[RFC][PATCH 01/17] x86/alternative: Unconditional custom return thunk

From: Peter Zijlstra
Date: Wed Aug 09 2023 - 03:27:23 EST


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.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/x86/include/asm/nospec-branch.h | 4 ----
arch/x86/kernel/alternative.c | 2 --
2 files changed, 6 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -347,11 +347,7 @@ 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);
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -698,9 +698,7 @@ void __init_or_module noinline apply_ret

#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.