Cannot "make install" with DESTDIR set

[kohlschuetter]

System information

Distribution Name | Alpine Linux
Distribution Version | 3.21.2
Kernel Version | 6.12.9 (host) 6.12.11 (target)
Architecture | x86_64
OpenZFS Version | 2.3.0 / master

Describe the problem you're observing

"make install" with a specified "DESTDIR=" may fail when installing to a root that contains an existing lib/modules structure, or may do the wrong thing when run as root (because we may affect the running kernel, not the target kernel)

Describe how to reproduce the problem

1. Build the Linux kernel, install modules into KDIR (e.g., if KDIR=/tmp/kernel-out/, there would be /tmp/kernel-out/lib/modules/6.12.11/, /tmp/kernel-out/boot/System.map-6.12.11 etc.)
2. Build and install zfs with "DESTDIR=$KDIR make install"

Include any warning/errors/backtraces from the system logs

...
sysmap=/tmp/kernel-out/boot/System.map-6.12.11; \
{ [ -f "$sysmap" ] && [ $(wc -l < "$sysmap") -ge 100 ]; } || \
	sysmap=/tmp/kernel-out/usr/lib/debug/boot/System.map-6.12.11; \
if [ -f $sysmap ]; then \
	depmod -ae -F $sysmap 6.12.11 ; \
fi
depmod: WARNING: could not open modules.order at /tmp/kernel-out/lib/modules/6.12.11: No such file or directory
depmod: WARNING: could not open modules.builtin at /tmp/kernel-out/lib/modules/6.12.11: No such file or directory
depmod: WARNING: could not open modules.builtin.modinfo at /tmp/kernel-out/lib/modules/6.12.11: No such file or directory
...

This is due to the makefile (module/Makefile) explicitly deleting all files starting with modules.* (see "Remove extraneous build products when packaging")

Even after disabling these lines, the installation does not succeed:

...
sysmap=/tmp/kernel-out/boot/System.map-6.12.11; \
{ [ -f "$sysmap" ] && [ $(wc -l < "$sysmap") -ge 100 ]; } || \
	sysmap=/tmp/kernel-out/usr/lib/debug/boot/System.map-6.12.11; \
if [ -f $sysmap ]; then \
	depmod -ae -F $sysmap 6.12.11 ; \
fi
depmod: ERROR: could not open directory /lib/modules/6.12.11: No such file or directory
depmod: FATAL: could not search modules: No such file or directory
...

Here, we try to run depmod on the wrong kernel (the host system's kernel, not the one under DESTDIR / not using the modules in INSTALL_MOD_PATH)

Adding -b $(INSTALL_MOD_PATH) to depmod (INSTALL_MOD_PATH defaults to DESTDIR) does the trick.