random: remove get_random_bytes_arch() and add rng_has_arch_random()

The RNG incorporates RDRAND into its state at boot and every time it
reseeds, so there's no reason for callers to use it directly. The
hashing that the RNG does on it is preferable to using the bytes raw.

The only current use case of get_random_bytes_arch() is vsprintf's
siphash key for pointer hashing, which uses it to initialize the pointer
secret earlier than usual if RDRAND is available. In order to replace
this narrow use case, just expose whether RDRAND is mixed into the RNG,
with a new function called rng_has_arch_random(). With that taken care
of, there are no users of get_random_bytes_arch() left, so it can be
removed.

Later, if trust_cpu gets turned on by default (as most distros are
doing), this one use of rng_has_arch_random() can probably go away as
well.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Petr Mladek <pmladek@suse.com> # for vsprintf.c
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

include/linux/random.h

@@ -38,7 +38,6 @@ static inline int unregister_random_vmfork_notifier(struct notifier_block *nb) {
 #endif
 
 void get_random_bytes(void *buf, size_t len);
-size_t __must_check get_random_bytes_arch(void *buf, size_t len);
 u32 get_random_u32(void);
 u64 get_random_u64(void);
 static inline unsigned int get_random_int(void)
@@ -77,6 +76,7 @@ unsigned long randomize_page(unsigned long start, unsigned long range);
 
 int __init random_init(const char *command_line);
 bool rng_is_initialized(void);
+bool rng_has_arch_random(void);
 int wait_for_random_bytes(void);
 int register_random_ready_notifier(struct notifier_block *nb);
 int unregister_random_ready_notifier(struct notifier_block *nb);