[RFC][PATCH 2/2] x86: Clean up default rethunk warning

[Peter Zijlstra <peterz@infradead.org>]

Replace the funny __warn_thunk thing with a more regular
WARN_ON_ONCE(), and simplify the ifdeffery.

Notably this avoids RET from having recursive RETs (once from the
thunk and once from the C function) -- recursive RET makes my head
hurt for no good reason.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/entry/entry.S               |    3 ---
 arch/x86/include/asm/nospec-branch.h |    2 --
 arch/x86/kernel/cpu/bugs.c           |    5 -----
 arch/x86/lib/retpoline.S             |   20 ++++++++++++--------
 4 files changed, 12 insertions(+), 18 deletions(-)

--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -10,8 +10,6 @@
 #include <asm/segment.h>
 #include <asm/cache.h>
 
-#include "calling.h"
-
 .pushsection .noinstr.text, "ax"
 
 SYM_FUNC_START(entry_ibpb)
@@ -45,4 +43,3 @@ EXPORT_SYMBOL_GPL(mds_verw_sel);
 
 .popsection
 
-THUNK warn_thunk_thunk, __warn_thunk
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -387,8 +387,6 @@ extern void clear_bhb_loop(void);
 
 extern void (*x86_return_thunk)(void);
 
-extern void __warn_thunk(void);
-
 #ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING
 extern void call_depth_return_thunk(void);
 
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -3025,8 +3025,3 @@ ssize_t cpu_show_reg_file_data_sampling(
 	return cpu_show_common(dev, attr, buf, X86_BUG_RFDS);
 }
 #endif
-
-void __warn_thunk(void)
-{
-	WARN_ONCE(1, "Unpatched return thunk in use. This should not happen!\n");
-}
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -12,9 +12,14 @@
 #include <asm/percpu.h>
 #include <asm/frame.h>
 #include <asm/nops.h>
+#include <asm/bug.h>
 
-	.section .text..__x86.indirect_thunk
+#define WARN_ONCE							\
+	1: ALTERNATIVE "", "ud2", X86_FEATURE_ALWAYS ;			\
+	ASM_BUGTABLE_FLAGS(1b, 0, 0, BUGFLAG_WARNING | BUGFLAG_ONCE) ;	\
+	REACHABLE
 
+	.section .text..__x86.indirect_thunk
 
 .macro POLINE reg
 	ANNOTATE_INTRA_FUNCTION_CALL
@@ -382,16 +387,15 @@ SYM_FUNC_END(call_depth_return_thunk)
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
-#if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
-    defined(CONFIG_MITIGATION_SRSO) || \
-    defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
-	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
-		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
-#else
+
+#ifdef CONFIG_X86_64
+	WARN_ONCE
+#endif
+
 	ANNOTATE_UNRET_SAFE
 	ret
-#endif
 	int3
+
 SYM_CODE_END(__x86_return_thunk)
 EXPORT_SYMBOL(__x86_return_thunk)