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Unikraft ELF Loader

The ELF Loader is the integral part of the binary compatibility layer, that enables Unikraft to run unmodified Linux applications. Linux binaries (ELFS - Executable and Linking Format) are loaded by the ELF Loader, and then control is passed to them binary.

Follow the instructions below to set up, configure, build and run ELF Loader.

To get started immediately, you can use Unikraft's companion command-line companion tool, kraft. Start by running the interactive installer:

curl --proto '=https' --tlsv1.2 -sSf https://get.kraftkit.sh | sudo sh

Once installed, clone this repository and run kraft build:

git clone https://github.com/unikraft/app-elfloader elfloader
cd elfloader/
kraft build

This will guide you through an interactive build process where you can select one of the available targets (architecture/platform combinations). Otherwise, we recommend the elfloader-qemu-x86_64-initrd-strace target:

kraft build --target elfloader-qemu-x86_64-initrd-strace

Once built, you can instantiate the unikernel to run an existing dynamic executable. For that, clone the catalog of dynamic applications from the dynamic-apps repository, in the same directory where you cloned app-elfloader:

git clone https://github.com/unikraft/dynamic-apps

Run a simple helloworld binary, by running, while inside the elfloader directory:

kraft run --target elfloader-qemu-x86_64-initrd-strace --plat qemu --initrd ../dynamic-apps/lang/c/helloworld/:/ -- /helloworld

Because we used an strace target, the output of the program also consists of system calls invoked by the Linux binary after loading.

Quick Setup (aka TLDR)

For a quick setup, run the commands below. Note that you still need to install the requirements.

For building and running everything, follow the steps below.

git clone https://github.com/unikraft/dynamic-apps
git clone https://github.com/unikraft/app-elfloader elfloader
cd elfloader/
./scripts/setup.sh
wget https://raw.githubusercontent.com/unikraft/app-testing/staging/scripts/generate.py -O scripts/generate.py
chmod a+x scripts/generate.py
./scripts/generate.py
./scripts/build/make-qemu-x86_64-9pfs.sh
./scripts/run/qemu-x86_64-9pfs-helloworld-c.sh

This will configure and build the app-elfloader. After that, it will run the /helloworld ELF file from the dynamic-apps repository on top of the elfloader image:

Note: Close the linger QEMU VM by using Ctrl+a x. That is, press Ctrl and a at the same time, and then, separately, x.

Requirements

In order to set up, configure, build and run app-elfloader on Unikraft, the following packages are required:

  • build-essential / base-devel / @development-tools (the meta-package that includes make, gcc and other development-related packages)
  • sudo
  • flex
  • bison
  • git
  • wget
  • uuid-runtime
  • qemu-system-x86
  • qemu-kvm
  • sgabios

On Ubuntu/Debian or other apt-based distributions, run the following command to install the requirements:

sudo apt install -y --no-install-recommends \
  build-essential \
  sudo \
  libncurses-dev \
  libyaml-dev \
  flex \
  bison \
  git \
  wget \
  uuid-runtime \
  qemu-kvm \
  qemu-system-x86 \
  sgabios

Running the ELF loader app with QEMU might require networking support, depending on the Linux application used (such as Nginx, or Redis). For this to work properly a specific configuration must be enabled for QEMU. Run the commands below to enable that configuration (for the network bridge to work):

sudo mkdir /etc/qemu/
echo "allow all" | sudo tee /etc/qemu/bridge.conf

Set Up

The following repositories are required for app-elfloader:

  • The application repository (this repository): app-elfloader
  • The Unikraft core repository: unikraft
  • Library repositories:

Follow the steps below for the setup:

  1. First clone the dynamic-apps repository that contains pre-build ELFs to be used with elfloader:

    git clone https://github.com/unikraft/dynamic-apps dynamic-apps

  2. Now clone the app-elfloader repository in the elfloader/ directory, on the same level with the dynamic-apps repository clone:

    git clone https://github.com/unikraft/app-elfloader elfloader

    Enter the elfloader/ directory:

    cd elfloader/

ls -F

    This will show you the contents of the repository:

    arch_prctl.c  brk.c  Config.uk  elf_ctx.c  elf_load.c  elf_prog.h  example/  exportsyms.uk  libelf_helper.h  main.c  Makefile  Makefile.uk  README.md  support/

  3. While inside the elfloader/ directory, clone all required repositories by using the setup.sh script:

    ./scripts/setup.sh

  4. Use the tree command to inspect the contents of the workdir/ directory. It should print something like this:

    tree -F -L 2 workdir/

    The layout of the workdir/ directory should look something like this:

    workdir/
|-- libs/
|   |-- lwip/
|   |-- libelf/
`-- unikraft/
    |-- arch/
    |-- Config.uk
    |-- CONTRIBUTING.md
    |-- COPYING.md
    |-- include/
    |-- lib/
    |-- Makefile
    |-- Makefile.uk
    |-- plat/
    |-- README.md
    |-- support/
    `-- version.mk

9 directories, 7 files

Scripted Building and Running

To build and run Unikraft images, it's easiest to generate build and running scripts and use those.

First of all, grab the generate.py script and place it in the scripts/ directory by running:

wget https://raw.githubusercontent.com/unikraft/app-testing/staging/scripts/generate.py -O scripts/generate.py
chmod a+x scripts/generate.py

Now, run the generate.py script. You must run it in the root directory of this repository:

./scripts/generate.py

Running the script will generate build and run scripts in the scripts/build/ and the scripts/run/ directories:

scripts/
|-- build/
|   |-- kraft-fc-x86_64-initrd-debug.sh*
|   |-- kraft-fc-x86_64-initrd.sh*
|   |-- kraft-fc-x86_64-initrd-strace.sh*
|   |-- kraft-qemu-x86_64-9pfs-debug.sh*
|   |-- kraft-qemu-x86_64-9pfs.sh*
|   |-- kraft-qemu-x86_64-9pfs-strace.sh*
|   |-- kraft-qemu-x86_64-initrd-debug.sh*
|   |-- kraft-qemu-x86_64-initrd.sh*
|   |-- kraft-qemu-x86_64-initrd-strace.sh*
|   |-- make-fc-x86_64-initrd-debug.sh*
|   |-- make-fc-x86_64-initrd.sh*
|   |-- make-fc-x86_64-initrd-strace.sh*
|   |-- make-qemu-x86_64-9pfs-debug.sh*
|   |-- make-qemu-x86_64-9pfs.sh*
|   |-- make-qemu-x86_64-9pfs-strace.sh*
|   |-- make-qemu-x86_64-initrd-debug.sh*
|   |-- make-qemu-x86_64-initrd.sh*
|   `-- make-qemu-x86_64-initrd-strace.sh*
|-- generate.py*
|-- run/
|   |-- fc-x86_64-initrd-helloworld-c.json
|   |-- fc-x86_64-initrd-helloworld-c.sh*
|   |-- fc-x86_64-initrd-nginx.json
[...]
|   |-- kraft-qemu-x86_64-initrd-nginx.sh*
|   |-- kraft-qemu-x86_64-initrd-sqlite3.sh*
|   |-- kraft-qemu-x86_64-initrd-strace-helloworld-c.sh*
|   |-- kraft-qemu-x86_64-initrd-strace-nginx.sh*
|   |-- kraft-qemu-x86_64-initrd-strace-sqlite3.sh*
|   |-- qemu-x86_64-9pfs-helloworld-c.sh*
|   |-- qemu-x86_64-9pfs-nginx.sh*
|   |-- qemu-x86_64-9pfs-sqlite3.sh*
|   |-- qemu-x86_64-initrd-helloworld-c.sh*
|   |-- qemu-x86_64-initrd-nginx.sh*
|   `-- qemu-x86_64-initrd-sqlite3.sh*
|-- run.yaml
`-- setup.sh*

They are shell scripts, so you can use an editor or a text viewer to check their contents:

cat scripts/run/kraft-fc-x86_64-initrd-helloworld-c.sh

Now, invoke each script to build and run ELF loader. A sample build and run set of commands is:

./scripts/build/kraft-qemu-x86_64-9pfs-strace.sh
./scripts/run/kraft-qemu-x86_64-9pfs-strace-helloworld-c.sh
./scripts/run/kraft-qemu-x86_64-9pfs-strace-nginx.sh

Another one is:

./scripts/build/make-qemu-x86_64-initrd.sh
./scripts/run/qemu-x86_64-initrd-helloworld-c.sh
./scripts/run/qemu-x86_64-initrd-nginx.sh

Note that Firecracker only works with initrd (not 9pfs). And Firecracker networking is not yet upstream.

Detailed Steps

Configure

Configuring, building and running a Unikraft application depends on our choice of platform and architecture. Currently, supported platform and architecture for app-elfloader are QEMU (KVM), x86_64. Use the .config.elfloader_qemu-x86_64 configuration file together with make defconfig to create the configuration file:

UK_DEFCONFIG=$(pwd)/.config.elfloader_qemu-x86_64 make defconfig

This results in the creation of the .config file:

ls .config
.config

The .config file will be used in the build step.

Build

Building uses as input the .config file from above, and results in a unikernel image as output. The unikernel output image, together with intermediary build files, are stored in the build/ directory.

Clean Up

Before building after some changes had been made, you may need to clean up the build output.

Cleaning up is done with 3 possible commands:

  • make clean: cleans all actual build output files (binary files, including the unikernel image)
  • make properclean: removes the entire build/ directory
  • make distclean: removes the entire build/ directory and the .config file

Typically, you would use make properclean to remove all build artifacts, but keep the configuration file.

QEMU x86_64

Building for QEMU x86_64 assumes you did the QEMU x86_64 configuration step above. Build the Unikraft elfloader image for QEMU x86_64 by using the command below:

make -j $(nproc)

You can see a list of all the files processed by the build system:

[...]
  LD      elfloader_qemu-x86_64.dbg
  UKBI    elfloader_qemu-x86_64.dbg.bootinfo
  SCSTRIP elfloader_qemu-x86_64
  GZ      elfloader_qemu-x86_64.gz
make[1]: Leaving directory '/tmp/apps/app-elfloader/workdir/unikraft'

At the end of the build command, the elfloader_qemu-x86_64 unikernel image is generated. This image is to be used in the run step.

Executing ELF binaries

The elfloader currently supports statically-linked and dynamically-linked applications for Linux on x86_64, as long as they are compiled position independent (PIE). In most cases, such an application can be loaded from any virtual file system that is supported by Unikraft. For example, the application binary can be packaged with a CPIO initramdisk or handed over via a 9pfs host share. Please note that we use 9pfs in the following how-to. To load the application from another file system (e.g., initramdisk), you will need to follow equivalent steps.

Before we can launch an application we need to prepare a root file system that contains the ELF binary along with its library dependencies. You can use ldd (or probably musl-ldd for applications linked with musl) to list the shared libraries on which the application depends. Please note that the vDSO (here: linux-vdso.so.1) is a kernel-provided library that is not present on the host filesystem. Please ignore this file.

For a helloworld example application (here: /example/helloworld, compiled on Debian 11), ldd will likely look like the following:

$ ldd helloworld
	linux-vdso.so.1 (0x00007ffdd695d000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007efed259f000)
	/lib64/ld-linux-x86-64.so.2 (0x00007efed2787000)

Copy the library dependencies to the same subdirectories as reported by ldd. Please remember to also copy any additional and required configuration files to the root file system. In this example, the populated root filesystem will look like this:

rootfs/
├── lib
│   └── x86_64-linux-gnu
│       └── libc.so.6
├── lib64
│   └── ld-linux-x86-64.so.2
└── helloworld

Because the official dynamic loader maps the application and libraries into memory, the elfloader unikernel must be configured with posix-mmap, ukvmem, and vfscore. For 9pfs, also make sure that you configured vfscore to automatically mount a host shared filesystem: Under Library Configuration -> vfscore: Configuration select Automatically mount a root filesystem, set Default root filesystem to 9PFS, and ensure that Default root device is to fs0. This last option simplifies the use of the -e parameter of qemu-guest.

The application can then be started with:

# qemu-guest -k elfloader_kvm-x86_64 -e rootfs/ \
            -a "/helloworld <application arguments>"

NOTE: This command line example expects that you built your unikernel with Application Options -> Application name/path via command line (APPELFLOADER_CUSTOMAPPNAME).

HINT: Environment variables can be set through lib/posix-environ and lib/uklibparam. For this purpose, enable Library Configuration -> posix-environ and activate Parse kernel command line arguments. The variables can be handed over via the kernel command line with the Unikraft library parameter env.vars, for example:

# qemu-guest -k elfloader_kvm-x86_64 -e rootfs/ \
            -a "env.vars=[ LD_LIBRARY_PATH=/lib LD_SHOW_AUXV=1 ] -- /helloworld <application arguments>"

NOTE: At the moment, a program exit will not yet cause a shutdown of the elfloader unikernel. You need to manually terminate it. In case of qemu-guest, you can use CTRL + C.

Debugging

strace-like Output

Unikraft's syscall_shim provides the ability to print a strace-like message for every processed binary system call request on the kernel output. This option can be useful for understanding what code a system call handler returns to the application, and how the application interacts with the kernel. The setting can be found under Library Configuration -> syscall_shim -> Debugging: 'strace'-like messages for binary system calls.

GNU Debugger (gdb)

It is possible to debug elfloader together with the loaded application, and use the full set of debugging facilities for kernel and application at the same time. In principle, gdb must only be made aware of the runtime memory layout of elfloader with the loaded application. Thanks to the single address space layout, we gain easy debugability and full transparency.

Static-PIE ELF

As a first step, gdb is started with loading the symbols from the dbg image of the elfloader. We map the symbols of ELF application with the gdb command add-symbol-file by specifying the application (with debug symbols) and the base load address. If info messages are enabled in ukdebug, this base address will be messaged by the loader like this:

ELF program loaded to 0x400101000-0x4001c2a08 (793096 B), entry at 0x40010ad50

To this address (here: 0x400101000) you have to add the offset of the .text segment. You can use readelf -S to find it out. In our example it is 0x92a0 (output shortened):

$ readelf -S helloworld_static
Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [11] .text             PROGBITS         00000000000092a0  000092a0
       0000000000086eb0  0000000000000000  AX       0     0     32

The resulting address here is 0x40010a2a0. The symbols of the static helloworld program can then be loaded from gdb with the following command:

(gdb) add-symbol-file -readnow helloworld_static 0x40010A2A0

From this point you have symbol resolution in your debugger, for both the Unikraft elfloader and the loaded application.

NOTE: You can only set regular breakpoints within the application (break with GDB) after it got loaded into memory by elfloader (otherwise they will be ignored). The recommended procedure is:

  1. Set a breakpoint just after the application was loaded (e.g., the first system call that the application executes),
  2. Let the execution continue until the breakpoint is reached.
  3. Set the interesting breakpoints within application space.

Dynamically-linked ELF

The principle of runtime address space layout for dynamically linked executables is the same as for statically linked executables. The differences are that we have additionally loaded a dynamic loader together with the application and we have to load the symbols of each dependent dynamic library as well. For this purpose, we recommend to enable strace-like output in syscall_shim (read subsection: strace-like output). It is the dynamic loader that will for each library:

  1. Open the library.
  2. Parse the ELF header.
  3. Memory-map all needed sections into memory.
  4. Close the library file again.

For our Helloworld example compiled with glibc, this looks like the following for libc:

openat(AT_FDCWD, "/libc.so.6", O_RDONLY|O_CLOEXEC) = fd:3
read(fd:3, <out>"\x7FELF\x02\x01\x01\x03\x00\x00\x00\x00\x00\x00\x00\x00\x03\x00>\x00\x01\x00\x00\x00"..., 832) = 832
fstat(fd:3, va:0x40006ef18) = OK
mmap(NULL, 1918592, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, fd:3, 0) = va:0x8000005000
mmap(va:0x8000027000, 1417216, PROT_EXEC|PROT_READ, MAP_PRIVATE|MAP_DENYWRITE|MAP_FIXED, fd:3, 139264) = va:0x8000027000
mmap(va:0x8000181000, 323584, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE|MAP_FIXED, fd:3, 1556480) = va:0x8000181000
mmap(va:0x80001d0000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_DENYWRITE|MAP_FIXED, fd:3, 1875968) = va:0x80001d0000
mmap(va:0x80001d6000, 13952, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, fd:-1, 0) = va:0x80001d6000
close(fd:3) = OK

The virtual address returned by the first mmap operation is the virtual base address of the application that we need to note down. In this case, the virtual base address of libc.so.6 is 0x8000005000. Please do the same for each dynamically loaded library.

To load the application and library symbols appropriately, as described in the previous subsection, you must add the segment offset of the .text section to the virtual base address. This allows you to load the symbols from gdb with add-symbol-file.

NOTE: Regular breakpoints in shared libraries can only be set after the libraries have been loaded into memory. Since these are loaded by the dynamic loader and not directly by the elfloader, this is done with mmap system calls as shown in the console snippet above. The corresponding close system call (break uk_syscall_r_close) is a safe place to hop to before setting the actual breakpoints within a shared library.

Recommendation: gdb Setup Script

Because of the calculations, we recommend scripting the gdb setup so that any subsequent gdb debug session will be ready quickly. You can get inspiration from the following bash script, which provides a function that automatically determines the .text offset and adds it to a given base address. The advantage is that only the base addresses have to be noted from the Unikraft console output.

#!/bin/bash
# Host and port of the GDB server port (qemu)
GDBSRV=":1234"

# Generate a GDB command line for loading the symbols of an ELF executable/
# shared library.
# Usage: gdb-add-symbols "<ELF executable/library>" \
#                        "<base load address (hex, no leading '0x')>"
gdb-add-symbols()
{
	local LOAD_ELF="$1"
	local LOAD_ADDR="${2}"
	local LOAD_TADDR=
	local TEXT_OFFSET=

	# Hacky way to figure out the .text offset
	TEXT_OFFSET=$( readelf -S "${LOAD_ELF}" | grep '.text' | awk '{ print $5 }' )

	# Compute offset of .text section with base address
	LOAD_TADDR=$( printf 'obase=16;ibase=16;%s+%s\n' "${LOAD_ADDR^^}" "${TEXT_OFFSET^^}" | bc )

	# Generate GDB command
	printf 'add-symbol-file -readnow %s 0x%s' "${LOAD_ELF}" "${LOAD_TADDR}"
}

# Connect to $GDBSRV and set up gdb
# NOTE: The first block of instructions connects to the gdb port of the qemu
#       process and follows the CPU mode change while the guest is booting.
#       This is currently a requirement for using gdb with qemu for x86_64. For
#       other platforms and architectures, this may be different.
#       This block assumes that the guest is started in paused state (`-P` for
#       `qemu-guest`).
# NOTE: Please note that regular breakpoints within the application or a shared
#       library can only be set after they have been loaded into memory. Usually
#       this is done by elfloader for the application and dynamic loader or by
#       the dynamic loader for shared libraries.
# HINT: You can use the `directory` command to specify additional paths that
#       `gdb` will use to search for source files.
#       For example, if you run your dynamically linked application with
#       Debian's libc, you can install (`apt install glibc-source`) and
#       extract the glibc sources under /usr/src/glibc.
#         --eval-command="directory /usr/src/glibc/glibc-2.31"
exec gdb \
	--eval-command="target remote $GDBSRV" \
	--eval-command="hbreak _ukplat_entry" \
	--eval-command="continue" \
	--eval-command="disconnect" \
	--eval-command="set arch i386:x86-64:intel" \
	--eval-command="target remote $GDBSRV" \
	\
	--eval-command="$( gdb-add-symbols "rootfs/helloworld" "8000000000" )" \
	--eval-command="$( gdb-add-symbols "rootfs/libc.so.6" "8000005000" )"

Hint: Debug symbols of libraries installed from packages

If you run your dynamically-linked application with libraries installed via a package manager, you can check if a debug package is also available for installation. For example, Debian provides the debug symbols automatically for gdb with the following installation (root privileges required):

# apt install libc6-dbg

As soon as the Debian's libc.so.6 is loaded by gdb, the debugger will load the symbols provided by the Debian debug package.

Hint: Sources of libraries installed from packages

If you run your dynamically linked application with libraries installed via a package manager, you can check if a source package is available for installation. For example, the libc sources are available under Debian with the package glibc-source. After you extracted the installed sources archive under /usr/src/glibc, you can make these sources visible to gdb with (given that you use Debian's libc also for the application with elfloader):

(gdb) directory /usr/src/glibc/glibc-2.31

Hint: Duplicate symbols

Common symbols like main might exist in both the KVM image and the ELF application. You can be specific by referring to their respective source file if debug information is present: helloworld.c:main instead of just main. Alternatively, you can use info functions [regexp] to find the address of your symbol. For example, info function ^main$ prints the address of the main function. See "Examining the Symbol Table" for details.