[RFC 8/8] x86: Clean up default rethunk warning

Peter Zijlstra
2025-06-02T14:50:39Z

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@infradead.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 | 24 +++++++++++++++++-------
4 files changed, 17 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 */
@@ -53,8 +51,6 @@ 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 +417,21 @@ 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 \
+ 1: ALTERNATIVE "", "ud2", X86_FEATURE_ALWAYS ; \
+ ASM_BUGTABLE_FLAGS(1b, .Lwarn, \
+ 0, 0, BUGFLAG_WARNING | \
+ BUGFLAG_ONCE | BUGFLAG_FORMAT) ; \
+ ANNOTATE_REACHABLE
+
/*
* This function name is magical and is used by -mfunction-return=thunk-extern
* for the compiler to generate JMPs to it.
@@ -432,15 +448,9 @@ 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)