id = 1274
title = "Cannot debug init using \"qemu -s -S\" if init is compiled dynamically or if kvm is enabled"
state = "closed"
created_at = "2022-10-25T06:37:16.506Z"
closed_at = "2023-12-01T19:56:42.721Z"
labels = ["Documentation", "GDB", "accel: KVM"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/1274"
host-os = "Debian sid (as of 2022-10-22)"
host-arch = "x86_64"
qemu-version = "QEMU emulator version 7.1.0 (Debian 1:7.1+dfsg-2+b1)"
guest-os = "Debian sid (as of 2022-10-22)"
guest-arch = "x86_64"
description = """I'm trying to connect from host to init process running in guest. I'm using this guide: https://qemu-project.gitlab.io/qemu/system/gdb.html . Everything works well, but there is two problems:
1. Debugging stops to work if I add "-enable-kvm"
2. Debugging stops to work if I remove "-static" when compiling init"""
reproduce = """I have absolutely fresh Debian sid system (as of 2022-10-22). I create the following file on it:
```c
#include <stdio.h>

int
main ()
{
  printf ("a\\n");
  printf ("b\\n");
  for (;;);
}
```

Then I compile it so: `gcc -static -g a.c`. Result is saved as `/root/a.out`. Then I run `sync; echo 3 > /proc/sys/vm/drop_caches; sync` to make sure this `/root/a.out` actually got to disk.

Then I start the host system inside of qemu using well-known `-snapshot /dev/sda` trick. Exact command is here:

```bash
qemu-system-x86_64 -daemonize -m 300M -s -S -kernel /vmlinuz -initrd /initrd.img -snapshot -append "root=/dev/sda init=/root/a.out" -drive file=/dev/sda,format=raw
```

(As you guessed, my disk has no partitions, it directly stores ext4 filesystem.)

Then I type on host `gdb ./a.out`. And then inside of gdb I type `target remote localhost:1234`, then `br 7` (line 7 is `printf ("b\\n")`, then `c`. Then guest OS boots and reaches init (i. e. `/root/a.out`). And then gdb actually pauses on line 7. I. e. everything works!

But if I add `-enable-kvm` to qemu command line OR remove `-static` from gcc command line, then breakpoint doesn't work, i. e. gdb doesn't pause on breakpoint, the execution continues and the execution fails to infinite loop."""
additional = "n/a"