One new feature in kmod 34
is related to lazily loading the decompression libraries. In other words,
dlopen them when/if they are needed. Although it’s desired for a tool like
modinfo to be able to inspect a .ko.xz, .ko.zst or .ko.gz module, other
daemons linking to libkmod would benefit from never loading them. This is the
case for systemd-udevd that will load the kernel modules during boot when they
are requested by the kernel.
Testing on Archlinux, it goes from this:
$ cat /proc/$(pidof systemd-udevd)/maps | grep -e libz -e liblzma
7f27a9ae8000-7f27a9aeb000 r--p 00000000 00:19 15656 /usr/lib/liblzma.so.5.6.4
7f27a9aeb000-7f27a9b0e000 r-xp 00003000 00:19 15656 /usr/lib/liblzma.so.5.6.4
7f27a9b0e000-7f27a9b19000 r--p 00026000 00:19 15656 /usr/lib/liblzma.so.5.6.4
7f27a9b19000-7f27a9b1a000 r--p 00031000 00:19 15656 /usr/lib/liblzma.so.5.6.4
7f27a9b1a000-7f27a9b1b000 rw-p 00032000 00:19 15656 /usr/lib/liblzma.so.5.6.4
7f27a9b1b000-7f27a9b27000 r--p 00000000 00:19 15985 /usr/lib/libzstd.so.1.5.7
7f27a9b27000-7f27a9bed000 r-xp 0000c000 00:19 15985 /usr/lib/libzstd.so.1.5.7
7f27a9bed000-7f27a9bfe000 r--p 000d2000 00:19 15985 /usr/lib/libzstd.so.1.5.7
7f27a9bfe000-7f27a9bff000 r--p 000e3000 00:19 15985 /usr/lib/libzstd.so.1.5.7
7f27a9bff000-7f27a9c00000 rw-p 000e4000 00:19 15985 /usr/lib/libzstd.so.1.5.7
7f27aa892000-7f27aa895000 r--p 00000000 00:19 7852 /usr/lib/libz.so.1.3.1
7f27aa895000-7f27aa8a3000 r-xp 00003000 00:19 7852 /usr/lib/libz.so.1.3.1
7f27aa8a3000-7f27aa8a9000 r--p 00011000 00:19 7852 /usr/lib/libz.so.1.3.1
7f27aa8a9000-7f27aa8aa000 r--p 00017000 00:19 7852 /usr/lib/libz.so.1.3.1
7f27aa8aa000-7f27aa8ab000 rw-p 00018000 00:19 7852 /usr/lib/libz.so.1.3.1
$
to this:
$ cat /proc/$(pidof systemd-udevd)/maps | grep -e libz -e liblzma
$
… even if all modules in Archlinux are zstd-compressed.
systemd itself started doing this a few releases ago and published https://systemd.io/ELF_PACKAGE_METADATA/. That spec is also used for this new support in kmod to annotate what libraries are possibly dlopen’ed. However although it prevented libkmod from being loaded in other binaries, it didn’t prevent all these decompression libraries from being mapped in systemd-udevd since it uses libkmod.
One might wonder why not even libzstd.so is mapped. That’s because when
loading the modules and the kernel supports decompressing that format, libkmod
just skips any decompression: it opens the file and passes the descriptor
to the Linux kernel via finit_module.
/sys/module/compression shows what algorithm the Linux kernel can use for
module decompression.
In kmod 34 all that is needed is to setup the build with -Ddlopen=all.
More fine-grained options are also supported, allowing to specify individual
libraries to dlopen. It may go away in a future release if distros just choose
an all-or-nothing support.