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

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Date:   Thu, 18 Apr 2019 07:36:25 -0700
From:   Lucas De Marchi <lucas.de.marchi@...il.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        linux-api@...r.kernel.org,
        linux-modules <linux-modules@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH v2] moduleparam: Save information about built-in modules
 in separate file

On Thu, Apr 18, 2019 at 6:52 AM Jessica Yu <jeyu@...nel.org> wrote:
>
> +++ Masahiro Yamada [18/04/19 20:10 +0900]:
> >On Sat, Apr 6, 2019 at 9:15 PM Alexey Gladkov <gladkov.alexey@...il.com> wrote:
> >>
> >> Problem:
> >>
> >> 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.
> >>
> >> Proposal:
> >>
> >> 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.license=GPL
> >> ext4.description=Fourth Extended Filesystem
> >> ext4.author=Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others
> >> ext4.alias=fs-ext4
> >> ext4.alias=ext3
> >> ext4.alias=fs-ext3
> >> ext4.alias=ext2
> >> ext4.alias=fs-ext2
> >> md_mod.alias=block-major-9-*
> >> md_mod.alias=md
> >> md_mod.description=MD RAID framework
> >> md_mod.license=GPL
> >> md_mod.parmtype=create_on_open:bool
> >> md_mod.parmtype=start_dirty_degraded:int
> >> ...
> >>
> >> v2:
> >>  * 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

I agree with modules.builtin.modinfo

> 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.

I think it's ok with the current format. It's simple enough. I would
delay merging it until
the userspace counterpart is implemented though. So to make sure we
have everything we need.
I can work on that in ~1 or 2 weeks.

thanks
Lucas De Marchi

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



-- 
Lucas De Marchi