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;
 * Add MODULE_VERSION to modinfo that is saved to the kernel.builtin;
 * Fix build warnings on powerpc.

v3:
 * Rename output file to modules.builtin.modinfo as suggested by Masahiro Yamada;
 * Update Documentation/dontdiff, Documentation/kbuild/kbuild.txt.

Co-Developed-by: Gleb Fotengauer-Malinovskiy <redacted>
Signed-off-by: Gleb Fotengauer-Malinovskiy <redacted>
Signed-off-by: Alexey Gladkov <redacted>
---
 .gitignore                        |  1 +
 Documentation/dontdiff            |  1 +
 Documentation/kbuild/kbuild.txt   |  5 +++++
 Makefile                          |  2 ++
 include/asm-generic/vmlinux.lds.h |  1 +
 include/linux/module.h            |  1 +
 include/linux/moduleparam.h       | 12 +++++-------
 scripts/link-vmlinux.sh           |  4 ++++
 8 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/.gitignore b/.gitignore
index a20ac26aa2f5..8ef2c87703b3 100644
--- a/.gitignore
+++ b/.gitignore

@@ -45,6 +45,7 @@
 *.xz
 Module.symvers
 modules.builtin
+modules.builtin.modinfo

 #
 # Top-level generic files

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index c8cf45362bd6..41ef7cb043c1 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh

@@ -226,6 +226,10 @@ modpost_link vmlinux.o
 # modpost vmlinux.o to check for section mismatches
 ${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o

+info MODINFO modules.builtin.modinfo
+"${OBJCOPY}" -j .modinfo -O binary vmlinux.o modules.builtin.modinfo
+chmod 444 modules.builtin.modinfo