A freshly built kernel cannot RAM-boot against the modules already on the device

scope: generic · severity: trap · confidence: proven · subsystem: build

Symptom — you RAM-boot a kernel you just built and the phone comes up almost. It pings, the USB gadget enumerates, ssh is refused. No panic, no oops, nothing on netconsole. It reads exactly like a bad kernel, and the obvious next move – suspect the change you just made – is wrong.

Check /proc/version before anything else. If the build number is not the one on the flashed slot, you are here.

Cause — a RAM boot replaces the kernel and nothing else. The initramfs and /lib/modules on the device still belong to the kernel that was flashed. Rebuilding moves the build id and the BTF, so every module is refused:

failed to validate module [uinput] BTF: -22
modprobe: ERROR: could not insert 'uinput': Invalid argument

On a device whose initramfs needs a module to mount root, that is fatal rather than merely degraded. taimen keeps its pmOS install as a subpartition image inside /dev/sda13, so the initramfs must losetup it, so it needs loop.ko. Without it: failed to mount subpartitions! and then the debug shell – which is a perfectly friendly place, once you know you are in it (see initramfs-is-not-frozen).

The widespread half-belief that this is a CONFIG-change hazard is what makes it bite. A config edit does move every module’s module_layout CRC, and that is true and separate. But BTF and the build id move on every kernel rebuild, config touched or not. There is no such thing as a source-only kernel change that keeps the installed modules loadable.

What to do — a DTS-only RAM boot is unaffected and stays the cheap rung; it does not rebuild the kernel. For anything that rebuilds Image.gz, use the rung that regenerates the initramfs and pushes matching modules (porthole build fast). If the device cannot RAM-boot at all, say so once in the profile and let the tool refuse:

PORTHOLE_RAMBOOT_NEEDS_MODULES=loop

and porthole build boot --kernel will stop before it strands the phone, naming the rung to use instead.