocation = ec->memlocation % wrap; + } + return i; +} + +/*************************************************************************** + * Start of entropy processing logic + ***************************************************************************/ + +/** + * Stuck test by checking the: + * 1st derivation of the jitter measurement (time delta) + * 2nd derivation of the jitter measurement (delta of time deltas) + * 3rd derivation of the jitter measurement (delta of delta of time deltas) + * + * All values must always be non-zero. + * + * Input: + * @ec Reference to entropy collector + * @current_delta Jitter time delta + * + * @return + * 0 jitter measurement not stuck (good bit) + * 1 jitter measurement stuck (reject bit) + */ +static void jent_stuck(struct rand_data *ec, __u64 current_delta) +{ + __s64 delta2 = ec->last_delta - current_delta; + __s64 delta3 = delta2 - ec->last_delta2; + + ec->last_delta = current_delta; + ec->last_delta2 = delta2; + + if (!current_delta || !delta2 || !delta3) + ec->stuck = 1; +} + +/** + * This is the heart of the entropy generation: calculate time deltas and + * use the CPU jitter in the time deltas. The jitter is folded into one + * bit. You can call this function the "random bit generator" as it + * produces one random bit per invocation. + * + * WARNING: ensure that ->prev_time is primed before using the output + * of this function! This can be done by calling this function + * and not using its result. + * + * Input: + * @entropy_collector Reference to entropy collector + * + * @return One random bit + */ +static __u64 jent_measure_jitter(struct rand_data *ec) +{ + __u64 time = 0; + __u64 data = 0; + __u64 current_delta = 0; + + /* Invoke one noise source before time measurement to add variations */ + jent_memaccess(ec, 0); + + /* + * Get time stamp and calculate time delta to previous + * invocation to measure the timing variations + */ + jent_get_nstime(&time); + current_delta = time - ec->prev_time; + ec->prev_time = time; + + /* Now call the next noise sources which also folds the data */ + jent_fold_time(ec, current_delta, &data, 0); + + /* + * Check whether we have a stuck measurement. The enforcement + * is performed after the stuck value has been mixed into the + * entropy pool. + */ + jent_stuck(ec, current_delta); + + return data; +} + +/** + * Von Neuman unbias as explained in RFC 4086 section 4.2. As shown in the + * documentation of that RNG, the bits from jent_measure_jitter are considered + * independent which implies that the Von Neuman unbias operation is applicable. + * A proof of the Von-Neumann unbias operation to remove skews is given in the + * document "A proposal for: Functionality classes for random number + * generators", version 2.0 by Werner Schindler, section 5.4.1. + * + * Input: + * @entropy_collector Reference to entropy collector + * + * @return One random bit + */ +static __u64 jent_unbiased_bit(struct rand_data *entropy_collector) +{ + do { + __u64 a = jent_measure_jitter(entropy_collector); + __u64 b = jent_measure_jitter(entropy_collector); + + if (a == b) + continue; + if (1 == a) + return 1; + else + return 0; + } while (1); +} + +/** + * Shuffle the pool a bit by mixing some value with a bijective function (XOR) + * into the pool. + * + * The function generates a mixer value that depends on the bits set and the + * location of the set bits in the random number generated by the entropy + * source. Therefore, based on the generated random number, this mixer value + * can have 2**64 different values. That mixer value is initialized with the + * first two SHA-1 constants. After obtaining the mixer value, it is XORed into + * the random number. + * + * The mixer value is not assumed to contain any entropy. But due to the XOR + * operation, it can also not destroy any entropy present in the entropy pool. + * + * Input: + * @entropy_collector Reference to entropy collector + */ +static void jent_stir_pool(struct rand_data *entropy_collector) +{ + /* + * to shut up GCC on 32 bit, we have to initialize the 64 variable + * with two 32 bit variables + */ + union c { + __u64 u64; + __u32 u32[2]; + }; + /* + * This constant is derived from the first two 32 bit initialization + * vectors of SHA-1 as defined in FIPS 180-4 section 5.3.1 + */ + union c constant; + /* + * The start value of the mixer variable is derived from the third + * and fourth 32 bit initialization vector of SHA-1 as defined in + * FIPS 180-4 section 5.3.1 + */ + union c mixer; + unsigned int i = 0; + + /* + * Store the SHA-1 constants in reverse order to make up the 64 bit + * value -- this applies to a little endian system, on a big endian + * system, it reverses as expected. But this really does not matter + * as we do not rely on the specific numbers. We just pick the SHA-1 + * constants as they have a good mix of bit set and unset. + */ + constant.u32[1] = 0x67452301; + constant.u32[0] = 0xefcdab89; + mixer.u32[1] = 0x98badcfe; + mixer.u32[0] = 0x10325476; + + for (i = 0; i < DATA_SIZE_BITS; i++) { + /* + * get the i-th bit of the input random number and only XOR + * the constant into the mixer value when that bit is set + */ + if ((entropy_collector->data >> i) & 1) + mixer.u64 ^= constant.u64; + mixer.u64 = jent_rol64(mixer.u64, 1); + } + entropy_collector->data ^= mixer.u64; +} + +/** + * Generator of one 64 bit random number + * Function fills rand_data->data + * + * Input: + * @ec Reference to entropy collector + */ +static void jent_gen_entropy(struct rand_data *ec) +{ + unsigned int k = 0; + + /* priming of the ->prev_time value */ + jent_measure_jitter(ec); + + while (1) { + __u64 data = 0; + + if (ec->disable_unbias == 1) + data = jent_measure_jitter(ec); + else + data = jent_unbiased_bit(ec); + + /* enforcement of the jent_stuck test */ + if (ec->stuck) { + /* + * We only mix in the bit considered not appropriate + * without the LSFR. The reason is that if we apply + * the LSFR and we do not rotate, the 2nd bit with LSFR + * will cancel out the first LSFR application on the + * bad bit. + * + * And we do not rotate as we apply the next bit to the + * current bit location again. + */ + ec->data ^= data; + ec->stuck = 0; + continue; + } + + /* + * Fibonacci LSFR with polynom of + * x^64 + x^61 + x^56 + x^31 + x^28 + x^23 + 1 which is + * primitive according to + * http://poincare.matf.bg.ac.rs/~ezivkovm/publications/primpol1.pdf + * (the shift values are the polynom values minus one + * due to counting bits from 0 to 63). As the current + * position is always the LSB, the polynom only needs + * to shift data in from the left without wrap. + */ + ec->data ^= data; + ec->data ^= ((ec->data >> 63) & 1); + ec->data ^= ((ec->data >> 60) & 1); + ec->data ^= ((ec->data >> 55) & 1); + ec->data ^= ((ec->data >> 30) & 1); + ec->data ^= ((ec->data >> 27) & 1); + ec->data ^= ((ec->data >> 22) & 1); + ec->data = jent_rol64(ec->data, 1); + + /* + * We multiply the loop value with ->osr to obtain the + * oversampling rate requested by the caller + */ + if (++k >= (DATA_SIZE_BITS * ec->osr)) + break; + } + if (ec->stir) + jent_stir_pool(ec); +} + +/** + * The continuous test required by FIPS 140-2 -- the function automatically + * primes the test if needed. + * + * Return: + * 0 if FIPS test passed + * < 0 if FIPS test failed + */ +static void jent_fips_test(struct rand_data *ec) +{ + if (!jent_fips_enabled()) + return; + + /* prime the FIPS test */ + if (!ec->old_data) { + ec->old_data = ec->data; + jent_gen_entropy(ec); + } + + if (ec->data == ec->old_data) + jent_panic("jitterentropy: Duplicate output detected\n"); + + ec->old_data = ec->data; +} + +/** + * Entry function: Obtain entropy for the caller. + * + * This function invokes the entropy gathering logic as often to generate + * as many bytes as requested by the caller. The entropy gathering logic + * creates 64 bit per invocation. + * + * This function truncates the last 64 bit entropy value output to the exact + * size specified by the caller. + * + * Input: + * @ec Reference to entropy collector + * @data pointer to buffer for storing random data -- buffer must already + * exist + * @len size of the buffer, specifying also the requested number of random + * in bytes + * + * @return 0 when request is fulfilled or an error + * + * The following error codes can occur: + * -1 entropy_collector is NULL + */ +int jent_read_entropy(struct rand_data *ec, unsigned char *data, + unsigned int len) +{ + unsigned char *p = data; + + if (!ec) + return -1; + + while (0 < len) { + unsigned int tocopy; + + jent_gen_entropy(ec); + jent_fips_test(ec); + if ((DATA_SIZE_BITS / 8) < len) + tocopy = (DATA_SIZE_BITS / 8); + else + tocopy = len; + jent_memcpy(p, &ec->data, tocopy); + + len -= tocopy; + p += tocopy; + } + + return 0; +} + +/*************************************************************************** + * Initialization logic + ***************************************************************************/ + +struct rand_data *jent_entropy_collector_alloc(unsigned int osr, + unsigned int flags) +{ + struct rand_data *entropy_collector; + + entropy_collector = jent_zalloc(sizeof(struct rand_data)); + if (!entropy_collector) + return NULL; + + if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) { + /* Allocate memory for adding variations based on memory + * access + */ + entropy_collector->mem = jent_zalloc(JENT_MEMORY_SIZE); + if (!entropy_collector->mem) { + jent_zfree(entropy_collector); + return NULL; + } + entropy_collector->memblocksize = JENT_MEMORY_BLOCKSIZE; + entropy_collector->memblocks = JENT_MEMORY_BLOCKS; + entropy_collector->memaccessloops = JENT_MEMORY_ACCESSLOOPS; + } + + /* verify and set the oversampling rate */ + if (0 == osr) + osr = 1; /* minimum sampling rate is 1 */ + entropy_collector->osr = osr; + + entropy_collector->stir = 1; + if (flags & JENT_DISABLE_STIR) + entropy_collector->stir = 0; + if (flags & JENT_DISABLE_UNBIAS) + entropy_collector->disable_unbias = 1; + + /* fill the data pad with non-zero values */ + jent_gen_entropy(entropy_collector); + + return entropy_collector; +} + +void jent_entropy_collector_free(struct rand_data *entropy_collector) +{ + jent_zfree(entropy_collector->mem); + entropy_collector->mem = NULL; + jent_zfree(entropy_collector); + entropy_collector = NULL; +} + +int jent_entropy_init(void) +{ + int i; + __u64 delta_sum = 0; + __u64 old_delta = 0; + int time_backwards = 0; + int count_var = 0; + int count_mod = 0; + + /* We could perform statistical tests here, but the problem is + * that we only have a few loop counts to do testing. These + * loop counts may show some slight skew and we produce + * false positives. + * + * Moreover, only old systems show potentially problematic + * jitter entropy that could potentially be caught here. But + * the RNG is intended for hardware that is available or widely + * used, but not old systems that are long out of favor. Thus, + * no statistical tests. + */ + + /* + * We could add a check for system capabilities such as clock_getres or + * check for CONFIG_X86_TSC, but it does not make much sense as the + * following sanity checks verify that we have a high-resolution + * timer. + */ + /* + * TESTLOOPCOUNT needs some loops to identify edge systems. 100 is + * definitely too little. + */ +#define TESTLOOPCOUNT 300 +#define CLEARCACHE 100 + for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) { + __u64 time = 0; + __u64 time2 = 0; + __u64 folded = 0; + __u64 delta = 0; + unsigned int lowdelta = 0; + + jent_get_nstime(&time); + jent_fold_time(NULL, time, &folded, 1<<MIN_FOLD_LOOP_BIT); + jent_get_nstime(&time2); + + /* test whether timer works */ + if (!time || !time2) + return JENT_ENOTIME; + delta = time2 - time; + /* + * test whether timer is fine grained enough to provide + * delta even when called shortly after each other -- this + * implies that we also have a high resolution timer + */ + if (!delta) + return JENT_ECOARSETIME; + + /* + * up to here we did not modify any variable that will be + * evaluated later, but we already performed some work. Thus we + * already have had an impact on the caches, branch prediction, + * etc. with the goal to clear it to get the worst case + * measurements. + */ + if (CLEARCACHE > i) + continue; + + /* test whether we have an increasing timer */ + if (!(time2 > time)) + time_backwards++; + + /* + * Avoid modulo of 64 bit integer to allow code to compile + * on 32 bit architectures. + */ + lowdelta = time2 - time; + if (!(lowdelta % 100)) + count_mod++; + + /* + * ensure that we have a varying delta timer which is necessary + * for the calculation of entropy -- perform this check + * only after the first loop is executed as we need to prime + * the old_data value + */ + if (i) { + if (delta != old_delta) + count_var++; + if (delta > old_delta) + delta_sum += (delta - old_delta); + else + delta_sum += (old_delta - delta); + } + old_delta = delta; + } + + /* + * we allow up to three times the time running backwards. + * CLOCK_REALTIME is affected by adjtime and NTP operations. Thus, + * if such an operation just happens to interfere with our test, it + * should not fail. The value of 3 should cover the NTP case being + * performed during our test run. + */ + if (3 < time_backwards) + return JENT_ENOMONOTONIC; + /* Error if the time variances are always identical */ + if (!delta_sum) + return JENT_EVARVAR; + + /* + * Variations of deltas of time must on average be larger + * than 1 to ensure the entropy estimation + * implied with 1 is preserved + */ + if (delta_sum <= 1) + return JENT_EMINVARVAR; + + /* + * Ensure that we have variations in the time stamp below 10 for at + * least 10% of all checks -- on some platforms, the counter + * increments in multiples of 100, but not always + */ + if ((TESTLOOPCOUNT/10 * 9) < count_mod) + return JENT_ECOARSETIME; + + return 0; +} |