rtos: string: Add memory barrier to memset_s for security

Add Xtensa memory barrier (memw) instruction after memset() in
memset_s() implementation to prevent compiler dead store elimination
(DSE) optimization from removing the memory clearing operation.

When optimization flags like -O2 are enabled, compilers may perform dead
store elimination and incorrectly remove memset() calls used for
security purposes to scrub sensitive data from memory. This is critical
for confidential data handling where memory must be reliably cleared
after use.

The memory barrier ensures the memset operation completes and cannot be
optimized away, satisfying secure memory scrubbing requirements for
cryptographic operations and sensitive data processing.

Additionally, the patch removes the check for the return value of
memset. The standard C library memset always returns the pointer passed
as its first argument and does not indicate errors through its return
value. Error handling for a NULL destination is already performed
earlier in the function, so the return value check is unnecessary and
can be safely omitted.

Signed-off-by: Tomasz Leman <tomasz.m.leman@intel.com>

Author: tmleman

zephyr/include/rtos/string.h

@@ -69,8 +69,14 @@ static inline int memset_s(void *dest, size_t dest_size, int data, size_t count)
 	if (count > dest_size)
 		return -EINVAL;
 
-	if (!memset(dest, data, count))
-		return -ENOMEM;
+	memset(dest, data, count);
+	/*
+	 * Prevent compiler from optimizing away the memset.
+	 * Memory barrier prevents dead store elimination.
+	 */
+#if defined(CONFIG_XTENSA)
+	__asm__ __volatile__("memw" ::: "memory");
+#endif
 
 	return 0;
 }