lx_emul: implement __get_random_u32_below

The implementation was already part of the Zynq's sd_card driver and
since other ARM drivers need it as well, promote it to the common
shadow library.

Issue genodelabs/genode-allwinner#21.

repos/dde_linux/src/app/wireguard/lx_emul.c

@@ -39,18 +39,6 @@ int wait_for_random_bytes(void)
 
 #include <linux/prandom.h>
 
-u32 prandom_u32(void)
-{
-	return lx_emul_random_gen_u32();
-}
-
-
-u32 __get_random_u32_below(u32 ceil)
-{
-	return lx_emul_random_gen_u32();
-}
-
-
 u8 get_random_u8(void)
 {
 	u8 ret;

repos/dde_linux/src/lib/lx_emul/shadow/drivers/char/random.c

@@ -60,3 +60,30 @@ bool rng_is_initialized(void)
 {
 	return 1;
 }
+
+
+u32 __get_random_u32_below(u32 ceil)
+{
+	/**
+	 * Returns a random number from the half-open interval [0, ceil)
+	 * with uniform distribution.
+	 *
+	 * The idea here is to split [0, 2^32) into #ceil bins. By dividing a random
+	 * number from the 32-bit interval, we can determine into which bin the number
+	 * fell.
+	 */
+
+	/* determine divisor to determine bin number by dividing 2^32 by ceil */
+	u32 div = 0x100000000ULL / ceil;
+
+	/**
+	 * In case the above division has a remainder, we will end up with an
+	 * additional (but smaller) bin at the end of the 32-bit interval. We'll
+	 * discard the result if the number fell into this bin and repeat.
+	 */
+	u32 result = ceil;
+	while (result >= ceil)
+		result = lx_emul_random_gen_u32() / div;
+
+	return result;
+}

repos/dde_linux/src/test/driver_time/dummies.c

@@ -22,14 +22,6 @@ void register_syscore_ops(struct syscore_ops * ops)
 }
 
 
-#include <linux/random.h>
-
-u32 __get_random_u32_below(u32 ceil)
-{
-	lx_emul_trace_and_stop(__func__);
-}
-
-
 #include <linux/interrupt.h>
 
 DEFINE_STATIC_KEY_FALSE(force_irqthreads_key);

repos/pc/src/drivers/framebuffer/intel/pc/dummies.c

@@ -422,14 +422,6 @@ void pci_disable_msi(struct pci_dev *dev)
 }
 
 
-#include <linux/random.h>
-
-u32 __get_random_u32_below(u32 ceil)
-{
-	lx_emul_trace_and_stop(__func__);
-}
-
-
 #include <asm/smp.h>
 
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);

repos/pc/src/drivers/usb_host/pc/dummies.c

@@ -85,14 +85,6 @@ int __printk_ratelimit(const char * func)
 }
 
 
-#include <linux/prandom.h>
-
-u32 prandom_u32(void)
-{
-	lx_emul_trace_and_stop(__func__);
-}
-
-
 #include <linux/pci.h>
 
 void pci_disable_device(struct pci_dev * dev)
@@ -101,14 +93,6 @@ void pci_disable_device(struct pci_dev * dev)
 }
 
 
-#include <linux/random.h>
-
-u32 __get_random_u32_below(u32 ceil)
-{
-	lx_emul_trace_and_stop(__func__);
-}
-
-
 #include <asm/smp.h>
 
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);

repos/pc/src/lib/pc_wifi/dummies.c

@@ -386,14 +386,6 @@ void ieee80211_free_led_names(struct ieee80211_local * local)
 pteval_t __default_kernel_pte_mask __read_mostly = ~0;
 
 
-#include <linux/random.h>
-
-u32 __get_random_u32_below(u32 ceil)
-{
-	lx_emul_trace_and_stop(__func__);
-}
-
-
 u16 get_random_u16(void)
 {
 	lx_emul_trace_and_stop(__func__);