[tip: x86/urgent] x86/ibt: Suppress spurious ENDBR

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     25e73b7e3f72a25aa30cbb2eecb49036e0acf066
Gitweb:        https://git.kernel.org/tip/25e73b7e3f72a25aa30cbb2eecb49036e0acf066
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Wed, 02 Aug 2023 12:55:46 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Tue, 12 Sep 2023 17:50:53 +02:00

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@xxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230802110323.016197440@xxxxxxxxxxxxx
---
 arch/x86/include/asm/linkage.h      | 8 ++++++++
 include/linux/instruction_pointer.h | 5 +++++
 2 files changed, 13 insertions(+)

diff --git a/arch/x86/include/asm/linkage.h b/arch/x86/include/asm/linkage.h
index 97a3de7..5ff49fd 100644
--- 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 */
diff --git a/include/linux/instruction_pointer.h b/include/linux/instruction_pointer.h
index cda1f70..aa0b3ff 100644
--- 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 */