[Mellanox] [Smartswitch] Kill stale initramfs udevd process before updating udevd rules

[tirupatihemanth]

Why I did it

On SONiC SmartSwitch platforms with DPUs, systemd-udevd crashes with SIGABRT on every reboot when DPU firmware initialization is slow. During the initramfs boot phase, a standalone systemd-udevd daemon is started to handle device discovery. If DPU firmware takes longer than the 60-second udevadm settle timeout (BlueField-3 DPUs can take 120 seconds each in the failure case when they are stuck), the initramfs cannot stop this udevd before switch_root. The stale process survives into the real system but is never chrooted into the overlayfs root, leaving it with a broken filesystem view. When dpu-udev-manager.sh writes udev rules, the stale udevd detects the change and crashes on an assertion in systemd's chase() path resolution (assert(path_is_absolute(p)) at chase.c:648), because dir_fd_is_root() returns false for a process whose root still points to the initramfs rootfs rather than the overlayfs.

This triggers a systemd issue : systemd/systemd#29559 which maintainers doesn't consider as a bug from systemd side. Raising this fix for our usecase.

Core was generated by `/usr/lib/systemd/systemd-udevd --daemon --resolve-names=never'.
Program terminated with signal SIGABRT, Aborted.
#0  0x00007f29fe7f695c in ?? () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007f29fe7f695c in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007f29fe7a1cc2 in raise () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007f29fe78a4ac in abort () from /lib/x86_64-linux-gnu/libc.so.6
#3  0x00007f29fea50c11 in ?? () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#4  0x00007f29feb94a8b in chase () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#5  0x00007f29feb956e2 in chase_and_opendir () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#6  0x00007f29feb9a609 in conf_files_list_strv () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#7  0x00007f29fea913e8 in config_get_stats_by_path () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#8  0x0000559f295519cf in ?? ()
#9  0x0000559f29553a77 in ?? ()
#10 0x00007f29fec36055 in ?? () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#11 0x00007f29fec3668d in sd_event_dispatch () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#12 0x00007f29fec394a8 in sd_event_run () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#13 0x00007f29fec396c7 in sd_event_loop () from /usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-257.so
#14 0x0000559f29545820 in ?? ()
#15 0x00007f29fe78bca8 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#16 0x00007f29fe78bd65 in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
#17 0x0000559f29545c51 in ?? ()

How I did it

Added a kill_stale_udevd() function to dpu-udev-manager.sh that runs before writing the udev rules. It identifies the systemd-managed udevd PID via systemctl show, then kills any other systemd-udevd --daemon process that doesn't match -- these are leftover initramfs instances. If no stale process exists (e.g. DPUs are healthy and the initramfs udevd exited cleanly), the function is a no-op.

How to verify it

• Deploy the image on a SmartSwitch with DPUs in a state where firmware initialization times out (>60s per DPU) by stopping image installation before firmware install step
• Reboot the switch
• Verify no new systemd-udevd coredumps in /var/core/
• Verify the stale process was killed: journalctl -b 0 | grep dpu-udev-manager should show killing stale initramfs udevd PID (systemd udevd is PID )
• Verify systemd-udevd.service is healthy: systemctl status systemd-udevd should show active (running)
• Verify DPU udev rules were written: cat /etc/udev/rules.d/92-midplane-intf.rules should contain the DPU interface naming rules

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

This PR addresses SmartSwitch reboot instability on DPU-based platforms by preventing a stale initramfs systemd-udevd process from surviving switch_root and crashing when udev rules are updated in the real root filesystem.

Changes:

• Added kill_stale_udevd() to detect and terminate leftover initramfs systemd-udevd --daemon processes before writing DPU udev rules.
• Updated the script header (shebang and copyright/SPDX metadata).