3 files changed, 125 insertions, 7 deletions
diff --git a/pkg/build/linux.go b/pkg/build/linux.go
index f08455c39..be2341311 100644
--- a/pkg/build/linux.go
+++ b/pkg/build/linux.go
@@ -31,20 +31,20 @@ func (linux linux) build(params *Params) error {
 		return err
 	}
 	kernelPath := filepath.Join(params.KernelDir, filepath.FromSlash(kernelBin(params.TargetArch)))
-	if fileInfo, err := os.Stat(params.UserspaceDir); err == nil && !fileInfo.IsDir() && params.VMType == "qemu" {
+	if fileInfo, err := os.Stat(params.UserspaceDir); err == nil && fileInfo.IsDir() {
+		// The old way of assembling the image from userspace dir.
+		// It should be removed once all syzbot instances are switched.
+		return linux.createImage(params, kernelPath)
+	}
+	if params.VMType == "qemu" {
 		// If UserspaceDir is a file (image) and we use qemu, we just copy image and kernel to the output dir
 		// assuming that qemu will use injected kernel boot. In this mode we also assume password/key-less ssh.
-		// In future it would be good to switch to accepting complete disk image always and just replacing
-		// kernel in it for GCE VMs (assembling image from userspace files in createImage isn't reasonable).
 		if err := osutil.CopyFile(kernelPath, filepath.Join(params.OutputDir, "kernel")); err != nil {
 			return err
 		}
 		return osutil.CopyFile(params.UserspaceDir, filepath.Join(params.OutputDir, "image"))
 	}
-	if err := linux.createImage(params, kernelPath); err != nil {
-		return err
-	}
-	return nil
+	return embedLinuxKernel(params, kernelPath)
 }
 
 func (linux linux) sign(params *Params) (string, error) {
diff --git a/pkg/build/linux_linux.go b/pkg/build/linux_linux.go
new file mode 100644
index 000000000..05a1036e0
--- /dev/null
+++ b/pkg/build/linux_linux.go
@@ -0,0 +1,104 @@
+// Copyright 2021 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package build
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"syscall"
+	"unsafe"
+
+	"github.com/google/syzkaller/pkg/osutil"
+	"golang.org/x/sys/unix"
+)
+
+// embedLinuxKernel copies a new kernel into an existing disk image.
+// There are several assumptions about the image:
+// - the image is ext4 (may be inferred from image name if necessary, e.g. "image.btrfs")
+// - the data is on partition 1 (we could see what partitions we got and use the last one)
+// - ssh works without password (we don't copy the key)
+// - cmdline file is not supported (should be moved to kernel config)
+// - the kernel is stored in the image in /vmlinuz file.
+func embedLinuxKernel(params *Params, kernelPath string) error {
+	if params.CmdlineFile != "" {
+		return fmt.Errorf("cmdline file is not supported for linux images")
+	}
+	tempDir, err := ioutil.TempDir("", "syz-build")
+	if err != nil {
+		return err
+	}
+	defer os.RemoveAll(tempDir)
+	imageFile := filepath.Join(tempDir, "image")
+	if err := osutil.CopyFile(params.UserspaceDir, imageFile); err != nil {
+		return err
+	}
+	loop, loopFile, err := linuxSetupLoop(imageFile)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		unix.IoctlGetInt(loop, unix.LOOP_CLR_FD)
+		unix.Close(loop)
+	}()
+	mountDir := filepath.Join(tempDir, "mnt")
+	if err := osutil.MkdirAll(mountDir); err != nil {
+		return err
+	}
+	if err := unix.Mount(loopFile+"p1", mountDir, "ext4", 0, ""); err != nil {
+		return err
+	}
+	defer unix.Unmount(mountDir, 0)
+	if err := osutil.CopyFile(kernelPath, filepath.Join(mountDir, "vmlinuz")); err != nil {
+		return err
+	}
+	if params.SysctlFile != "" {
+		if err := osutil.CopyFile(params.SysctlFile, filepath.Join(mountDir, "etc", "sysctl.conf")); err != nil {
+			return err
+		}
+	}
+	if err := unix.Unmount(mountDir, 0); err != nil {
+		return err
+	}
+	return osutil.CopyFile(imageFile, filepath.Join(params.OutputDir, "image"))
+}
+
+func linuxSetupLoop(imageFile string) (int, string, error) {
+	image, err := unix.Open(imageFile, unix.O_RDWR, 0)
+	if err != nil {
+		return 0, "", fmt.Errorf("failed to open %v: %v", imageFile, err)
+	}
+	defer unix.Close(image)
+	loopControl, err := unix.Open("/dev/loop-control", unix.O_RDWR, 0)
+	if err != nil {
+		return 0, "", fmt.Errorf("failed to open /dev/loop-control: %v", err)
+	}
+	defer unix.Close(loopControl)
+	loopIndex, err := unix.IoctlRetInt(loopControl, unix.LOOP_CTL_GET_FREE)
+	if err != nil {
+		return 0, "", fmt.Errorf("LOOP_CTL_GET_FREE failed: %v", err)
+	}
+	loopFile := fmt.Sprintf("/dev/loop%v", loopIndex)
+	loop, err := unix.Open(loopFile, unix.O_RDWR, 0)
+	if err != nil {
+		return 0, "", fmt.Errorf("failed to open %v: %v", loopFile, err)
+	}
+	if err := unix.IoctlSetInt(loop, unix.LOOP_SET_FD, image); err != nil {
+		unix.Close(loop)
+		return 0, "", fmt.Errorf("LOOP_SET_FD failed: %v", err)
+	}
+	info := &unix.LoopInfo64{
+		Flags: unix.LO_FLAGS_PARTSCAN,
+	}
+	for i := 0; i < len(imageFile); i++ {
+		info.File_name[i] = imageFile[i]
+	}
+	if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, uintptr(loop), unix.LOOP_SET_STATUS64,
+		uintptr(unsafe.Pointer(info))); err != 0 {
+		unix.Close(loop)
+		return 0, "", fmt.Errorf("LOOP_SET_STATUS64 failed: %v", err)
+	}
+	return loop, loopFile, nil
+}
diff --git a/pkg/build/linux_nolinux.go b/pkg/build/linux_nolinux.go
new file mode 100644
index 000000000..1bffbeaae
--- /dev/null
+++ b/pkg/build/linux_nolinux.go
@@ -0,0 +1,14 @@
+// Copyright 2021 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+// +build !linux
+
+package build
+
+import (
+	"errors"
+)
+
+func embedLinuxKernel(params *Params, kernelPath string) error {
+	return errors.New("building linux image is only supported on linux")
+}