[PATCH 12/12] config.mak.uname: add a note about CSPRNG_METHOD for Linux

Commit 05cd988dce ("wrapper: add a helper to generate numbers from a CSPRNG", 2022-01-17) added a csprng_bytes() function which used one of several interfaces to provide a source of cryptographically secure pseudorandom numbers. The CSPRNG_METHOD make variable was provided to determine the choice of available 'backends' for the source of random bytes.

Commit 05cd988dce did not set CSPRNG_METHOD in the Linux section of the config.mak.uname file, so it defaults to using '/dev/urandom' as the source of random bytes. The 'backend' values which could be used on Linux, in order of preference, are 'arc4random', 'getrandom' or 'getentropy' ('openssl' is an option, but seems to be discouraged).

The arc4random routines (ar4random_buf() is the one actually used) were added to glibc in version 2.36, while both getrandom() and getentropy() were included in 2.25. So, some of the more up-to-date distributions of Linux (eg Debian 12, Ubuntu 24.04) would be able to use the preferred 'arc4random' setting.

If the meson build system is used on a newer platform, then they will be configured to use 'arc4random', whereas the make build will currently default to using '/dev/urandom'. Add a note to the config.mak.uname file, in the Linux section, to prompt make users to override CSPRNG_METHOD in the config.mak file, if appropriate.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
---
 config.mak.uname | 2 ++
 1 file changed, 2 insertions(+)