gdb / qemu kernel debug notes

run.sh

# notes from when I had trouble with kexec some time back. possibly useful to others.

# disable kaslr or boot with nokaslr, or no symbols will match and no breakpoints will be hit
#
# hbreak used to be necessary? not any more

qemu-system-x86_64 --version
#QEMU emulator version 4.0.0
gdb --version
#GNU gdb (Gentoo 8.3 vanilla) 8.3

#update these
kernel=$GOPATH/src/github.com/u-root/u-root/work/i/arch/x86/boot/bzImage
uinteg=/tmp/uroot-integration646325419

#in one console window
qemu-system-x86_64 -machine accel=tcg -nographic                        \
    -kernel $kernel                                                     \
    -append 'nokaslr console=ttyS0 earlyprintk=ttyS0'                   \
    -initrd $uinteg/initramfs.cpio                                      \
    -drive file=fat:rw:$uinteg,if=none,id=tmpdir                        \
    -device ich9-ahci,id=ahci -device ide-drive,drive=tmpdir,bus=ahci.0 \
    -device virtio-rng-pci                                              \
    -s -S

#in another console window
gdb vmlinux -ex 'target remote localhost:1234' \
            -ex 'b __x64_sys_kexec_load'       \
            -ex 'b __x64_sys_kexec_file_load'  \
            -ex 'b __x64_sys_execve'           \
            -ex 'b __x64_sys_execveat'         \
            -ex 'b do_syscall_64'              \
            -ex 'b do_execve_file'             \
            -ex 'b open_exec'