Description:
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- Built PHP on a system where the SYS_getrandom macro is defined
- Ran the executable on a Kernel that doesn't implement this syscall
- php_random_bytes() is unable to generate random bytes (always returns FAILURE)

I stumbled upon this issue by running a "php:7-alpine" Docker container (from https://hub.docker.com/_/php/) on a Debian 8 with a 3.16 kernel. The getrandom syscall is apparently implemented in linux 3.17.

The problem became visible when session_regenerate_id() failed with the following error:

    Failed to create new session ID: user (path: /sessions)

Which I traced to php_random_bytes() returning FAILURE.

Expected result:
----------------
php_random_bytes() could fallback to other methods, such as readon /dev/urandom, when the "getrandom" syscall is not implemented by the kernel.

Actual result:
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php_random_bytes() calls syscall(SYS_getrandom, ...). This returns -1, with errno=38. php_random_bytes() returns FAILURE.