Subject: [PATCH bpf-next v4 02/27] x86/ftrace: Remove fault protection code in prepare_ftrace_return

From: Jiri Olsa <[email-masked]>
Date: Thu, 26 Aug 2021 21:38:57 +0200

From: "Steven Rostedt (VMware)" <[email-masked]>

Removing the fault protection code when writing return_hooker
to stack. As Steven noted:

> That protection was there from the beginning due to being "paranoid",
> considering ftrace was bricking network cards. But that protection
> would not have even protected against that.

Signed-off-by: Steven Rostedt (VMware) <[email-masked]>
Signed-off-by: Jiri Olsa <[email-masked]>
---
 arch/x86/kernel/ftrace.c | 38 +++-----------------------------------
 1 file changed, 3 insertions(+), 35 deletions(-)

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 1b3ce3b4a2a2..c555624da989 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -625,12 +625,10 @@ int ftrace_disable_ftrace_graph_caller(void)
  * Hook the return address and push it in the stack of return addrs
  * in current thread info.
  */
-void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
+void prepare_ftrace_return(unsigned long ip, unsigned long *parent,
 			   unsigned long frame_pointer)
 {
 	unsigned long return_hooker = (unsigned long)&return_to_handler;
-	unsigned long old;
-	int faulted;
 
 	/*
 	 * When resuming from suspend-to-ram, this function can be indirectly
@@ -650,37 +648,7 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		return;
 
-	/*
-	 * Protect against fault, even if it shouldn't
-	 * happen. This tool is too much intrusive to
-	 * ignore such a protection.
-	 */
-	asm volatile(
-		"1: " _ASM_MOV " (%[parent]), %[old]\n"
-		"2: " _ASM_MOV " %[return_hooker], (%[parent])\n"
-		"   movl $0, %[faulted]\n"
-		"3:\n"
-
-		".section .fixup, \"ax\"\n"
-		"4: movl $1, %[faulted]\n"
-		"   jmp 3b\n"
-		".previous\n"
-
-		_ASM_EXTABLE(1b, 4b)
-		_ASM_EXTABLE(2b, 4b)
-
-		: [old] "=&r" (old), [faulted] "=r" (faulted)
-		: [parent] "r" (parent), [return_hooker] "r" (return_hooker)
-		: "memory"
-	);
-
-	if (unlikely(faulted)) {
-		ftrace_graph_stop();
-		WARN_ON(1);
-		return;
-	}
-
-	if (function_graph_enter(old, self_addr, frame_pointer, parent))
-		*parent = old;
+	if (!function_graph_enter(*parent, ip, frame_pointer, parent))
+		*parent = return_hooker;
 }
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
-- 
2.31.1