linux-kernel - [PATCH 1/2] x86/ibt: Suppress spurious ENDBR

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Date:   Wed, 02 Aug 2023 12:55:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        jpoimboe@...nel.org, andrew.cooper3@...rix.com,
        David.Kaplan@....com
Subject: [PATCH 1/2] x86/ibt: Suppress spurious ENDBR

It was reported that under certain circumstances GCC emits ENDBR
instructions for _THIS_IP_ usage. Specifically, when it appears at the
start of a basic block -- but not elsewhere.

Since _THIS_IP_ is never used for control flow, these ENDBR
instructions are completely superfluous. Override the _THIS_IP_
definition for x86_64 to avoid this.

Less ENDBR instructions is better.

Fixes: 156ff4a544ae ("x86/ibt: Base IBT bits")
Reported-by: David Kaplan <David.Kaplan@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/linkage.h      |    8 ++++++++
 include/linux/instruction_pointer.h |    5 +++++
 2 files changed, 13 insertions(+)

--- a/arch/x86/include/asm/linkage.h
+++ b/arch/x86/include/asm/linkage.h
@@ -8,6 +8,14 @@
 #undef notrace
 #define notrace __attribute__((no_instrument_function))
 
+#ifdef CONFIG_64BIT
+/*
+ * The generic version tends to create spurious ENDBR instructions under
+ * certain conditions.
+ */
+#define _THIS_IP_ ({ unsigned long __here; asm ("lea 0(%%rip), %0" : "=r" (__here)); __here; })
+#endif
+
 #ifdef CONFIG_X86_32
 #define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))
 #endif /* CONFIG_X86_32 */
--- a/include/linux/instruction_pointer.h
+++ b/include/linux/instruction_pointer.h
@@ -2,7 +2,12 @@
 #ifndef _LINUX_INSTRUCTION_POINTER_H
 #define _LINUX_INSTRUCTION_POINTER_H
 
+#include <asm/linkage.h>
+
 #define _RET_IP_		(unsigned long)__builtin_return_address(0)
+
+#ifndef _THIS_IP_
 #define _THIS_IP_  ({ __label__ __here; __here: (unsigned long)&&__here; })
+#endif
 
 #endif /* _LINUX_INSTRUCTION_POINTER_H */