Re: [PATCH v2] moduleparam: Save information about built-in modules in separate file

From: Jessica Yu
Date: Thu Apr 18 2019 - 09:52:49 EST

+++ Masahiro Yamada [18/04/19 20:10 +0900]:
On Sat, Apr 6, 2019 at 9:15 PM Alexey Gladkov <gladkov.alexey@xxxxxxxxx> wrote:


When a kernel module is compiled as a separate module, some important
information about the kernel module is available via .modinfo section of
the module. In contrast, when the kernel module is compiled into the
kernel, that information is not available.

Information about built-in modules is necessary in the following cases:

1. When it is necessary to find out what additional parameters can be
passed to the kernel at boot time.

2. When you need to know which module names and their aliases are in
the kernel. This is very useful for creating an initrd image.


The proposed patch does not remove .modinfo section with module
information from the vmlinux at the build time and saves it into a
separate file after kernel linking. So, the kernel does not increase in
size and no additional information remains in it. Information is stored
in the same format as in the separate modules (null-terminated string
array). Because the .modinfo section is already exported with a separate
modules, we are not creating a new API.

It can be easily read in the userspace:

$ tr '\0' '\n' < kernel.builtin
ext4.softdep=pre: crc32c
ext4.description=Fourth Extended Filesystem Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others
md_mod.description=MD RAID framework

* Extract modinfo from vmlinux.o as suggested by Masahiro Yamada;
* Rename output file to kernel.builtin;

Sorry, I do not get why you renamed
"kernel.builtin.modinfo" to "kernel.builtin".

If you drop "modinfo", we do not understand
what kind information is contained in it.

I think "kernel" and "builtin" have
a quite similar meaning here.

How about "builtin.modinfo" for example?

It is shorter, and it is clear enough
that it contains module_info.

I agree that the name kernel.builtin is unclear in what kind of
information it contains. Apologies for not having clarified this in
the previous review.

Since kbuild already produces "modules.order" and "modules.builtin"
files, why not just name it "modules.builtin.modinfo" to keep the
names consistent with what is already there? It clearly conveys that
this file contains modinfo for builtin modules.

I'll leave it up to Lucas to decide if the file format is OK for kmod.
With the modinfo dump, kmod could then decide what to do with the
information, append to modules.alias{,.bin}, etc.

Also, I think this file needs to be documented in Documentation/kbuild/kbuild.txt.