pkg/build: add a new way of building linux images

Add a new, simpler procedure:
working bootable image is accepted as an input
and we only replace the kernel in it.
This just looks much saner and the right way to do it
(instead of assembling the image from bits and shelling out
to mkfs, fdisk, grub, etc). The new way also works inside of
containers much better.

1 files changed, 104 insertions, 0 deletions

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
+}