3 files changed, 138 insertions, 1 deletions
diff --git a/pkg/sentry/fsimpl/sys/BUILD b/pkg/sentry/fsimpl/sys/BUILD
index 1b548ccd4..f9b232da6 100644
--- a/pkg/sentry/fsimpl/sys/BUILD
+++ b/pkg/sentry/fsimpl/sys/BUILD
@@ -5,17 +5,22 @@ licenses(["notice"])
 go_library(
     name = "sys",
     srcs = [
+        "kcov.go",
         "sys.go",
     ],
     visibility = ["//pkg/sentry:internal"],
     deps = [
         "//pkg/abi/linux",
         "//pkg/context",
+        "//pkg/coverage",
+        "//pkg/sentry/arch",
         "//pkg/sentry/fsimpl/kernfs",
         "//pkg/sentry/kernel",
         "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/memmap",
         "//pkg/sentry/vfs",
         "//pkg/syserror",
+        "//pkg/usermem",
     ],
 )
 
diff --git a/pkg/sentry/fsimpl/sys/kcov.go b/pkg/sentry/fsimpl/sys/kcov.go
new file mode 100644
index 000000000..92710d877
--- /dev/null
+++ b/pkg/sentry/fsimpl/sys/kcov.go
@@ -0,0 +1,116 @@
+// Copyright 2020 The gVisor Authors.
+//
+// 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 sys
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+func (fs *filesystem) newKcovFile(ctx context.Context, creds *auth.Credentials) *kernfs.Dentry {
+	k := &kcovInode{}
+	k.InodeAttrs.Init(creds, 0, 0, fs.NextIno(), linux.S_IFREG|0600)
+	d := &kernfs.Dentry{}
+	d.Init(k)
+	return d
+}
+
+// kcovInode implements kernfs.Inode.
+type kcovInode struct {
+	kernfs.InodeAttrs
+	kernfs.InodeNoopRefCount
+	kernfs.InodeNotSymlink
+	kernfs.InodeNotDirectory
+}
+
+func (i *kcovInode) Open(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	k := kernel.KernelFromContext(ctx)
+	if k == nil {
+		panic("KernelFromContext returned nil")
+	}
+	fd := &kcovFD{
+		inode: i,
+		kcov:  k.NewKcov(),
+	}
+
+	if err := fd.vfsfd.Init(fd, opts.Flags, rp.Mount(), vfsd, &vfs.FileDescriptionOptions{
+		DenyPRead:  true,
+		DenyPWrite: true,
+	}); err != nil {
+		return nil, err
+	}
+	return &fd.vfsfd, nil
+}
+
+type kcovFD struct {
+	vfs.FileDescriptionDefaultImpl
+	vfs.NoLockFD
+
+	vfsfd vfs.FileDescription
+	inode *kcovInode
+	kcov  *kernel.Kcov
+}
+
+// Ioctl implements vfs.FileDescriptionImpl.Ioctl.
+func (fd *kcovFD) Ioctl(ctx context.Context, uio usermem.IO, args arch.SyscallArguments) (uintptr, error) {
+	cmd := uint32(args[1].Int())
+	arg := args[2].Uint64()
+	switch uint32(cmd) {
+	case linux.KCOV_INIT_TRACE:
+		return 0, fd.kcov.InitTrace(arg)
+	case linux.KCOV_ENABLE:
+		return 0, fd.kcov.EnableTrace(ctx, uint8(arg))
+	case linux.KCOV_DISABLE:
+		if arg != 0 {
+			// This arg is unused; it should be 0.
+			return 0, syserror.EINVAL
+		}
+		return 0, fd.kcov.DisableTrace(ctx)
+	default:
+		return 0, syserror.ENOTTY
+	}
+}
+
+// ConfigureMmap implements vfs.FileDescriptionImpl.ConfigureMmap.
+func (fd *kcovFD) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) error {
+	return fd.kcov.ConfigureMMap(ctx, opts)
+}
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *kcovFD) Release(ctx context.Context) {
+	// kcov instances have reference counts in Linux, but this seems sufficient
+	// for our purposes.
+	fd.kcov.Reset()
+}
+
+// SetStat implements vfs.FileDescriptionImpl.SetStat.
+func (fd *kcovFD) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
+	creds := auth.CredentialsFromContext(ctx)
+	fs := fd.vfsfd.VirtualDentry().Mount().Filesystem()
+	return fd.inode.SetStat(ctx, fs, creds, opts)
+}
+
+// Stat implements vfs.FileDescriptionImpl.Stat.
+func (fd *kcovFD) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	return fd.inode.Stat(ctx, fd.vfsfd.Mount().Filesystem(), opts)
+}
diff --git a/pkg/sentry/fsimpl/sys/sys.go b/pkg/sentry/fsimpl/sys/sys.go
index 393feb802..1f042d9f7 100644
--- a/pkg/sentry/fsimpl/sys/sys.go
+++ b/pkg/sentry/fsimpl/sys/sys.go
@@ -21,6 +21,7 @@ import (
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/coverage"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/kernfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
@@ -73,7 +74,7 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 		}),
 		"firmware": fs.newDir(creds, defaultSysDirMode, nil),
 		"fs":       fs.newDir(creds, defaultSysDirMode, nil),
-		"kernel":   fs.newDir(creds, defaultSysDirMode, nil),
+		"kernel":   kernelDir(ctx, fs, creds),
 		"module":   fs.newDir(creds, defaultSysDirMode, nil),
 		"power":    fs.newDir(creds, defaultSysDirMode, nil),
 	})
@@ -94,6 +95,21 @@ func cpuDir(ctx context.Context, fs *filesystem, creds *auth.Credentials) *kernf
 	return fs.newDir(creds, defaultSysDirMode, children)
 }
 
+func kernelDir(ctx context.Context, fs *filesystem, creds *auth.Credentials) *kernfs.Dentry {
+	// If kcov is available, set up /sys/kernel/debug/kcov. Technically, debugfs
+	// should be mounted at debug/, but for our purposes, it is sufficient to
+	// keep it in sys.
+	var children map[string]*kernfs.Dentry
+	if coverage.KcovAvailable() {
+		children = map[string]*kernfs.Dentry{
+			"debug": fs.newDir(creds, linux.FileMode(0700), map[string]*kernfs.Dentry{
+				"kcov": fs.newKcovFile(ctx, creds),
+			}),
+		}
+	}
+	return fs.newDir(creds, defaultSysDirMode, children)
+}
+
 // Release implements vfs.FilesystemImpl.Release.
 func (fs *filesystem) Release(ctx context.Context) {
 	fs.Filesystem.VFSFilesystem().VirtualFilesystem().PutAnonBlockDevMinor(fs.devMinor)