[ruby-core:122566] [Ruby Bug#21448] Random.urandom may fail to fall back to reading /dev/urandom on Linux < 3.17

Issue #21448 has been updated by nobu (Nobuyoshi Nakada).

Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.2: REQUIRED, 3.3: REQUIRED, 3.4: REQUIRED

Unfortunately, despite the same signature, `getentropy` is different thing in different layers depending on the platform.

* `getentropy(3)` - a library function on Linux:

    > The `getentropy()` function is implemented using `getrandom(2)`.

* `getentropy(2)` - a system call on macOS:

    > Unlike the random(4) pseudo-devices, it is not vulnerable to file descriptor exhaustion attacks and is available when sandboxed or in a chroot, making it more reliable for security-critical applications.

I think we should use `getentropy()` only on macOS.

It could be solved by either of the following patches.

```diff
diff --git i/configure.ac w/configure.ac
index 7270bd5e8b9..144e11fcb57 100644
--- i/configure.ac
+++ w/configure.ac
@@ -2137,3 +2137,3 @@ AC_CHECK_FUNCS(getcwd)
 AC_CHECK_FUNCS(getegid)
-AC_CHECK_FUNCS(getentropy)
+AS_CASE([$target_os], [darwin*], [AC_CHECK_FUNCS(getentropy)])
 AC_CHECK_FUNCS(geteuid)
```

```diff
diff --git i/random.c w/random.c
index f6f63e4cffe..7423f48b1a5 100644
--- i/random.c
+++ w/random.c
@@ -440,3 +440,3 @@ random_init(int argc, VALUE *argv, VALUE obj)
 
-#ifdef HAVE_GETENTROPY
+#if defined(HAVE_GETENTROPY) && defined(__APPLE__)
 # define MAX_SEED_LEN_PER_READ 256
```

----------------------------------------
Bug #21448: Random.urandom may fail to fall back to reading /dev/urandom on Linux < 3.17
https://bugs.ruby-lang.org/issues/21448#change-113796

* Author: rhenium (Kazuki Yamaguchi)
* Status: Open
* Backport: 3.2: REQUIRED, 3.3: REQUIRED, 3.4: REQUIRED
----------------------------------------
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 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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