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Add support for getrandom()

Joseph Richeyrequested to merge
joerichey94/cryptsetup:getrandom into master

Fixes #161 (closed)

Adds support for using the getrandom(2) syscall in cryptsetup. This change is rather large but mainly consists of refactoring lib/random.c to reduce the amount of code that is different between versions that call getrandom() and versions read from /dev/[u]random. As discussed in the issue, this change does not alter the --use-[u]random flags or the CRYPT_RNG_[U]RANDOM libcryptsetup options.

This MR consists of 4 major parts (for more details see the individual commits):

  1. The first two commits change the behavior of _get_random() (which reads from /dev/random). It now no longer prints the percentage progress of key generation and prints the warning message if it will block at all (instead of waiting 5 seconds before warning).
  2. The third commit adds a _getrandom() emulation function, which acts like the getrandom() syscall by reading from /dev/random or /dev/urandom.
  3. The forth commit refactors all the current RNG logic to go through _getrandom()
  4. The final three commits actually implement calling the getrandom() syscall, using a compile time switch to have _getrandom() just call the getrandom() syscall instead of reading from files.

The option to enable cryptsetup is off by default (for now). The code was manually tested with all 4 possible RNG options. This MR doesn't include minor stylistic improvements (see joerichey94/cryptsetup!3 and joerichey94/cryptsetup!4). It also leaves all the FIPS related behavior the same.

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