Add Fs controls

Add Fs controls and implement "cat" command.

PiperOrigin-RevId: 388812540

2 files changed, 96 insertions, 0 deletions

diff --git a/pkg/sentry/control/BUILD b/pkg/sentry/control/BUILD
index 9fb8a054d..7ee237c9f 100644
--- a/pkg/sentry/control/BUILD
+++ b/pkg/sentry/control/BUILD
@@ -6,6 +6,7 @@ go_library(
     name = "control",
     srcs = [
         "control.go",
+        "fs.go",
         "lifecycle.go",
         "logging.go",
         "pprof.go",
@@ -17,6 +18,7 @@ go_library(
     ],
     deps = [
         "//pkg/abi/linux",
+        "//pkg/context",
         "//pkg/fd",
         "//pkg/log",
         "//pkg/sentry/fdimport",
@@ -36,6 +38,7 @@ go_library(
         "//pkg/sync",
         "//pkg/tcpip/link/sniffer",
         "//pkg/urpc",
+        "//pkg/usermem",
     ],
 )
 
diff --git a/pkg/sentry/control/fs.go b/pkg/sentry/control/fs.go
new file mode 100644
index 000000000..d19b21f2d
--- /dev/null
+++ b/pkg/sentry/control/fs.go
@@ -0,0 +1,93 @@
+// Copyright 2021 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 control
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/urpc"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// CatOpts contains options for the Cat RPC call.
+type CatOpts struct {
+	// Files are the filesystem paths for the files to cat.
+	Files []string `json:"files"`
+
+	// FilePayload contains the destination for output.
+	urpc.FilePayload
+}
+
+// Fs includes fs-related functions.
+type Fs struct {
+	Kernel *kernel.Kernel
+}
+
+// Cat is a RPC stub which prints out and returns the content of the files.
+func (f *Fs) Cat(o *CatOpts, _ *struct{}) error {
+	// Create an output stream.
+	if len(o.FilePayload.Files) != 1 {
+		return ErrInvalidFiles
+	}
+
+	output := o.FilePayload.Files[0]
+	for _, file := range o.Files {
+		if err := cat(f.Kernel, file, output); err != nil {
+			return fmt.Errorf("cannot read from file %s: %v", file, err)
+		}
+	}
+
+	return nil
+}
+
+// fileReader encapsulates a fs.File and provides an io.Reader interface.
+type fileReader struct {
+	ctx  context.Context
+	file *fs.File
+}
+
+// Read implements io.Reader.Read.
+func (f *fileReader) Read(p []byte) (int, error) {
+	n, err := f.file.Readv(f.ctx, usermem.BytesIOSequence(p))
+	return int(n), err
+}
+
+func cat(k *kernel.Kernel, path string, output *os.File) error {
+	ctx := k.SupervisorContext()
+	mns := k.GlobalInit().Leader().MountNamespace()
+	root := mns.Root()
+	defer root.DecRef(ctx)
+
+	remainingTraversals := uint(fs.DefaultTraversalLimit)
+	d, err := mns.FindInode(ctx, root, nil, path, &remainingTraversals)
+	if err != nil {
+		return fmt.Errorf("cannot find file %s: %v", path, err)
+	}
+	defer d.DecRef(ctx)
+
+	file, err := d.Inode.GetFile(ctx, d, fs.FileFlags{Read: true})
+	if err != nil {
+		return fmt.Errorf("cannot get file for path %s: %v", path, err)
+	}
+	defer file.DecRef(ctx)
+
+	_, err = io.Copy(output, &fileReader{ctx: ctx, file: file})
+	return err
+}