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

Thomas Gleixner
2020-03-08T23:24:03Z

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@linutronix.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