1/*
2 * Non-physical true random number generator based on timing jitter --
3 * Jitter RNG standalone code.
4 *
5 * Copyright Stephan Mueller <smueller@chronox.de>, 2015
6 *
7 * Design
8 * ======
9 *
10 * See http://www.chronox.de/jent.html
11 *
12 * License
13 * =======
14 *
15 * Redistribution and use in source and binary forms, with or without
16 * modification, are permitted provided that the following conditions
17 * are met:
18 * 1. Redistributions of source code must retain the above copyright
19 * notice, and the entire permission notice in its entirety,
20 * including the disclaimer of warranties.
21 * 2. Redistributions in binary form must reproduce the above copyright
22 * notice, this list of conditions and the following disclaimer in the
23 * documentation and/or other materials provided with the distribution.
24 * 3. The name of the author may not be used to endorse or promote
25 * products derived from this software without specific prior
26 * written permission.
27 *
28 * ALTERNATIVELY, this product may be distributed under the terms of
29 * the GNU General Public License, in which case the provisions of the GPL2 are
30 * required INSTEAD OF the above restrictions. (This clause is
31 * necessary due to a potential bad interaction between the GPL and
32 * the restrictions contained in a BSD-style copyright.)
33 *
34 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
35 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
36 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF
37 * WHICH ARE HEREBY DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE
38 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
39 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
40 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
41 * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
42 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
43 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
44 * USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH
45 * DAMAGE.
46 */
47
48/*
49 * This Jitterentropy RNG is based on the jitterentropy library
50 * version 1.1.0 provided at http://www.chronox.de/jent.html
51 */
52
53#ifdef __OPTIMIZE__
54 #error "The CPU Jitter random number generator must not be compiled with optimizations. See documentation. Use the compiler switch -O0 for compiling jitterentropy.c."
55#endif
56
57typedef unsigned long long __u64;
58typedef long long __s64;
59typedef unsigned int __u32;
60#define NULL ((void *) 0)
61
62/* The entropy pool */
63struct rand_data {
64 /* all data values that are vital to maintain the security
65 * of the RNG are marked as SENSITIVE. A user must not
66 * access that information while the RNG executes its loops to
67 * calculate the next random value. */
68 __u64 data; /* SENSITIVE Actual random number */
69 __u64 old_data; /* SENSITIVE Previous random number */
70 __u64 prev_time; /* SENSITIVE Previous time stamp */
71#define DATA_SIZE_BITS ((sizeof(__u64)) * 8)
72 __u64 last_delta; /* SENSITIVE stuck test */
73 __s64 last_delta2; /* SENSITIVE stuck test */
74 unsigned int stuck:1; /* Time measurement stuck */
75 unsigned int osr; /* Oversample rate */
76 unsigned int stir:1; /* Post-processing stirring */
77 unsigned int disable_unbias:1; /* Deactivate Von-Neuman unbias */
78#define JENT_MEMORY_BLOCKS 64
79#define JENT_MEMORY_BLOCKSIZE 32
80#define JENT_MEMORY_ACCESSLOOPS 128
81#define JENT_MEMORY_SIZE (JENT_MEMORY_BLOCKS*JENT_MEMORY_BLOCKSIZE)
82 unsigned char *mem; /* Memory access location with size of
83 * memblocks * memblocksize */
84 unsigned int memlocation; /* Pointer to byte in *mem */
85 unsigned int memblocks; /* Number of memory blocks in *mem */
86 unsigned int memblocksize; /* Size of one memory block in bytes */
87 unsigned int memaccessloops; /* Number of memory accesses per random
88 * bit generation */
89};
90
91/* Flags that can be used to initialize the RNG */
92#define JENT_DISABLE_STIR (1<<0) /* Disable stirring the entropy pool */
93#define JENT_DISABLE_UNBIAS (1<<1) /* Disable the Von-Neuman Unbiaser */
94#define JENT_DISABLE_MEMORY_ACCESS (1<<2) /* Disable memory access for more
95 * entropy, saves MEMORY_SIZE RAM for
96 * entropy collector */
97
98/* -- error codes for init function -- */
99#define JENT_ENOTIME 1 /* Timer service not available */
100#define JENT_ECOARSETIME 2 /* Timer too coarse for RNG */
101#define JENT_ENOMONOTONIC 3 /* Timer is not monotonic increasing */
102#define JENT_EMINVARIATION 4 /* Timer variations too small for RNG */
103#define JENT_EVARVAR 5 /* Timer does not produce variations of
104 * variations (2nd derivation of time is
105 * zero). */
106#define JENT_EMINVARVAR 6 /* Timer variations of variations is tooi
107 * small. */
108
109/***************************************************************************
110 * Helper functions
111 ***************************************************************************/
112
113void jent_get_nstime(__u64 *out);
114__u64 jent_rol64(__u64 word, unsigned int shift);
115void *jent_zalloc(unsigned int len);
116void jent_zfree(void *ptr);
117int jent_fips_enabled(void);
118void jent_panic(char *s);
119void jent_memcpy(void *dest, const void *src, unsigned int n);
120
121/**
122 * Update of the loop count used for the next round of
123 * an entropy collection.
124 *
125 * Input:
126 * @ec entropy collector struct -- may be NULL
127 * @bits is the number of low bits of the timer to consider
128 * @min is the number of bits we shift the timer value to the right at
129 * the end to make sure we have a guaranteed minimum value
130 *
131 * @return Newly calculated loop counter
132 */
133static __u64 jent_loop_shuffle(struct rand_data *ec,
134 unsigned int bits, unsigned int min)
135{
136 __u64 time = 0;
137 __u64 shuffle = 0;
138 unsigned int i = 0;
139 unsigned int mask = (1<<bits) - 1;
140
141 jent_get_nstime(&time);
142 /*
143 * mix the current state of the random number into the shuffle
144 * calculation to balance that shuffle a bit more
145 */
146 if (ec)
147 time ^= ec->data;
148 /*
149 * we fold the time value as much as possible to ensure that as many
150 * bits of the time stamp are included as possible
151 */
152 for (i = 0; (DATA_SIZE_BITS / bits) > i; i++) {
153 shuffle ^= time & mask;
154 time = time >> bits;
155 }
156
157 /*
158 * We add a lower boundary value to ensure we have a minimum
159 * RNG loop count.
160 */
161 return (shuffle + (1<<min));
162}
163
164/***************************************************************************
165 * Noise sources
166 ***************************************************************************/
167
168/**
169 * CPU Jitter noise source -- this is the noise source based on the CPU
170 * execution time jitter
171 *
172 * This function folds the time into one bit units by iterating
173 * through the DATA_SIZE_BITS bit time value as follows: assume our time value
174 * is 0xabcd
175 * 1st loop, 1st shift generates 0xd000
176 * 1st loop, 2nd shift generates 0x000d
177 * 2nd loop, 1st shift generates 0xcd00
178 * 2nd loop, 2nd shift generates 0x000c
179 * 3rd loop, 1st shift generates 0xbcd0
180 * 3rd loop, 2nd shift generates 0x000b
181 * 4th loop, 1st shift generates 0xabcd
182 * 4th loop, 2nd shift generates 0x000a
183 * Now, the values at the end of the 2nd shifts are XORed together.
184 *
185 * The code is deliberately inefficient and shall stay that way. This function
186 * is the root cause why the code shall be compiled without optimization. This
187 * function not only acts as folding operation, but this function's execution
188 * is used to measure the CPU execution time jitter. Any change to the loop in
189 * this function implies that careful retesting must be done.
190 *
191 * Input:
192 * @ec entropy collector struct -- may be NULL
193 * @time time stamp to be folded
194 * @loop_cnt if a value not equal to 0 is set, use the given value as number of
195 * loops to perform the folding
196 *
197 * Output:
198 * @folded result of folding operation
199 *
200 * @return Number of loops the folding operation is performed
201 */
202static __u64 jent_fold_time(struct rand_data *ec, __u64 time,
203 __u64 *folded, __u64 loop_cnt)
204{
205 unsigned int i;
206 __u64 j = 0;
207 __u64 new = 0;
208#define MAX_FOLD_LOOP_BIT 4
209#define MIN_FOLD_LOOP_BIT 0
210 __u64 fold_loop_cnt =
211 jent_loop_shuffle(ec, MAX_FOLD_LOOP_BIT, MIN_FOLD_LOOP_BIT);
212
213 /*
214 * testing purposes -- allow test app to set the counter, not
215 * needed during runtime
216 */
217 if (loop_cnt)
218 fold_loop_cnt = loop_cnt;
219 for (j = 0; j < fold_loop_cnt; j++) {
220 new = 0;
221 for (i = 1; (DATA_SIZE_BITS) >= i; i++) {
222 __u64 tmp = time << (DATA_SIZE_BITS - i);
223
224 tmp = tmp >> (DATA_SIZE_BITS - 1);
225 new ^= tmp;
226 }
227 }
228 *folded = new;
229 return fold_loop_cnt;
230}
231
232/**
233 * Memory Access noise source -- this is a noise source based on variations in
234 * memory access times
235 *
236 * This function performs memory accesses which will add to the timing
237 * variations due to an unknown amount of CPU wait states that need to be
238 * added when accessing memory. The memory size should be larger than the L1
239 * caches as outlined in the documentation and the associated testing.
240 *
241 * The L1 cache has a very high bandwidth, albeit its access rate is usually
242 * slower than accessing CPU registers. Therefore, L1 accesses only add minimal
243 * variations as the CPU has hardly to wait. Starting with L2, significant
244 * variations are added because L2 typically does not belong to the CPU any more
245 * and therefore a wider range of CPU wait states is necessary for accesses.
246 * L3 and real memory accesses have even a wider range of wait states. However,
247 * to reliably access either L3 or memory, the ec->mem memory must be quite
248 * large which is usually not desirable.
249 *
250 * Input:
251 * @ec Reference to the entropy collector with the memory access data -- if
252 * the reference to the memory block to be accessed is NULL, this noise
253 * source is disabled
254 * @loop_cnt if a value not equal to 0 is set, use the given value as number of
255 * loops to perform the folding
256 *
257 * @return Number of memory access operations
258 */
259static unsigned int jent_memaccess(struct rand_data *ec, __u64 loop_cnt)
260{
261 unsigned char *tmpval = NULL;
262 unsigned int wrap = 0;
263 __u64 i = 0;
264#define MAX_ACC_LOOP_BIT 7
265#define MIN_ACC_LOOP_BIT 0
266 __u64 acc_loop_cnt =
267 jent_loop_shuffle(ec, MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT);
268
269 if (NULL == ec || NULL == ec->mem)
270 return 0;
271 wrap = ec->memblocksize * ec->memblocks;
272
273 /*
274 * testing purposes -- allow test app to set the counter, not
275 * needed during runtime
276 */
277 if (loop_cnt)
278 acc_loop_cnt = loop_cnt;
279
280 for (i = 0; i < (ec->memaccessloops + acc_loop_cnt); i++) {
281 tmpval = ec->mem + ec->memlocation;
282 /*
283 * memory access: just add 1 to one byte,
284 * wrap at 255 -- memory access implies read
285 * from and write to memory location
286 */
287 *tmpval = (*tmpval + 1) & 0xff;
288 /*
289 * Addition of memblocksize - 1 to pointer
290 * with wrap around logic to ensure that every
291 * memory location is hit evenly
292 */
293 ec->memlocation = ec->memlocation + ec->memblocksize - 1;
294 ec->memlocation = ec->memlocation % wrap;
295 }
296 return i;
297}
298
299/***************************************************************************
300 * Start of entropy processing logic
301 ***************************************************************************/
302
303/**
304 * Stuck test by checking the:
305 * 1st derivation of the jitter measurement (time delta)
306 * 2nd derivation of the jitter measurement (delta of time deltas)
307 * 3rd derivation of the jitter measurement (delta of delta of time deltas)
308 *
309 * All values must always be non-zero.
310 *
311 * Input:
312 * @ec Reference to entropy collector
313 * @current_delta Jitter time delta
314 *
315 * @return
316 * 0 jitter measurement not stuck (good bit)
317 * 1 jitter measurement stuck (reject bit)
318 */
319static void jent_stuck(struct rand_data *ec, __u64 current_delta)
320{
321 __s64 delta2 = ec->last_delta - current_delta;
322 __s64 delta3 = delta2 - ec->last_delta2;
323
324 ec->last_delta = current_delta;
325 ec->last_delta2 = delta2;
326
327 if (!current_delta || !delta2 || !delta3)
328 ec->stuck = 1;
329}
330
331/**
332 * This is the heart of the entropy generation: calculate time deltas and
333 * use the CPU jitter in the time deltas. The jitter is folded into one
334 * bit. You can call this function the "random bit generator" as it
335 * produces one random bit per invocation.
336 *
337 * WARNING: ensure that ->prev_time is primed before using the output
338 * of this function! This can be done by calling this function
339 * and not using its result.
340 *
341 * Input:
342 * @entropy_collector Reference to entropy collector
343 *
344 * @return One random bit
345 */
346static __u64 jent_measure_jitter(struct rand_data *ec)
347{
348 __u64 time = 0;
349 __u64 data = 0;
350 __u64 current_delta = 0;
351
352 /* Invoke one noise source before time measurement to add variations */
353 jent_memaccess(ec, 0);
354
355 /*
356 * Get time stamp and calculate time delta to previous
357 * invocation to measure the timing variations
358 */
359 jent_get_nstime(&time);
360 current_delta = time - ec->prev_time;
361 ec->prev_time = time;
362
363 /* Now call the next noise sources which also folds the data */
364 jent_fold_time(ec, current_delta, &data, 0);
365
366 /*
367 * Check whether we have a stuck measurement. The enforcement
368 * is performed after the stuck value has been mixed into the
369 * entropy pool.
370 */
371 jent_stuck(ec, current_delta);
372
373 return data;
374}
375
376/**
377 * Von Neuman unbias as explained in RFC 4086 section 4.2. As shown in the
378 * documentation of that RNG, the bits from jent_measure_jitter are considered
379 * independent which implies that the Von Neuman unbias operation is applicable.
380 * A proof of the Von-Neumann unbias operation to remove skews is given in the
381 * document "A proposal for: Functionality classes for random number
382 * generators", version 2.0 by Werner Schindler, section 5.4.1.
383 *
384 * Input:
385 * @entropy_collector Reference to entropy collector
386 *
387 * @return One random bit
388 */
389static __u64 jent_unbiased_bit(struct rand_data *entropy_collector)
390{
391 do {
392 __u64 a = jent_measure_jitter(entropy_collector);
393 __u64 b = jent_measure_jitter(entropy_collector);
394
395 if (a == b)
396 continue;
397 if (1 == a)
398 return 1;
399 else
400 return 0;
401 } while (1);
402}
403
404/**
405 * Shuffle the pool a bit by mixing some value with a bijective function (XOR)
406 * into the pool.
407 *
408 * The function generates a mixer value that depends on the bits set and the
409 * location of the set bits in the random number generated by the entropy
410 * source. Therefore, based on the generated random number, this mixer value
411 * can have 2**64 different values. That mixer value is initialized with the
412 * first two SHA-1 constants. After obtaining the mixer value, it is XORed into
413 * the random number.
414 *
415 * The mixer value is not assumed to contain any entropy. But due to the XOR
416 * operation, it can also not destroy any entropy present in the entropy pool.
417 *
418 * Input:
419 * @entropy_collector Reference to entropy collector
420 */
421static void jent_stir_pool(struct rand_data *entropy_collector)
422{
423 /*
424 * to shut up GCC on 32 bit, we have to initialize the 64 variable
425 * with two 32 bit variables
426 */
427 union c {
428 __u64 u64;
429 __u32 u32[2];
430 };
431 /*
432 * This constant is derived from the first two 32 bit initialization
433 * vectors of SHA-1 as defined in FIPS 180-4 section 5.3.1
434 */
435 union c constant;
436 /*
437 * The start value of the mixer variable is derived from the third
438 * and fourth 32 bit initialization vector of SHA-1 as defined in
439 * FIPS 180-4 section 5.3.1
440 */
441 union c mixer;
442 unsigned int i = 0;
443
444 /*
445 * Store the SHA-1 constants in reverse order to make up the 64 bit
446 * value -- this applies to a little endian system, on a big endian
447 * system, it reverses as expected. But this really does not matter
448 * as we do not rely on the specific numbers. We just pick the SHA-1
449 * constants as they have a good mix of bit set and unset.
450 */
451 constant.u32[1] = 0x67452301;
452 constant.u32[0] = 0xefcdab89;
453 mixer.u32[1] = 0x98badcfe;
454 mixer.u32[0] = 0x10325476;
455
456 for (i = 0; i < DATA_SIZE_BITS; i++) {
457 /*
458 * get the i-th bit of the input random number and only XOR
459 * the constant into the mixer value when that bit is set
460 */
461 if ((entropy_collector->data >> i) & 1)
462 mixer.u64 ^= constant.u64;
463 mixer.u64 = jent_rol64(mixer.u64, 1);
464 }
465 entropy_collector->data ^= mixer.u64;
466}
467
468/**
469 * Generator of one 64 bit random number
470 * Function fills rand_data->data
471 *
472 * Input:
473 * @ec Reference to entropy collector
474 */
475static void jent_gen_entropy(struct rand_data *ec)
476{
477 unsigned int k = 0;
478
479 /* priming of the ->prev_time value */
480 jent_measure_jitter(ec);
481
482 while (1) {
483 __u64 data = 0;
484
485 if (ec->disable_unbias == 1)
486 data = jent_measure_jitter(ec);
487 else
488 data = jent_unbiased_bit(ec);
489
490 /* enforcement of the jent_stuck test */
491 if (ec->stuck) {
492 /*
493 * We only mix in the bit considered not appropriate
494 * without the LSFR. The reason is that if we apply
495 * the LSFR and we do not rotate, the 2nd bit with LSFR
496 * will cancel out the first LSFR application on the
497 * bad bit.
498 *
499 * And we do not rotate as we apply the next bit to the
500 * current bit location again.
501 */
502 ec->data ^= data;
503 ec->stuck = 0;
504 continue;
505 }
506
507 /*
508 * Fibonacci LSFR with polynom of
509 * x^64 + x^61 + x^56 + x^31 + x^28 + x^23 + 1 which is
510 * primitive according to
511 * http://poincare.matf.bg.ac.rs/~ezivkovm/publications/primpol1.pdf
512 * (the shift values are the polynom values minus one
513 * due to counting bits from 0 to 63). As the current
514 * position is always the LSB, the polynom only needs
515 * to shift data in from the left without wrap.
516 */
517 ec->data ^= data;
518 ec->data ^= ((ec->data >> 63) & 1);
519 ec->data ^= ((ec->data >> 60) & 1);
520 ec->data ^= ((ec->data >> 55) & 1);
521 ec->data ^= ((ec->data >> 30) & 1);
522 ec->data ^= ((ec->data >> 27) & 1);
523 ec->data ^= ((ec->data >> 22) & 1);
524 ec->data = jent_rol64(ec->data, 1);
525
526 /*
527 * We multiply the loop value with ->osr to obtain the
528 * oversampling rate requested by the caller
529 */
530 if (++k >= (DATA_SIZE_BITS * ec->osr))
531 break;
532 }
533 if (ec->stir)
534 jent_stir_pool(ec);
535}
536
537/**
538 * The continuous test required by FIPS 140-2 -- the function automatically
539 * primes the test if needed.
540 *
541 * Return:
542 * 0 if FIPS test passed
543 * < 0 if FIPS test failed
544 */
545static void jent_fips_test(struct rand_data *ec)
546{
547 if (!jent_fips_enabled())
548 return;
549
550 /* prime the FIPS test */
551 if (!ec->old_data) {
552 ec->old_data = ec->data;
553 jent_gen_entropy(ec);
554 }
555
556 if (ec->data == ec->old_data)
557 jent_panic("jitterentropy: Duplicate output detected\n");
558
559 ec->old_data = ec->data;
560}
561
562/**
563 * Entry function: Obtain entropy for the caller.
564 *
565 * This function invokes the entropy gathering logic as often to generate
566 * as many bytes as requested by the caller. The entropy gathering logic
567 * creates 64 bit per invocation.
568 *
569 * This function truncates the last 64 bit entropy value output to the exact
570 * size specified by the caller.
571 *
572 * Input:
573 * @ec Reference to entropy collector
574 * @data pointer to buffer for storing random data -- buffer must already
575 * exist
576 * @len size of the buffer, specifying also the requested number of random
577 * in bytes
578 *
579 * @return 0 when request is fulfilled or an error
580 *
581 * The following error codes can occur:
582 * -1 entropy_collector is NULL
583 */
584int jent_read_entropy(struct rand_data *ec, unsigned char *data,
585 unsigned int len)
586{
587 unsigned char *p = data;
588
589 if (!ec)
590 return -1;
591
592 while (0 < len) {
593 unsigned int tocopy;
594
595 jent_gen_entropy(ec);
596 jent_fips_test(ec);
597 if ((DATA_SIZE_BITS / 8) < len)
598 tocopy = (DATA_SIZE_BITS / 8);
599 else
600 tocopy = len;
601 jent_memcpy(p, &ec->data, tocopy);
602
603 len -= tocopy;
604 p += tocopy;
605 }
606
607 return 0;
608}
609
610/***************************************************************************
611 * Initialization logic
612 ***************************************************************************/
613
614struct rand_data *jent_entropy_collector_alloc(unsigned int osr,
615 unsigned int flags)
616{
617 struct rand_data *entropy_collector;
618
619 entropy_collector = jent_zalloc(sizeof(struct rand_data));
620 if (!entropy_collector)
621 return NULL;
622
623 if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
624 /* Allocate memory for adding variations based on memory
625 * access
626 */
627 entropy_collector->mem = jent_zalloc(JENT_MEMORY_SIZE);
628 if (!entropy_collector->mem) {
629 jent_zfree(entropy_collector);
630 return NULL;
631 }
632 entropy_collector->memblocksize = JENT_MEMORY_BLOCKSIZE;
633 entropy_collector->memblocks = JENT_MEMORY_BLOCKS;
634 entropy_collector->memaccessloops = JENT_MEMORY_ACCESSLOOPS;
635 }
636
637 /* verify and set the oversampling rate */
638 if (0 == osr)
639 osr = 1; /* minimum sampling rate is 1 */
640 entropy_collector->osr = osr;
641
642 entropy_collector->stir = 1;
643 if (flags & JENT_DISABLE_STIR)
644 entropy_collector->stir = 0;
645 if (flags & JENT_DISABLE_UNBIAS)
646 entropy_collector->disable_unbias = 1;
647
648 /* fill the data pad with non-zero values */
649 jent_gen_entropy(entropy_collector);
650
651 return entropy_collector;
652}
653
654void jent_entropy_collector_free(struct rand_data *entropy_collector)
655{
656 jent_zfree(entropy_collector->mem);
657 entropy_collector->mem = NULL;
658 jent_zfree(entropy_collector);
659 entropy_collector = NULL;
660}
661
662int jent_entropy_init(void)
663{
664 int i;
665 __u64 delta_sum = 0;
666 __u64 old_delta = 0;
667 int time_backwards = 0;
668 int count_var = 0;
669 int count_mod = 0;
670
671 /* We could perform statistical tests here, but the problem is
672 * that we only have a few loop counts to do testing. These
673 * loop counts may show some slight skew and we produce
674 * false positives.
675 *
676 * Moreover, only old systems show potentially problematic
677 * jitter entropy that could potentially be caught here. But
678 * the RNG is intended for hardware that is available or widely
679 * used, but not old systems that are long out of favor. Thus,
680 * no statistical tests.
681 */
682
683 /*
684 * We could add a check for system capabilities such as clock_getres or
685 * check for CONFIG_X86_TSC, but it does not make much sense as the
686 * following sanity checks verify that we have a high-resolution
687 * timer.
688 */
689 /*
690 * TESTLOOPCOUNT needs some loops to identify edge systems. 100 is
691 * definitely too little.
692 */
693#define TESTLOOPCOUNT 300
694#define CLEARCACHE 100
695 for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) {
696 __u64 time = 0;
697 __u64 time2 = 0;
698 __u64 folded = 0;
699 __u64 delta = 0;
700 unsigned int lowdelta = 0;
701
702 jent_get_nstime(&time);
703 jent_fold_time(NULL, time, &folded, 1<<MIN_FOLD_LOOP_BIT);
704 jent_get_nstime(&time2);
705
706 /* test whether timer works */
707 if (!time || !time2)
708 return JENT_ENOTIME;
709 delta = time2 - time;
710 /*
711 * test whether timer is fine grained enough to provide
712 * delta even when called shortly after each other -- this
713 * implies that we also have a high resolution timer
714 */
715 if (!delta)
716 return JENT_ECOARSETIME;
717
718 /*
719 * up to here we did not modify any variable that will be
720 * evaluated later, but we already performed some work. Thus we
721 * already have had an impact on the caches, branch prediction,
722 * etc. with the goal to clear it to get the worst case
723 * measurements.
724 */
725 if (CLEARCACHE > i)
726 continue;
727
728 /* test whether we have an increasing timer */
729 if (!(time2 > time))
730 time_backwards++;
731
732 /*
733 * Avoid modulo of 64 bit integer to allow code to compile
734 * on 32 bit architectures.
735 */
736 lowdelta = time2 - time;
737 if (!(lowdelta % 100))
738 count_mod++;
739
740 /*
741 * ensure that we have a varying delta timer which is necessary
742 * for the calculation of entropy -- perform this check
743 * only after the first loop is executed as we need to prime
744 * the old_data value
745 */
746 if (i) {
747 if (delta != old_delta)
748 count_var++;
749 if (delta > old_delta)
750 delta_sum += (delta - old_delta);
751 else
752 delta_sum += (old_delta - delta);
753 }
754 old_delta = delta;
755 }
756
757 /*
758 * we allow up to three times the time running backwards.
759 * CLOCK_REALTIME is affected by adjtime and NTP operations. Thus,
760 * if such an operation just happens to interfere with our test, it
761 * should not fail. The value of 3 should cover the NTP case being
762 * performed during our test run.
763 */
764 if (3 < time_backwards)
765 return JENT_ENOMONOTONIC;
766 /* Error if the time variances are always identical */
767 if (!delta_sum)
768 return JENT_EVARVAR;
769
770 /*
771 * Variations of deltas of time must on average be larger
772 * than 1 to ensure the entropy estimation
773 * implied with 1 is preserved
774 */
775 if (delta_sum <= 1)
776 return JENT_EMINVARVAR;
777
778 /*
779 * Ensure that we have variations in the time stamp below 10 for at
780 * least 10% of all checks -- on some platforms, the counter
781 * increments in multiples of 100, but not always
782 */
783 if ((TESTLOOPCOUNT/10 * 9) < count_mod)
784 return JENT_ECOARSETIME;
785
786 return 0;
787}
788