Kernel: Add a more expressive API for getting random bytes

We now have these API's in <Kernel/Random.h>:

    - get_fast_random_bytes(u8* buffer, size_t buffer_size)
    - get_good_random_bytes(u8* buffer, size_t buffer_size)
    - get_fast_random<T>()
    - get_good_random<T>()

Internally they both use x86 RDRAND if available, otherwise they fall
back to the same LCG we had in RandomDevice all along.

The main purpose of this patch is to give kernel code a way to better
express its needs for random data.

Randomness is something that will require a lot more work, but this is
hopefully a step in the right direction.

1 files changed, 41 insertions, 0 deletions

diff --git a/Kernel/Random.cpp b/Kernel/Random.cpp
new file mode 100644
index 0000000000..d517ae5cef
--- /dev/null
+++ b/Kernel/Random.cpp
@@ -0,0 +1,41 @@
+#include <Kernel/Arch/i386/CPU.h>
+#include <Kernel/Random.h>
+#include <Kernel/Devices/RandomDevice.h>
+
+static u32 random32()
+{
+    if (g_cpu_supports_rdrand) {
+        u32 value = 0;
+        asm volatile(
+            "1%=:\n"
+            "rdrand %0\n"
+            "jnc 1%=\n"
+            : "=r"(value));
+        return value;
+    }
+    // FIXME: This sucks lol
+    static u32 next = 1;
+    next = next * 1103515245 + 12345;
+    return next;
+}
+
+void get_good_random_bytes(u8* buffer, size_t buffer_size)
+{
+    union {
+        u8 bytes[4];
+        u32 value;
+    } u;
+    size_t offset = 4;
+    for (size_t i = 0; i < buffer_size; ++i) {
+        if (offset >= 4) {
+            u.value = random32();
+            offset = 0;
+        }
+        buffer[i] = u.bytes[offset++];
+    }
+}
+
+void get_fast_random_bytes(u8* buffer, size_t buffer_size)
+{
+    return get_good_random_bytes(buffer, buffer_size);
+}