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random(4): Translate a comment requirement into a compile-time invariant

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In various places, random represents the set of sources as a 32-bit word
bitmask.  It assumes all sources fit within this, i.e., the maximum valid
source number is 31.

There was a comment specifying this limitation, but we can actually refuse
to compile if our assumption is violated instead.  We still have a few spare
random source slots, but sooner or later someone may need to convert the
masks used from raw 32-bit words to bitset(9) APIs.

This prevents some kinds of developer foot-shooting when adding new random
sources.  No functional change.

Reviewed by:	delphij, markm
Approved by:	secteam (delphij)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D16982
This commit is contained in:
Conrad Meyer 2018-10-20 20:49:37 +00:00
parent e41793db2c
commit b0dee75e64
1 changed files with 2 additions and 3 deletions
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5
sys/sys/random.h
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@ -57,9 +57,6 @@ read_random(void *a __unused, u_int b __unused)
* Note: if you add or remove members of random_entropy_source, remember to
* also update the strings in the static array random_source_descr[] in
* random_harvestq.c.
*
* NOTE: complain loudly to markm@ or on the lists if this enum gets more than 32
* distinct values (0-31)! ENTROPYSOURCE may be == 32, but not > 32.
*/
enum random_entropy_source {
RANDOM_START = 0,
@ -92,6 +89,8 @@ enum random_entropy_source {
RANDOM_PURE_DARN,
ENTROPYSOURCE
};
_Static_assert(ENTROPYSOURCE <= 32,
"hardcoded assumption that values fit in a typical word-sized bitset");
#define RANDOM_HARVEST_EVERYTHING_MASK ((1 << (RANDOM_ENVIRONMENTAL_END + 1)) - 1)
#define RANDOM_HARVEST_PURE_MASK (((1 << ENTROPYSOURCE) - 1) & (-1UL << RANDOM_PURE_START))