x86/alternative: Make custom return thunk unconditional

There is infrastructure to rewrite return thunks to point to any
random thunk one desires, unwrap that from CALL_THUNKS, which up to
now was the sole user of that.

  [ bp: Make the thunks visible on 32-bit and add ifdeffery for the
    32-bit builds. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814121148.775293785@infradead.org

Author: Peter Zijlstra

arch/x86/include/asm/nospec-branch.h

@@ -341,17 +341,18 @@ extern retpoline_thunk_t __x86_indirect_thunk_array[];
 extern retpoline_thunk_t __x86_indirect_call_thunk_array[];
 extern retpoline_thunk_t __x86_indirect_jump_thunk_array[];
 
+#ifdef CONFIG_RETHUNK
 extern void __x86_return_thunk(void);
+#else
+static inline void __x86_return_thunk(void) {}
+#endif
+
 extern void zen_untrain_ret(void);
 extern void srso_untrain_ret(void);
 extern void srso_untrain_ret_alias(void);
 extern void entry_ibpb(void);
 
-#ifdef CONFIG_CALL_THUNKS
 extern void (*x86_return_thunk)(void);
-#else
-#define x86_return_thunk	(&__x86_return_thunk)
-#endif
 
 #ifdef CONFIG_CALL_DEPTH_TRACKING
 extern void __x86_return_skl(void);

arch/x86/kernel/alternative.c

@@ -687,10 +687,6 @@ void __init_or_module noinline apply_retpolines(s32 *start, s32 *end)
 
 #ifdef CONFIG_RETHUNK
 
-#ifdef CONFIG_CALL_THUNKS
-void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
-#endif
-
 /*
  * Rewrite the compiler generated return thunk tail-calls.
  *

arch/x86/kernel/cpu/bugs.c

@@ -63,6 +63,8 @@ EXPORT_SYMBOL_GPL(x86_pred_cmd);
 
 static DEFINE_MUTEX(spec_ctrl_mutex);
 
+void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
+
 /* Update SPEC_CTRL MSR and its cached copy unconditionally */
 static void update_spec_ctrl(u64 val)
 {