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| -rw-r--r-- | Documentation/filesystems/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/filesystems/ramfs-rootfs-initramfs.rst (renamed from Documentation/filesystems/ramfs-rootfs-initramfs.txt) | 54 |
2 files changed, 33 insertions, 22 deletions
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst index 08883a481a76..b8689d082911 100644 --- a/Documentation/filesystems/index.rst +++ b/Documentation/filesystems/index.rst @@ -83,5 +83,6 @@ Documentation for filesystem implementations. overlayfs proc qnx6 + ramfs-rootfs-initramfs virtiofs vfat diff --git a/Documentation/filesystems/ramfs-rootfs-initramfs.txt b/Documentation/filesystems/ramfs-rootfs-initramfs.rst index 97d42ccaa92d..6c576e241d86 100644 --- a/Documentation/filesystems/ramfs-rootfs-initramfs.txt +++ b/Documentation/filesystems/ramfs-rootfs-initramfs.rst @@ -1,5 +1,11 @@ -ramfs, rootfs and initramfs +.. SPDX-License-Identifier: GPL-2.0 + +=========================== +Ramfs, rootfs and initramfs +=========================== + October 17, 2005 + Rob Landley <rob@landley.net> ============================= @@ -99,14 +105,14 @@ out of that. All this differs from the old initrd in several ways: - The old initrd was always a separate file, while the initramfs archive is - linked into the linux kernel image. (The directory linux-*/usr is devoted - to generating this archive during the build.) + linked into the linux kernel image. (The directory ``linux-*/usr`` is + devoted to generating this archive during the build.) - The old initrd file was a gzipped filesystem image (in some file format, such as ext2, that needed a driver built into the kernel), while the new initramfs archive is a gzipped cpio archive (like tar only simpler, - see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst). The - kernel's cpio extraction code is not only extremely small, it's also + see cpio(1) and Documentation/driver-api/early-userspace/buffer-format.rst). + The kernel's cpio extraction code is not only extremely small, it's also __init text and data that can be discarded during the boot process. - The program run by the old initrd (which was called /initrd, not /init) did @@ -139,7 +145,7 @@ and living in usr/Kconfig) can be used to specify a source for the initramfs archive, which will automatically be incorporated into the resulting binary. This option can point to an existing gzipped cpio archive, a directory containing files to be archived, or a text file -specification such as the following example: +specification such as the following example:: dir /dev 755 0 0 nod /dev/console 644 0 0 c 5 1 @@ -175,12 +181,12 @@ or extracting your own preprepared cpio files to feed to the kernel build (instead of a config file or directory). The following command line can extract a cpio image (either by the above script -or by the kernel build) back into its component files: +or by the kernel build) back into its component files:: cpio -i -d -H newc -F initramfs_data.cpio --no-absolute-filenames The following shell script can create a prebuilt cpio archive you can -use in place of the above config file: +use in place of the above config file:: #!/bin/sh @@ -202,14 +208,17 @@ use in place of the above config file: exit 1 fi -Note: The cpio man page contains some bad advice that will break your initramfs -archive if you follow it. It says "A typical way to generate the list -of filenames is with the find command; you should give find the -depth option -to minimize problems with permissions on directories that are unwritable or not -searchable." Don't do this when creating initramfs.cpio.gz images, it won't -work. The Linux kernel cpio extractor won't create files in a directory that -doesn't exist, so the directory entries must go before the files that go in -those directories. The above script gets them in the right order. +.. Note:: + + The cpio man page contains some bad advice that will break your initramfs + archive if you follow it. It says "A typical way to generate the list + of filenames is with the find command; you should give find the -depth + option to minimize problems with permissions on directories that are + unwritable or not searchable." Don't do this when creating + initramfs.cpio.gz images, it won't work. The Linux kernel cpio extractor + won't create files in a directory that doesn't exist, so the directory + entries must go before the files that go in those directories. + The above script gets them in the right order. External initramfs images: -------------------------- @@ -236,9 +245,10 @@ An initramfs archive is a complete self-contained root filesystem for Linux. If you don't already understand what shared libraries, devices, and paths you need to get a minimal root filesystem up and running, here are some references: -http://www.tldp.org/HOWTO/Bootdisk-HOWTO/ -http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html -http://www.linuxfromscratch.org/lfs/view/stable/ + +- http://www.tldp.org/HOWTO/Bootdisk-HOWTO/ +- http://www.tldp.org/HOWTO/From-PowerUp-To-Bash-Prompt-HOWTO.html +- http://www.linuxfromscratch.org/lfs/view/stable/ The "klibc" package (http://www.kernel.org/pub/linux/libs/klibc) is designed to be a tiny C library to statically link early userspace @@ -255,7 +265,7 @@ name lookups, even when otherwise statically linked.) A good first step is to get initramfs to run a statically linked "hello world" program as init, and test it under an emulator like qemu (www.qemu.org) or -User Mode Linux, like so: +User Mode Linux, like so:: cat > hello.c << EOF #include <stdio.h> @@ -326,8 +336,8 @@ the above threads) is: explained his reasoning: - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html + - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1550.html + - http://www.uwsg.iu.edu/hypermail/linux/kernel/0112.2/1638.html and, most importantly, designed and implemented the initramfs code. |