linux-kernel - [PATCH v1] moduleparam: Save information about built-in modules in separate file

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Message-ID: <20190306165425.GD14017@MacBook-PC.fortress>
Date:   Wed, 6 Mar 2019 17:54:25 +0100
From:   Alexey Gladkov <gladkov.alexey@...il.com>
To:     linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-api@...r.kernel.org
Cc:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        "Dmitry V. Levin" <ldv@...linux.org>
Subject: [PATCH v1] moduleparam: Save information about built-in modules in
 separate file

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.modinfo
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
...

Signed-off-by: Alexey Gladkov <gladkov.alexey@...il.com>
Signed-off-by: Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>
---
 Makefile                    |  1 +
 include/linux/moduleparam.h | 12 +++++-------
 scripts/link-vmlinux.sh     |  8 ++++++++
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index d5713e7b1e50..971102194c92 100644
--- a/Makefile
+++ b/Makefile
@@ -1288,6 +1288,7 @@ _modinst_:
 	fi
 	@cp -f $(objtree)/modules.order $(MODLIB)/
 	@cp -f $(objtree)/modules.builtin $(MODLIB)/
+	@cp -f $(objtree)/kernel.builtin.modinfo $(MODLIB)/
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst

 # This depmod is only for convenience to give the initial
diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
index ba36506db4fb..5ba250d9172a 100644
--- a/include/linux/moduleparam.h
+++ b/include/linux/moduleparam.h
@@ -10,23 +10,21 @@
    module name. */
 #ifdef MODULE
 #define MODULE_PARAM_PREFIX /* empty */
+#define __MODULE_INFO_PREFIX /* empty */
 #else
 #define MODULE_PARAM_PREFIX KBUILD_MODNAME "."
+/* We cannot use MODULE_PARAM_PREFIX because some modules override it. */
+#define __MODULE_INFO_PREFIX KBUILD_MODNAME "."
 #endif

 /* Chosen so that structs with an unsigned long line up. */
 #define MAX_PARAM_PREFIX_LEN (64 - sizeof(unsigned long))

-#ifdef MODULE
 #define __MODULE_INFO(tag, name, info)					  \
 static const char __UNIQUE_ID(name)[]					  \
   __used __attribute__((section(".modinfo"), unused, aligned(1)))	  \
-  = __stringify(tag) "=" info
-#else  /* !MODULE */
-/* This struct is here for syntactic coherency, it is not used */
-#define __MODULE_INFO(tag, name, info)					  \
-  struct __UNIQUE_ID(name) {}
-#endif
+  = __MODULE_INFO_PREFIX __stringify(tag) "=" info
+
 #define __MODULE_PARM_TYPE(name, _type)					  \
   __MODULE_INFO(parmtype, name##type, #name ":" _type)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index c8cf45362bd6..399d7e4d11ec 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -258,10 +258,12 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then

 	# step 1
 	vmlinux_link "" .tmp_vmlinux1
+	"${OBJCOPY}" -R .modinfo .tmp_vmlinux1
 	kallsyms .tmp_vmlinux1 .tmp_kallsyms1.o

 	# step 2
 	vmlinux_link .tmp_kallsyms1.o .tmp_vmlinux2
+	"${OBJCOPY}" -R .modinfo .tmp_vmlinux2
 	kallsyms .tmp_vmlinux2 .tmp_kallsyms2.o

 	# step 3
@@ -273,6 +275,7 @@ if [ -n "${CONFIG_KALLSYMS}" ]; then
 		kallsyms_vmlinux=.tmp_vmlinux3

 		vmlinux_link .tmp_kallsyms2.o .tmp_vmlinux3
+		"${OBJCOPY}" -R .modinfo .tmp_vmlinux3

 		kallsyms .tmp_vmlinux3 .tmp_kallsyms3.o
 	fi
@@ -281,6 +284,11 @@ fi
 info LD vmlinux
 vmlinux_link "${kallsymso}" vmlinux

+info MODINFO kernel.builtin.modinfo
+"${OBJCOPY}" -j .modinfo -O binary vmlinux kernel.builtin.modinfo
+chmod 444 kernel.builtin.modinfo
+"${OBJCOPY}" -R .modinfo vmlinux
+
 if [ -n "${CONFIG_BUILDTIME_EXTABLE_SORT}" ]; then
 	info SORTEX vmlinux
 	sortextable vmlinux
-- 
2.19.2

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