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: 156ff4a ("x86/ibt: Base IBT bits")
Reported-by: David Kaplan <David.Kaplan@amd.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230802110323.016197440@infradead.org

Author: Peter Zijlstra

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 */

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 */