linux-kernel - [PATCH v2 35/36] x86/entry: Split and inline prepare_exit_to

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon,  5 Oct 2015 17:48:23 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 35/36] x86/entry: Split and inline prepare_exit_to_usermode

GCC is unable to properly optimize functions that have a very short
likely case and a longer and register-heavier cold part -- it fails
to sink all of the register saving and stack frame setup code into
the unlikely part.

Help it out with prepare_exit_to_usermode by splitting it into two
parts and inline the hot part.

Saves 6-8 cycles for compat syscalls.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/common.c | 43 ++++++++++++++++++++++++++++---------------
 1 file changed, 28 insertions(+), 15 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 3d44c83f2769..d6b8f4fd8274 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -218,14 +218,12 @@ long syscall_trace_enter(struct pt_regs *regs)
 		return syscall_trace_enter_phase2(regs, arch, phase1_result);
 }
 
-/* Called with IRQs disabled. */
-__visible void prepare_exit_to_usermode(struct pt_regs *regs)
-{
-	if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
-		local_irq_disable();
-
-	lockdep_sys_exit();
+#define EXIT_TO_USERMODE_LOOP_FLAGS				\
+	(_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |	\
+	 _TIF_NEED_RESCHED | _TIF_USER_RETURN_NOTIFY)
 
+static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags)
+{
 	/*
 	 * In order to return to user mode, we need to have IRQs off with
 	 * none of _TIF_SIGPENDING, _TIF_NOTIFY_RESUME, _TIF_USER_RETURN_NOTIFY,
@@ -235,14 +233,6 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs)
 	 * work to clear some of the flags can sleep.
 	 */
 	while (true) {
-		u32 cached_flags =
-			READ_ONCE(pt_regs_to_thread_info(regs)->flags);
-
-		if (!(cached_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME |
-				      _TIF_UPROBE | _TIF_NEED_RESCHED |
-				      _TIF_USER_RETURN_NOTIFY)))
-			break;
-
 		/* We have work to do. */
 		local_irq_enable();
 
@@ -266,7 +256,30 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs)
 
 		/* Disable IRQs and retry */
 		local_irq_disable();
+
+		cached_flags = READ_ONCE(pt_regs_to_thread_info(regs)->flags);
+
+		if (!(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
+			break;
+
 	}
+}
+
+/* Called with IRQs disabled. */
+__visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
+{
+	u32 cached_flags;
+
+	if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
+		local_irq_disable();
+
+	lockdep_sys_exit();
+
+	cached_flags =
+		READ_ONCE(pt_regs_to_thread_info(regs)->flags);
+
+	if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
+		exit_to_usermode_loop(regs, cached_flags);
 
 	user_enter();
 }
-- 
2.4.3

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/