Detect broken RDRAND during initialization.

Some CPUs advertise RDRAND in CPUID, but return
0xFFFFFFFF unconditionally.  To avoid locking up
later, test RDRAND during initialization, and if
it returns 0xFFFFFFFF, mark it as nonexistent.

Also fix a possible segmentation fault in CPUID check.

This commit is a squashed backport of the following
commits on the master branch:

  * 0e5bbcaa16
  * 4d36b0287d
  * 8086314026

random_seed.c

@@ -26,19 +26,8 @@
 static void do_cpuid(int regs[], int h)
 {
 	/* clang-format off */
-    __asm__ __volatile__(
-#if defined __x86_64__
-                         "pushq %%rbx;\n"
-#else
-                         "pushl %%ebx;\n"
-#endif
-                         "cpuid;\n"
-#if defined __x86_64__
-                         "popq %%rbx;\n"
-#else
-                         "popl %%ebx;\n"
-#endif
-                         : "=a"(regs[0]), [ebx] "=r"(regs[1]), "=c"(regs[2]), "=d"(regs[3])
+    __asm__ __volatile__("cpuid"
+                         : "=a"(regs[0]), "=b"(regs[1]), "=c"(regs[2]), "=d"(regs[3])
                          : "a"(h));
 	/* clang-format on */
 }
@@ -54,12 +43,51 @@ static void do_cpuid(int regs[], int h)
 
 #if HAS_X86_CPUID
 
+static int get_rdrand_seed(void);
+
+/* Valid values are -1 (haven't tested), 0 (no), and 1 (yes). */
+static int _has_rdrand = -1;
+
 static int has_rdrand(void)
 {
-	// CPUID.01H:ECX.RDRAND[bit 30] == 1
+	if (_has_rdrand != -1)
+	{
+		return _has_rdrand;
+	}
+
+	/* CPUID.01H:ECX.RDRAND[bit 30] == 1 */
 	int regs[4];
 	do_cpuid(regs, 1);
-	return (regs[2] & (1 << 30)) != 0;
+	if (!(regs[2] & (1 << 30)))
+	{
+		_has_rdrand = 0;
+		return 0;
+	}
+
+	/*
+	 * Some CPUs advertise RDRAND in CPUID, but return 0xFFFFFFFF
+	 * unconditionally. To avoid locking up later, test RDRAND here. If over
+	 * 3 trials RDRAND has returned the same value, declare it broken.
+	 * Example CPUs are AMD Ryzen 3000 series
+	 * and much older AMD APUs, such as the E1-1500
+	 * https://github.com/systemd/systemd/issues/11810
+	 * https://linuxreviews.org/RDRAND_stops_returning_random_values_on_older_AMD_CPUs_after_suspend
+	 */
+	_has_rdrand = 0;
+	int prev = get_rdrand_seed();
+	for (int i = 0; i < 3; i++)
+	{
+		int temp = get_rdrand_seed();
+		if (temp != prev)
+		{
+			_has_rdrand = 1;
+			break;
+		}
+
+		prev = temp;
+	}
+
+	return _has_rdrand;
 }
 
 #endif
@@ -74,7 +102,7 @@ static int get_rdrand_seed(void)
 {
 	DEBUG_SEED("get_rdrand_seed");
 	int _eax;
-	// rdrand eax
+	/* rdrand eax */
 	/* clang-format off */
 	__asm__ __volatile__("1: .byte 0x0F\n"
 	                     "   .byte 0xC7\n"
@@ -114,7 +142,7 @@ static int get_rdrand_seed(void)
 	DEBUG_SEED("get_rdrand_seed");
 	int _eax;
 retry:
-	// rdrand eax
+	/* rdrand eax */
 	__asm _emit 0x0F __asm _emit 0xC7 __asm _emit 0xF0
 	__asm jnc retry
 	__asm mov _eax, eax
@@ -188,6 +216,10 @@ static int get_dev_random_seed(void)
 
 /* clang-format off */
 #include <windows.h>
+
+/* Caution: these blank lines must remain so clang-format doesn't reorder
+   includes to put windows.h after wincrypt.h */
+
 #include <wincrypt.h>
 /* clang-format on */
 #ifndef __GNUC__