linux-kernel - [PATCH 6/7] x86/speculation: Annotate retpoline return instructions

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Message-Id: <20200402082220.808-7-alexandre.chartre@oracle.com>
Date:   Thu,  2 Apr 2020 10:22:19 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        peterz@...radead.org, jthierry@...hat.com, tglx@...utronix.de,
        alexandre.chartre@...cle.com
Subject: [PATCH 6/7] x86/speculation: Annotate retpoline return instructions

With retpoline, the return instruction (ret) is used to branch to an
address stored on the stack. Provide a macro to annotate such trampoline
returns so they can be properly handled by objtool, and use this macro
to annotate retpoline return instructions.

Signed-off-by: Alexandre Chartre <alexandre.chartre@...cle.com>
---
 arch/x86/include/asm/nospec-branch.h | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index a2885f801e13..9ae6dde2f10b 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -91,6 +91,20 @@
 	call \dst
 .endm
 
+/*
+ * Retpoline return instruction. This should be used as a substitute
+ * for the ret instruction when doing a trampoline return. It is
+ * similar to the ret instruction but it tells objtool this is a
+ * trampoline return.
+ */
+.macro RETPOLINE_RET
+	.Lannotate_\@:
+	.pushsection .discard.retpoline_ret
+	_ASM_PTR .Lannotate_\@
+	.popsection
+	ret
+.endm
+
 /*
  * These are the bare retpoline primitives for indirect jmp and call.
  * Do not use these directly; they only exist to make the ALTERNATIVE
@@ -104,7 +118,7 @@
 	jmp	.Lspec_trap_\@
 .Ldo_rop_\@:
 	mov	\reg, (%_ASM_SP)
-	ret
+	RETPOLINE_RET
 .endm
 
 /*
-- 
2.18.2