From d02b74a5dcfed4bfc8f2f8e545bca4d2afabb296 Mon Sep 17 00:00:00 2001
From: Googler <noreply@google.com>
Date: Fri, 27 Apr 2018 10:37:02 -0700
Subject: Check in gVisor.

PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463
---
 pkg/sentry/kernel/rseq.go | 118 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 pkg/sentry/kernel/rseq.go

(limited to 'pkg/sentry/kernel/rseq.go')

diff --git a/pkg/sentry/kernel/rseq.go b/pkg/sentry/kernel/rseq.go
new file mode 100644
index 000000000..635372993
--- /dev/null
+++ b/pkg/sentry/kernel/rseq.go
@@ -0,0 +1,118 @@
+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package kernel
+
+import (
+	"gvisor.googlesource.com/gvisor/pkg/sentry/hostcpu"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
+	"gvisor.googlesource.com/gvisor/pkg/syserror"
+)
+
+// Restartable sequences, as described in https://lwn.net/Articles/650333/.
+
+// RSEQCriticalRegion describes a restartable sequence critical region.
+type RSEQCriticalRegion struct {
+	// When a task in this thread group has its CPU preempted (as defined by
+	// platform.ErrContextCPUPreempted) or has a signal delivered to an
+	// application handler while its instruction pointer is in CriticalSection,
+	// set the instruction pointer to Restart and application register r10 (on
+	// amd64) to the former instruction pointer.
+	CriticalSection usermem.AddrRange
+	Restart         usermem.Addr
+}
+
+// RSEQAvailable returns true if t supports restartable sequences.
+func (t *Task) RSEQAvailable() bool {
+	return t.k.useHostCores && t.k.Platform.DetectsCPUPreemption()
+}
+
+// RSEQCriticalRegion returns a copy of t's thread group's current restartable
+// sequence.
+func (t *Task) RSEQCriticalRegion() RSEQCriticalRegion {
+	return *t.tg.rscr.Load().(*RSEQCriticalRegion)
+}
+
+// SetRSEQCriticalRegion replaces t's thread group's restartable sequence.
+//
+// Preconditions: t.RSEQAvailable() == true.
+func (t *Task) SetRSEQCriticalRegion(rscr RSEQCriticalRegion) error {
+	// These checks are somewhat more lenient than in Linux, which (bizarrely)
+	// requires rscr.CriticalSection to be non-empty and rscr.Restart to be
+	// outside of rscr.CriticalSection, even if rscr.CriticalSection.Start == 0
+	// (which disables the critical region).
+	if rscr.CriticalSection.Start == 0 {
+		rscr.CriticalSection.End = 0
+		rscr.Restart = 0
+		t.tg.rscr.Store(&rscr)
+		return nil
+	}
+	if rscr.CriticalSection.Start >= rscr.CriticalSection.End {
+		return syserror.EINVAL
+	}
+	if rscr.CriticalSection.Contains(rscr.Restart) {
+		return syserror.EINVAL
+	}
+	// TODO: check that rscr.CriticalSection and rscr.Restart are in
+	// the application address range, for consistency with Linux
+	t.tg.rscr.Store(&rscr)
+	return nil
+}
+
+// RSEQCPUAddr returns the address that RSEQ will keep updated with t's CPU
+// number.
+//
+// Preconditions: The caller must be running on the task goroutine.
+func (t *Task) RSEQCPUAddr() usermem.Addr {
+	return t.rseqCPUAddr
+}
+
+// SetRSEQCPUAddr replaces the address that RSEQ will keep updated with t's CPU
+// number.
+//
+// Preconditions: t.RSEQAvailable() == true. The caller must be running on the
+// task goroutine. t's AddressSpace must be active.
+func (t *Task) SetRSEQCPUAddr(addr usermem.Addr) error {
+	t.rseqCPUAddr = addr
+	if addr != 0 {
+		if err := t.rseqCopyOutCPU(); err != nil {
+			t.rseqCPUAddr = 0
+			t.rseqCPU = -1
+			return syserror.EINVAL // yes, EINVAL, not err or EFAULT
+		}
+	} else {
+		t.rseqCPU = -1
+	}
+	return nil
+}
+
+// Preconditions: The caller must be running on the task goroutine. t's
+// AddressSpace must be active.
+func (t *Task) rseqCopyOutCPU() error {
+	t.rseqCPU = int32(hostcpu.GetCPU())
+	buf := t.CopyScratchBuffer(4)
+	usermem.ByteOrder.PutUint32(buf, uint32(t.rseqCPU))
+	_, err := t.CopyOutBytes(t.rseqCPUAddr, buf)
+	return err
+}
+
+// Preconditions: The caller must be running on the task goroutine.
+func (t *Task) rseqInterrupt() {
+	rscr := t.tg.rscr.Load().(*RSEQCriticalRegion)
+	if ip := t.Arch().IP(); rscr.CriticalSection.Contains(usermem.Addr(ip)) {
+		t.Debugf("Interrupted RSEQ critical section at %#x; restarting at %#x", ip, rscr.Restart)
+		t.Arch().SetIP(uintptr(rscr.Restart))
+		t.Arch().SetRSEQInterruptedIP(ip)
+	}
+}
-- 
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