[RFC/INCOMPLETE 05/13] x86/entry: Add enter_from_user_mode and use it in syscalls

From: Andy Lutomirski
Date: Tue Jun 16 2015 - 16:18:04 EST

Changing the x86 context tracking hooks is dangerous because there
are no good checks that we track our context correctly. Add a
helper to check that we're actually in CONTEXT_USER when we enter
from user mode and wire it up for syscall entries.

Subsequent patches will wire this up for all non-NMI entries as
well. NMIs are their own special beast and cannot currently switch
overall context tracking state. Instead, they have their own
special RCU hooks.

This is a tiny speedup if !CONFIG_CONTEXT_TRACKING (removes a
branch) and a tiny slowdown if CONFIG_CONTEXT_TRACING (adds a layer
of indirection). Eventually, we should fix up the core context
tracking code to supply a function that does what we want (and can
be much simpler than user_exit), which will enable us to get rid of
the extra call.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
arch/x86/entry/common.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 348465473e55..54af0df50080 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -28,6 +28,15 @@
#include <trace/events/syscalls.h>

+/* Called on entry from user mode with IRQs off. */
+asmlinkage __visible void enter_from_user_mode(void)
+ context_tracking_assert_state(CONTEXT_USER);
+ user_exit();
static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
#ifdef CONFIG_X86_64
@@ -65,14 +74,16 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
work = ACCESS_ONCE(current_thread_info()->flags) &

* If TIF_NOHZ is set, we are required to call user_exit() before
* doing anything that could touch RCU.
if (work & _TIF_NOHZ) {
- user_exit();
+ enter_from_user_mode();
work &= ~_TIF_NOHZ;


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