[5.4.1089, arm64] eBPF opensnoop does not display PATH

[Francis Laniel]

Hi.

FIrst, I hope you are fine and the same for your relatives.

The actual kernel used on AKS arm64 (i.e. 5.4.1089) suffers from this problem:
https://github.com/iovisor/bcc/issues/2253
As a consequence, opensnoop does not display PATH:
# Run the following from Canonical:UbuntuServer:18_04-daily-lts-arm64:18.04.202208290
$ uname -a
Linux francis-vm-arm64-ubuntu18vm 5.4.0-1089-azure #94~18.04.1-Ubuntu SMP Fri Aug 5 12:36:48 UTC 2022 aarch64 aarch64 aarch64 GNU/Linux
$ lsb_release -rd
Description: Ubuntu 18.04.6 LTS
Release: 18.04
$ git clone --recurse-submodules https://github.com/iovisor/bcc
Linux francis-vm-arm64-ubuntu18vm 5.4.0-1089-azure #94~18.04.1-Ubuntu SMP Fri Aug 5 12:36:48 UTC 2022 aarch64 aarch64 aarch64 GNU/Linux
$ sudo sh -c 'apt update && apt install -qy clang-10 llvm-10 make gcc pkg-config libelf-dev libz-dev'
...
$ cd bcc/libbpf-tools
$ CLANG=clang-10 LLVM_STRIP=llvm-strip-10 make -j opensnoop
...
BINARY opensnoop
$ sudo ./opensnoop
PID COMM FD ERR PATH
1672 python3 3 0
9746 opensnoop 20 0
1672 python3 3 0
1672 python3 3 0
1672 python3 -1 2
1672 python3 3 0
1 systemd 18 0
1672 python3 6 0
1672 python3 3 0
1672 python3 3 0
1672 python3 3 0
1672 python3 3 0
1672 python3 3 0
^C
As you can see, nothing is printed for the PATH while normal behavior prints the path of the opened file:
$ uname -a
Linux pwmachine 5.15.0-46-generic #49~20.04.1-Ubuntu SMP Thu Aug 4 19:15:44 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
$ sudo ./opensnoop
PID COMM FD ERR PATH
2704 systemd 23 0 virtual
2704 systemd 22 0 misc
2704 systemd 23 0 fuse
2704 systemd 22 0 /sys/devices/virtual/misc/fuse/uevent
2704 systemd 22 0 /run/udev/data/c10:229
2704 systemd 22 0 /proc/2704/status
2704 systemd 22 0 /proc/2704/status
2704 systemd 22 0 /proc/2704/status
^C

This bug was fixed in upstream patch:
https://github.com/torvalds/linux/commit/6ae08ae3dea2cfa03dd3665a3c8475c2d429ef47
Sadly, this patch was not back ported, so it is not present in stable kernels.

I backported the patches myself (see attachment) and was able to build the kernel package with the following command:
sudo LANG=C $(dpkg-architecture -aarm64) CROSS_COMPILE=aarch64-linux-gnu- fakeroot debian/rules binary skipdbg=false
Sadly, I was not able to successfully boot it on Azure, either by installing the package or using kexec.
I suspect this is because my image is not signed.

Best regards and thank you in advance.