Debugging Btrfs with GDB

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This document explains how to debug Btrfs with GDB on UML(User Mode Linux) for beginners.

These are strategies for the beginners of debugging Btrfs:

  • Use a loop device to setup Btrfs filesystem without cost: You don't have to purchase a hard drive.
  • Use Btrfs userland commands to trigger ioctl execution, which enables you to easily break the execution of the kernel, for the first attempt.

Of course, after you grabbed the ideas described here, you can create another strategies to fit your needs :D

Building UML Kernel

First of all, get the Linux Kernel source code.

 git clone git://
 cd linux-2.6

Do make defconfig for UML.

 make defconfig ARCH=um

Enable Btrfs by selecting File systems -> Btrfs filesystem (EXPERIMENTAL) Unstable disk format.

 make menuconfig ARCH=um

Build the UML kernel.

 make ARCH=um

Now you can set break points at the Btrfs functions (btrfs_*) built in the kernel.

 gdb ./linux
 (gdb) b btrfs_ (hit tab key to completion)
 Display all 716 possibilities? (y or n) y
 btrfs_acl_chmod                            btrfs_lookup_block_group
 btrfs_add_block_group_cache                btrfs_lookup_csum
 btrfs_add_dead_root                        btrfs_lookup_csums_range
 btrfs_add_delayed_data_ref                 btrfs_lookup_dentry
 btrfs_add_delayed_extent_op                btrfs_lookup_dir_index_item

Building Btrfs Userland Commands

On Ubuntu, you can use apt-get to setup tools required for building the Btrfs userland commands. Execute the following command on your UML guest:

 apt-get install build-essential git-core uuid-dev libattr1-dev zlib1g-dev libacl1-dev e2fslibs-dev

Next, get the code and build it in your UML guest.

 git clone git://
 cd btrfs-progs

Preparing a Loop Device

Prepare a loop device for Btrfs. You must provide image file size at least 256 MB. Execute the following commands on your UML guest:

 dd if=/dev/zero of=btrfs.img bs=4k count=100000
 mkfs.btrfs btrfs.img
 losetup /dev/loop0 btrfs.img
 mount -t btrfs /dev/loop0 /mnt

Running the Kernel and Setting a Break Point

Run the UML kernel and attach the kernel with GDB. And then, set a break point. For example, if you want to stop the execution of the kernel when a subvolume is being created, do like that:

 (gdb) b btrfs_ioctl
 Breakpoint 1 at 0x816b3a2: file fs/btrfs/ctree.h, line 1988.
 (gdb) c

Executing Userland Command

Run the userland command. For example, if you want to break the execution where you set the break point previously, run like this:

 btrfs-progs-unstable/btrfs sub create /mnt/subvol

Breaks it. Now, you can investigate with GDB.

 Breakpoint 1, btrfs_ioctl (file=0x9fc7da0, cmd=1342215182, arg=3213134472)
     at fs/btrfs/ctree.h:1988
 1988            return file->f_path.dentry;
 (gdb) bt
 #0  btrfs_ioctl (file=0x9fc7da0, cmd=1342215182, arg=3213134472) at fs/btrfs/ctree.h:1988
 #1  0x080bfcc1 in vfs_ioctl (filp=0x9fc7da0, cmd=1342215182, arg=3213134472)
     at fs/ioctl.c:44
 #2  0x080c032e in do_vfs_ioctl (filp=0x9fc7da0, fd=3, cmd=1342215182, arg=3213134472)
     at fs/ioctl.c:597
 #3  0x080c0397 in sys_ioctl (fd=3, cmd=1342215182, arg=3213134472) at fs/ioctl.c:617
 #4  0x0805ab18 in handle_syscall (r=0x9f08260) at arch/um/kernel/skas/syscall.c:35
 #5  0x0806832d in userspace (regs=0x9f08260) at arch/um/os-Linux/skas/process.c:201
 #6  0x08058964 in fork_handler () at arch/um/kernel/process.c:181
 #7  0x00000000 in ?? ()
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