linux-kernel - [patch part-II V2 10/13] x86/entry/common: Split prepare_exit_to_usermode() and syscall_return

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Date:   Sun, 08 Mar 2020 23:24:09 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>
Subject: [patch part-II V2 10/13] x86/entry/common: Split prepare_exit_to_usermode() and syscall_return_slowpath()

Split the functions into traceable and probale parts and a part protected
from instrumentation.

This is required because after calling user_exit_irqsoff() kprobes and
tracepoints/function entry/exit which can be utilized by e.g. BPF are not
longer safe vs. RCU.

Preparatory step to move irq flags tracing to C code.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
V2: New patch to reduce the churn
---
 arch/x86/entry/common.c |   25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -206,7 +206,7 @@ static void exit_to_usermode_loop(struct
 }
 
 /* Called with IRQs disabled. */
-__visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
+static noinline void __prepare_exit_to_usermode(struct pt_regs *regs)
 {
 	struct thread_info *ti = current_thread_info();
 	u32 cached_flags;
@@ -245,11 +245,16 @@ static void exit_to_usermode_loop(struct
 	 */
 	ti->status &= ~(TS_COMPAT|TS_I386_REGS_POKED);
 #endif
+}
 
-	user_enter_irqoff();
+__visible inline notrace void prepare_exit_to_usermode(struct pt_regs *regs)
+{
+	__prepare_exit_to_usermode(regs);
 
+	user_enter_irqoff();
 	mds_user_clear_cpu_buffers();
 }
+NOKPROBE_SYMBOL(prepare_exit_to_usermode);
 
 #define SYSCALL_EXIT_WORK_FLAGS				\
 	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT |	\
@@ -277,11 +282,7 @@ static void syscall_slow_exit_work(struc
 		tracehook_report_syscall_exit(regs, step);
 }
 
-/*
- * Called with IRQs on and fully valid regs.  Returns with IRQs off in a
- * state such that we can immediately switch to user mode.
- */
-__visible inline void syscall_return_slowpath(struct pt_regs *regs)
+static void __syscall_return_slowpath(struct pt_regs *regs)
 {
 	struct thread_info *ti = current_thread_info();
 	u32 cached_flags = READ_ONCE(ti->flags);
@@ -302,8 +303,18 @@ static void syscall_slow_exit_work(struc
 		syscall_slow_exit_work(regs, cached_flags);
 
 	local_irq_disable();
+}
+
+/*
+ * Called with IRQs on and fully valid regs.  Returns with IRQs off in a
+ * state such that we can immediately switch to user mode.
+ */
+__visible inline notrace void syscall_return_slowpath(struct pt_regs *regs)
+{
+	__syscall_return_slowpath(regs);
 	prepare_exit_to_usermode(regs);
 }
+NOKPROBE_SYMBOL(syscall_return_slowpath);
 
 #ifdef CONFIG_X86_64
 static __always_inline