linux-kernel - [PATCH 10/11] x86: Clean up default rethunk warning

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Message-ID: <20250607095619.149445852@infradead.org>
Date: Sat, 07 Jun 2025 11:42:34 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
 peterz@...radead.org,
 kees@...nel.org,
 acarmina@...hat.com,
 jpoimboe@...nel.org,
 mark.rutland@....com,
 torvalds@...uxfoundation.org
Subject: [PATCH 10/11] x86: Clean up default rethunk warning

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

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

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/entry/entry.S               |    4 ----
 arch/x86/include/asm/nospec-branch.h |    2 --
 arch/x86/kernel/cpu/bugs.c           |    5 -----
 arch/x86/lib/retpoline.S             |   22 +++++++++++++++-------
 4 files changed, 15 insertions(+), 18 deletions(-)

--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -13,8 +13,6 @@
 #include <asm/cpufeatures.h>
 #include <asm/nospec-branch.h>
 
-#include "calling.h"
-
 .pushsection .noinstr.text, "ax"
 
 /* Clobbers AX, CX, DX */
@@ -61,8 +59,6 @@ EXPORT_SYMBOL_GPL(mds_verw_sel);
 
 .popsection
 
-THUNK warn_thunk_thunk, __warn_thunk
-
 /*
  * Clang's implementation of TLS stack cookies requires the variable in
  * question to be a TLS variable. If the variable happens to be defined as an
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -386,8 +386,6 @@ 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
@@ -3415,8 +3415,3 @@ ssize_t cpu_show_indirect_target_selecti
 	return cpu_show_common(dev, attr, buf, X86_BUG_ITS);
 }
 #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
@@ -12,6 +12,7 @@
 #include <asm/percpu.h>
 #include <asm/frame.h>
 #include <asm/nops.h>
+#include <asm/bug.h>
 
 	.section .text..__x86.indirect_thunk
 
@@ -416,6 +417,19 @@ EXPORT_SYMBOL(its_return_thunk)
 
 #endif /* CONFIG_MITIGATION_ITS */
 
+        .pushsection        .rodata.str1.1
+.Lwarn:
+        .string "Unpatched return thunk in use.\n"
+	.popsection
+
+/*
+ * Helper that will trip WARN_ONCE() after alternatives have ran.
+ */
+#define ALT_WARN_RETHUNK						\
+	ANNOTATE_REACHABLE ;						\
+	1: ALTERNATIVE "", "ud2", X86_FEATURE_ALWAYS ;			\
+	ASM_BUGTABLE_FLAGS(1b, .Lwarn, 0, 0, BUGFLAG_WARNING | BUGFLAG_ONCE )
+
 /*
  * This function name is magical and is used by -mfunction-return=thunk-extern
  * for the compiler to generate JMPs to it.
@@ -432,15 +446,9 @@ EXPORT_SYMBOL(its_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
+	ALT_WARN_RETHUNK
 	ANNOTATE_UNRET_SAFE
 	ret
-#endif
 	int3
 SYM_CODE_END(__x86_return_thunk)
 SYM_PIC_ALIAS(__x86_return_thunk)