Bug #21448
open
Random.urandom may fail to fall back to reading /dev/urandom on Linux < 3.17
Description
Origianlly reported for tmpdir: https://github.com/ruby/tmpdir/issues/50
On Linux, Random.urandom is expected to first attempt the getrandom(2) syscall (Linux >= 3.17), and fall back to reading from /dev/urandom if it is not supported.
In Ruby 3.1, commit 54c91185c9273b9699693910fa95383c86f2af22 replaced the fallback routine that read from /dev/urandom with a call to getentropy(3), if available at compile time.
On Linux, glibc 2.25 and musl 1.1.20 started to provide a getentropy(3) implementation based on getrandom(2). If Ruby is compiled with such a libc version but run on Linux 3.16 or earlier, both getrandom() and getentropy(3) fail. As a result, Random.urandom becomes unusable, even though /dev/urandom is still available.
I couldn't find the orignal issue the commit was intended to address, except that it appears to related to macOS. Is there a scenario on macOS where CCRandomGenerateBytes() or SecRandomCopyBytes() might fail, while getentropy() will still succeed?
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