linux-kernel - [tip:x86/asm] x86/entry: Add enter_from_user

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Date:	Tue, 7 Jul 2015 03:51:29 -0700
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	luto@...capital.net, linux-kernel@...r.kernel.org,
	brgerst@...il.com, peterz@...radead.org, mingo@...nel.org,
	vda.linux@...glemail.com, luto@...nel.org, hpa@...or.com,
	dvlasenk@...hat.com, riel@...hat.com,
	torvalds@...ux-foundation.org, bp@...en8.de, tglx@...utronix.de,
	keescook@...omium.org, oleg@...hat.com, fweisbec@...il.com
Subject: [tip:x86/asm] x86/entry: Add enter_from_user_mode()
  and use it in syscalls

Commit-ID:  feed36cde0a10adb957445a37e48f957f30b2273
Gitweb:     http://git.kernel.org/tip/feed36cde0a10adb957445a37e48f957f30b2273
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Fri, 3 Jul 2015 12:44:25 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Tue, 7 Jul 2015 10:59:06 +0200

x86/entry: Add enter_from_user_mode() and use it in syscalls

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@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: Denys Vlasenko <vda.linux@...glemail.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: paulmck@...ux.vnet.ibm.com
Link: http://lkml.kernel.org/r/853b42420066ec3fb856779cdc223a6dcb5d355b.1435952415.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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 917d0c3..9a327ee 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -28,6 +28,15 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
 
+#ifdef CONFIG_CONTEXT_TRACKING
+/* Called on entry from user mode with IRQs off. */
+__visible void enter_from_user_mode(void)
+{
+	CT_WARN_ON(ct_state() != CONTEXT_USER);
+	user_exit();
+}
+#endif
+
 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) &
 		_TIF_WORK_SYSCALL_ENTRY;
 
+#ifdef CONFIG_CONTEXT_TRACKING
 	/*
 	 * 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;
 	}
+#endif
 
 #ifdef CONFIG_SECCOMP
 	/*
--
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