How to debug the Linux kernel with GDB and QEMU?

Question

I'm new to kernel development and I would like to know how to run/debug the linux kernel using QEMU and gdb. I'm actually reading Robert Love's book but unfortunately it doesn't help the reader on how to install proper tools to run or debug the kernel... So what I did was to follow this tutorial http://opensourceforu.efytimes.com/2011/02/kernel-development-debugging-using-eclipse/. I'm using eclipse as an IDE to develop on the kernel but I wanted first to get it work under QEMU/gdb. So what I did so far was:

1) To compile the kernel with:

make defconfig (then setting the CONFIG_DEBUG_INFO=y in the .config)
make -j4

2) Once the compilation is over I run Qemu using:

qemu-system-x86_64 -s -S /dev/zero -kernel /arch/x86/boot/bzImage

which launch the kernel in "stopped" state

3) Thus I have to use gdb, I try the following command:

gdb ./vmlinux

which run it correctly but... Now I don't know what to do... I know that I have to use remote debugging on the port 1234 (default port used by Qemu), using the vmlinux as the symbol table file for debugging.

So my question is: What should I do to run the kernel on Qemu, attach my debugger to it and thus, get them work together to make my life easier with kernel development.

Answer 1

On Linux systems, vmlinux is a statically linked executable file that contains the Linux kernel in one of the object file formats supported by Linux, which includes ELF, COFF and a.out. The vmlinux file might be required for kernel debugging, symbol table generation or other operations, but must be made bootable before being used as an operating system kernel by adding a multiboot header, bootsector and setup routines.

An image of this initial root file system must be stored somewhere accessible by the Linux bootloader to the boot firmware of the computer. This can be the root file system itself, a boot image on an optical disc, a small partition on a local disk (a boot paratition, usually using ext4 or FAT file systems), or a TFTP server (on systems that can boot from Ethernet).

1. Compile linux kernel

   Build the kernel with this series applied, enabling CONFIG_DEBUG_INFO (but leave CONFIG_DEBUG_INFO_REDUCED off)

   • https://www.kernel.org/doc/html/latest/admin-guide/README.html
   • https://wiki.archlinux.org/index.php/Kernel/Traditional_compilation
   • https://lwn.net/Articles/533552/

2. Install GDB and Qemu

   sudo pacman -S gdb qemu
   

3. Create initramfs

   #!/bin/bash
   
   # Os     : Arch Linux
   # Kernel : 5.0.3
   
   INIT_DIR=$(pwd)
   BBOX_URL="https://busybox.net/downloads/busybox-1.30.1.tar.bz2"
   BBOX_FILENAME=$(basename ${BBOX_URL})
   BBOX_DIRNAME=$(basename ${BBOX_FILENAME} ".tar.bz2")
   RAM_FILENAME="${INIT_DIR}/initramfs.cpio.gz"
   
   function download_busybox {
       wget -c ${BBOX_URL} 2>/dev/null
   }
   
   function compile_busybox {
       tar xvf ${BBOX_FILENAME} && cd "${INIT_DIR}/${BBOX_DIRNAME}/"
       echo "[*] Settings > Build options > Build static binary (no shared libs)"
       echo "[!] Please enter to continue"
       read tmpvar
       make menuconfig && make -j2 && make install
   }
   
   function config_busybox {
       cd "${INIT_DIR}/${BBOX_DIRNAME}/"
       rm -rf initramfs/ && cp -rf _install/ initramfs/
       rm -f initramfs/linuxrc
       mkdir -p initramfs/{dev,proc,sys}
       sudo cp -a /dev/{null,console,tty,tty1,tty2,tty3,tty4} initramfs/dev/
   
   cat > "${INIT_DIR}/${BBOX_DIRNAME}/initramfs/init" << EOF
   #!/bin/busybox sh
   mount -t proc none /proc
   mount -t sysfs none /sys
   
   exec /sbin/init
   EOF
   
       chmod a+x initramfs/init
       cd "${INIT_DIR}/${BBOX_DIRNAME}/initramfs/"
       find . -print0 | cpio --null -ov --format=newc | gzip -9 > "${RAM_FILENAME}"
       echo "[*] output: ${RAM_FILENAME}"
   
   }
   
   download_busybox
   compile_busybox
   config_busybox
   

4. Boot Linux Kernel With Qemu

   #!/bin/bash
   
   KER_FILENAME="/home/debug/Projects/kernelbuild/linux-5.0.3/arch/x86/boot/bzImage"
   RAM_FILENAME="/home/debug/Projects/kerneldebug/initramfs.cpio.gz"
   
   qemu-system-x86_64 -s -kernel "${KER_FILENAME}" -initrd "${RAM_FILENAME}" -nographic -append "console=ttyS0"
   

   $ ./qemuboot_vmlinux.sh
   SeaBIOS (version 1.12.0-20181126_142135-anatol)
   
   
   iPXE (http://ipxe.org) 00:03.0 C980 PCI2.10 PnP PMM+07F92120+07EF2120 C980
   
   
   Booting from ROM...
   Probing EDD (edd=off to disable)... o
   [    0.019814] Spectre V2 : Spectre mitigation: LFENCE not serializing, switching to generic retpoline
   can't run '/etc/init.d/rcS': No such file or directory
   
   Please press Enter to activate this console.
   / #  uname -a
   Linux archlinux 5.0.3 #2 SMP PREEMPT Mon Mar 25 10:27:13 CST 2019 x86_64 GNU/Linux
   / #
   

5. Debug Linux Kernel With GDB

   ~/Projects/kernelbuild/linux-5.0.3 ➭ gdb vmlinux
   ...
   (gdb) target remote localhost:1234
   Remote debugging using localhost:1234
   0xffffffff89a4b852 in ?? ()
   (gdb) break start_kernel
   Breakpoint 1 at 0xffffffff826ccc08
   (gdb)
   Display all 190 possibilities? (y or n)
   (gdb) info functions
   All defined functions:
   
   Non-debugging symbols:
   0xffffffff81000000  _stext
   0xffffffff81000000  _text
   0xffffffff81000000  startup_64
   0xffffffff81000030  secondary_startup_64
   0xffffffff810000e0  verify_cpu
   0xffffffff810001e0  start_cpu0
   0xffffffff810001f0  __startup_64
   0xffffffff81000410  pvh_start_xen
   0xffffffff81001000  hypercall_page
   0xffffffff81001000  xen_hypercall_set_trap_table
   0xffffffff81001020  xen_hypercall_mmu_update
   0xffffffff81001040  xen_hypercall_set_gdt
   0xffffffff81001060  xen_hypercall_stack_switch
   0xffffffff81001080  xen_hypercall_set_callbacks
   0xffffffff810010a0  xen_hypercall_fpu_taskswitch
   0xffffffff810010c0  xen_hypercall_sched_op_compat
   0xffffffff810010e0  xen_hypercall_platform_op
   

Answer 2

I'd try:

(gdb) target remote localhost:1234
(gdb) continue

Using the '-s' option makes qemu listen on port tcp::1234, which you can connect to as localhost:1234 if you are on the same machine. Qemu's '-S' option makes Qemu stop execution until you give the continue command.

Best thing would probably be to have a look at a decent GDB tutorial to get along with what you are doing. This one looks quite nice.

Answer 3

Step-by-step procedure tested on Ubuntu 16.10 host

To get started from scratch quickly I've made a minimal fully automated QEMU + Buildroot example at: https://github.com/cirosantilli/linux-kernel-module-cheat/blob/c7bbc6029af7f4fab0a23a380d1607df0b2a3701/gdb-step-debugging.md Major steps are covered below.

First get a root filesystem rootfs.cpio.gz. If you need one, consider:

• a minimal init-only executable image: https://unix.stackexchange.com/questions/122717/custom-linux-distro-that-runs-just-one-program-nothing-else/238579#238579
• a Busybox interactive system: https://unix.stackexchange.com/questions/2692/what-is-the-smallest-possible-linux-implementation/203902#203902

Then on the Linux kernel:

git checkout v4.15
make mrproper
make x86_64_defconfig
cat <<EOF >.config-fragment
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_KERNEL=y
CONFIG_GDB_SCRIPTS=y
EOF
./scripts/kconfig/merge_config.sh .config .config-fragment
make -j"$(nproc)"
qemu-system-x86_64 -kernel arch/x86/boot/bzImage \
                   -initrd rootfs.cpio.gz -S -s \
                   -append nokaslr

On another terminal, from inside the Linux kernel tree, supposing you want to start debugging from start_kernel:

gdb \
    -ex "add-auto-load-safe-path $(pwd)" \
    -ex "file vmlinux" \
    -ex 'set arch i386:x86-64:intel' \
    -ex 'target remote localhost:1234' \
    -ex 'break start_kernel' \
    -ex 'continue' \
    -ex 'disconnect' \
    -ex 'set arch i386:x86-64' \
    -ex 'target remote localhost:1234'

and we are done!!

For kernel modules see: How to debug Linux kernel modules with QEMU?

For Ubuntu 14.04, GDB 7.7.1, hbreak was needed, break software breakpoints were ignored. Not the case anymore in 16.10. See also: https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/901944

The messy disconnect and what come after it are to work around the error:

Remote 'g' packet reply is too long: 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

Related threads:

• https://sourceware.org/bugzilla/show_bug.cgi?id=13984 might be a GDB bug
• Remote 'g' packet reply is too long
• http://wiki.osdev.org/QEMU_and_GDB_in_long_mode osdev.org is as usual an awesome source for these problems
• https://lists.nongnu.org/archive/html/qemu-discuss/2014-10/msg00069.html
• nokaslr: https://unix.stackexchange.com/questions/397939/turning-off-kaslr-to-debug-linux-kernel-using-qemu-and-gdb/421287#421287

Known limitations:

• the Linux kernel does not support (and does not even compile without patches) with -O0: How to de-optimize the Linux kernel to and compile it with -O0?
• GDB 7.11 will blow your memory on some types of tab completion, even after the max-completions fix: Tab completion interrupt for large binaries Likely some corner case which was not covered in that patch. So an ulimit -Sv 500000 is a wise action before debugging. Blew up specifically when I tab completed file<tab> for the filename argument of sys_execve as in: https://stackoverflow.com/a/42290593/895245

See also:

• https://github.com/torvalds/linux/blob/v4.9/Documentation/dev-tools/gdb-kernel-debugging.rst official Linux kernel "documentation"
• Linux kernel live debugging, how it's done and what tools are used?

Answer 4

As for me the best solution for debugging the kernel - is to use gdb from Eclipse environment. You should just set appropriate port for gdb (must be the same with one you specified in qemu launch string) in remote debugging section. Here is the manual: http://www.sw-at.com/blog/2011/02/11/linux-kernel-development-and-debugging-using-eclipse-cdt/

Answer 5

BjoernID's answer did not really work for me. After the first continuation, no breakpoint is reached and on interrupt, I would see lines such as:

0x0000000000000000 in ?? ()
(gdb) break rapl_pmu_init
Breakpoint 1 at 0xffffffff816631e7
(gdb) c
Continuing.
^CRemote 'g' packet reply is too long: 08793000000000002988d582000000002019[..]

I guess this has something to do with different CPU modes (real mode in BIOS vs. long mode when Linux has booted). Anyway, the solution is to run QEMU first without waiting (i.e. without -S):

qemu-system-x86_64 -enable-kvm -kernel arch/x86/boot/bzImage -cpu SandyBridge -s

In my case, I needed to break at something during boot, so after some deciseconds, I ran the gdb command. If you have more time (e.g. you need to debug a module that is loaded manually), then the timing doesn't really matter.

gdb allows you to specify commands that should be run when started. This makes automation a bit easier. To connect to QEMU (which should now already be started), break on a function and continue execution, use:

gdb -ex 'target remote localhost:1234' -ex 'break rapl_pmu_init' -ex c ./vmlinux

Answer 6

When you try to start vmlinux exe using gdb, then first thing on gdb is to issue cmds:

(gdb) target remote localhost:1234

(gdb) break start_kernel

(continue)

This will break the kernel at start_kernel.