Avoid libc surface under __BARE_METAL__

- __ASSERT halts via infinite loop instead of exit() (avoids newlib
  stdio teardown chain).
- hexdump / dump_trace become no-ops.
- New __weak rand_u32() helper: xorshift on bare-metal, libc rand()
  on hosted. randint() now uses it unconditionally so bare-metal
  callers get a working PRNG.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>

Author: sidcha

include/utils/assert.h

@@ -18,7 +18,24 @@ extern "C" {
 
 #include <zephyr/sys/__assert.h>
 
-#else /* __ZEPHYR__ */
+#elif defined(__BARE_METAL__)
+
+/*
+ * Bare-metal: avoid exit()/atexit cleanup (pulls in newlib stdio teardown).
+ * printf is expected to be provided by the platform; on failure, halt.
+ */
+#define __ASSERT_PRINT(fmt, ...) printf(fmt, ##__VA_ARGS__)
+#define __ASSERT(test, fmt, ...)                                          \
+	do {                                                              \
+		if (!(test)) {                                            \
+			__ASSERT_PRINT("ASSERTION FAIL [%s] @ %s:%d\n\t" fmt "\n", \
+				       STR(test), __FILE__, __LINE__,     \
+				       ##__VA_ARGS__);                    \
+			for (;;) { }                                      \
+		}                                                         \
+	} while (0)
+
+#else /* everything else expects libc */
 
 #define __ASSERT_PRINT(fmt, ...) printf(fmt, ##__VA_ARGS__)
 

include/utils/utils.h

@@ -200,10 +200,22 @@ typedef uint32_t tick_t;
 typedef uint64_t tick_t;
 #endif
 
+/**
+ * @brief Return a non-cryptographic random uint32.
+ *
+ * Bare-metal builds use an internal xorshift; hosted builds forward to
+ * libc rand(). Cryptographic callers must NOT use this.
+ *
+ * Defined as __weak: platforms that can do better (e.g. an STM32 RNG
+ * peripheral, an mbedTLS DRBG, /dev/urandom) should provide their own
+ * strong override.
+ */
+__weak uint32_t rand_u32(void);
+
 /**
  * @brief Return random number between 0 and `limit` both inclusive.
  *
- * Note: the random number generator must be pre-seeded.
+ * Backed by rand_u32(). Non-cryptographic.
  */
 int randint(int limit);
 

src/utils.c

@@ -19,15 +19,35 @@ _Static_assert(IS_ENABLED(_IS_ENABLED_SELFTEST_OFF) == 0,
 	       "IS_ENABLED(): undefined flag must evaluate to 0");
 #undef _IS_ENABLED_SELFTEST_ON
 
+__weak uint32_t rand_u32(void)
+{
+#ifdef __BARE_METAL__
+	/*
+	 * libc's rand() references _impure_ptr, which pulls newlib's
+	 * default FILE table (__sf) and the stdio teardown chain
+	 * (_close_r/_lseek_r/_read_r/_write_r) into the link even when
+	 * nothing else uses stdio. Replace it with a self-contained
+	 * xorshift — same non-cryptographic quality as rand(), zero libc
+	 * dependency.
+	 */
+	static uint32_t s_state = 0x9E3779B9u;
+	uint32_t x = s_state;
+
+	x ^= x << 13;
+	x ^= x >> 17;
+	x ^= x << 5;
+	s_state = x ? x : 1u;
+	return x;
+#else
+	return (uint32_t)rand();
+#endif
+}
+
 int randint(int limit)
 {
-	int r;
-	int divisor = RAND_MAX / (limit + 1);
+	uint32_t span = (uint32_t)limit + 1u;
 
-	do {
-		r = rand() / divisor;
-	} while (r > limit);
-	return r;
+	return (int)(rand_u32() % span);
 }
 
 uint32_t round_up_pow2(uint32_t v)
@@ -54,6 +74,18 @@ int num_digits_in_number(int num)
 	return digits;
 }
 
+#ifdef __BARE_METAL__
+
+__format_printf(3, 4)
+void hexdump(const void *p, size_t len, const char *fmt, ...)
+{
+	ARG_UNUSED(p);
+	ARG_UNUSED(len);
+	ARG_UNUSED(fmt);
+}
+
+#else
+
 __format_printf(3, 4)
 void hexdump(const void *p, size_t len, const char *fmt, ...)
 {
@@ -92,6 +124,8 @@ void hexdump(const void *p, size_t len, const char *fmt, ...)
 	printf("\n");
 }
 
+#endif /* __BARE_METAL__ */
+
 #if (defined(_WIN32) || defined(_WIN64))
 #define WIN32_LEAN_AND_MEAN
 #include <Windows.h>
@@ -283,6 +317,8 @@ void dump_trace(void)
 	puts("");
 	free(strings);
 }
+#elif defined(__BARE_METAL__)
+void dump_trace(void) { }
 #else
 void dump_trace(void)
 {