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blochl
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Joined: 28 Aug 2018
Posts: 16
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 Posted: Thu Oct 18, 2018 11:24 am Post subject: [Solved] Linking existing kernel with new initramfs |
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Hello,
I'm quite puzzled with the following problem: I have an xz-compressed bzImage, which contains an internal initramfs, and I want to make changes to the initramfs without re-compiling the kernel (because the changes might be done in a totally different environment from where the kernel was compiled).
Using an external initramfs which will be merged with the internal one at boot time is not an option, because I want a single bootable file (that can be signed) in the end.
I have read this and this tutorials about how a new initramfs might be inserted into an existing kernel, but the first one looks like it is written for someone who already knows what they are doing, and I can't figure out how to apply it to my case. The second one (section: "Combine the kernel, initramfs, and boot options") while really detailed, addresses a different scenario from what I have, and again, I can't figure out how to apply it to my case.
Can someone please explain (or point me to another tutorial) how given a bzImage with internal initramfs and a new initramfs I can get a bzImage linked only with the new initramfs?
I think that the solution should be similar to the tutorials above, I just can't get the exact procedure so far...
Last edited by blochl on Wed Oct 24, 2018 12:55 pm; edited 1 time in total |
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khayyam
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Joined: 07 Jun 2012
Posts: 6227
Location: Room 101
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| Posted: Thu Oct 18, 2018 1:39 pm Post subject: Re: Linking existing kernel with new initramfs |
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| blochl wrote: | | I have read this and this tutorials about how a new initramfs might be inserted into an existing kernel, but the first one looks like it is written for someone who already knows what they are doing, and I can't figure out how to apply it to my case. |
blochl ... the instructions from coreboot look pretty staightforward to me:
| Code: | # file initramfs
initramfs: ASCII cpio archive (SVR4 with no CRC)
# objcopy -I binary -O elf32-i386 -B i386 --redefine-sym _binary_initramfs_start=.init.ramfs -N _binary_initramfs_end -N _binary_initramfs_size initramfs initramfs.elf
# file initramfs.elf
initramfs.elf: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped
# cp /usr/src/linux/vmlinux .
# file vmlinux
vmlinux: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), statically linked, BuildID[sha1]=364adc71e0470c199d5cd55e6bcaeb9163943ce4, not stripped
# objcopy -I elf32-i386 -O elf32-i386 -R .init.ramfs --add-section .init.ramfs=initramfs.elf vmlinux vmlinux.withnewinitramfs
# ls -lh vmlinux*
-rwxr-xr-x 1 root root 10M 2018-04-28 19:46 vmlinux*
-rwxr-xr-x 1 root root 16M 2018-10-18 15:31 vmlinux.withnewinitramfs* |
The initramfs involved is much bigger than the one currently in use, and so the output seems about right, though I haven't attempted to boot it.
Any questions, just ask ... best ... khay |
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blochl
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Joined: 28 Aug 2018
Posts: 16
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| Posted: Thu Oct 18, 2018 5:41 pm Post subject: |
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Sorry, guess I wasn't specific enough. And thanks for the `file` clarifications, khayyam - that already made things a bit more clear. Though you use an uncompressed initramfs, and they use gzipped.
Currently my main showstopper is compression: as I said, I have an xz-compressed bzImage, while the instructions from coreboot require uncompressed vmlinux. Sure, I can get vmlinux with the in-tree script "scripts/extract-vmlinux", but then how can I compress it back?
On the other hand, in the second tutorial a compressed kernel is OK, but it requires the initramfs to be uncompressed. Sure, I can uncompress it before linking, but then the kernel will take much more space (as the initramfs will not be compressed after linking) and also they are not clear about what is "linuxx64.efi.stub" they refer to, if the kernel in their example is "vmlinuz-linux".
There are some more technical questions (like why the VMA addresses are set to these values (0x40000, 0x3000000)?) but the showstopper for me now is what to do with the compression. |
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blochl
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Joined: 28 Aug 2018
Posts: 16
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| Posted: Sat Oct 20, 2018 9:56 am Post subject: |
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Another problem: I don't even see the relevant section (".init.ramfs") in vmlinux (although it definitely has the initramfs linked-in). All I see is:
| Code: |
vmlinux: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00602000 ffffffff81000000 0000000001000000 00200000 2**12
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .notes 00000024 ffffffff81602000 0000000001602000 00802000 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 __ex_table 0000255c ffffffff81602030 0000000001602030 00802030 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .rodata 0010ee78 ffffffff81800000 0000000001800000 00a00000 2**12
CONTENTS, ALLOC, LOAD, DATA
4 .pci_fixup 00003300 ffffffff8190ee78 000000000190ee78 00b0ee78 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 __param 00002120 ffffffff81912178 0000000001912178 00b12178 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 __modver 00000d68 ffffffff81914298 0000000001914298 00b14298 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 .data 00086a80 ffffffff81a00000 0000000001a00000 00c00000 2**13
CONTENTS, ALLOC, LOAD, DATA
8 __bug_table 00009f60 ffffffff81a86a80 0000000001a86a80 00c86a80 2**0
CONTENTS, ALLOC, LOAD, DATA
9 .orc_unwind_ip 0006e860 ffffffff81a909e0 0000000001a909e0 00c909e0 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 .orc_unwind 000a5c90 ffffffff81aff242 0000000001aff242 00cff242 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
11 .orc_lookup 00018084 ffffffff81ba4ed4 0000000001ba4ed4 00da4ed2 2**0
ALLOC
12 .vvar 00001000 ffffffff81bbd000 0000000001bbd000 00dbd000 2**4
CONTENTS, ALLOC, LOAD, DATA
13 .init.text 0002b7bd ffffffff81bbe000 0000000001bbe000 00dbe000 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
14 .altinstr_aux 000007de ffffffff81be97bd 0000000001be97bd 00de97bd 2**0
CONTENTS, ALLOC, LOAD, READONLY, CODE
15 .init.data 00506678 ffffffff81bea000 0000000001bea000 00dea000 2**13
CONTENTS, ALLOC, LOAD, DATA
16 .x86_cpu_dev.init 00000018 ffffffff820f0678 00000000020f0678 012f0678 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
17 .altinstructions 00002db4 ffffffff820f0690 00000000020f0690 012f0690 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
18 .altinstr_replacement 00000abd ffffffff820f3444 00000000020f3444 012f3444 2**0
CONTENTS, ALLOC, LOAD, READONLY, CODE
19 .iommu_table 00000050 ffffffff820f3f08 00000000020f3f08 012f3f08 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
20 .apicdrivers 00000010 ffffffff820f3f58 00000000020f3f58 012f3f58 2**3
CONTENTS, ALLOC, LOAD, DATA
21 .exit.text 00001282 ffffffff820f3f68 00000000020f3f68 012f3f68 2**0
CONTENTS, ALLOC, LOAD, READONLY, CODE
22 .data..percpu 00000000 ffffffff820f6000 00000000020f51ea 012f51ea 2**0
CONTENTS, ALLOC, LOAD, DATA
23 .smp_locks 00000000 ffffffff820f6000 ffffffff820f6000 012f51ea 2**0
CONTENTS
24 .data_nosave 00000000 ffffffff820f6000 ffffffff820f6000 012f51ea 2**0
CONTENTS
25 .bss 00031000 ffffffff820f6000 00000000020f6000 012f51ea 2**12
ALLOC
26 .brk 0002c000 ffffffff82127000 0000000002127000 012f51ea 2**0
ALLOC
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khayyam
Watchman
Joined: 07 Jun 2012
Posts: 6227
Location: Room 101
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| Posted: Sat Oct 20, 2018 10:46 am Post subject: |
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blochl ...
I'm currently in the process of packing for a move next week, and so don't have time to see what might be done re compression. I know there is a script for this in the kernel sources, but it's not something I've done (and so would have to search and see what I came up with). Subsequent to the move I'll be offline until I'm able to afford service ... so, at this time I'm not in the best situation to help, sorry. Hopefully someone else will jump in.
good luck & best ... khay |
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blochl
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Joined: 28 Aug 2018
Posts: 16
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| Posted: Sat Oct 20, 2018 6:01 pm Post subject: |
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khay, I really appreciate that! Thanks! And good luck with your moving!
I couldn't find a script for that in the kernel sources, and as I saw the compression process is quite complicated: there are different headers that need to be prepended, etc...
I'll keep looking. Thanks for any additional help... |
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blochl
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Joined: 28 Aug 2018
Posts: 16
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| Posted: Sun Oct 21, 2018 7:05 pm Post subject: |
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OK, after more research into this, my goals have changed just a bit:
Instead of changing the embedded initramfs (which has some delicate parts in it) it would be much better to use the mechanism that exists in the kernel for merging an external initramfs with the internal one. But, in order to be signed, the external initrd (it loads as initrd, despite being an initramfs) should still be a part of the kernel. So I'm looking at the following now:
This, where the initrd is linked to the ready-made compressed kernel, with the following command (I modified it here for my case):
| Code: |
objcopy --add-section .initrd="external.initramfs" \
--change-section-vma .initrd=0x3000000 \
original-bzImage new-bzImage
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But this doesn't work. The system doesn't boot with such a kernel.
The output of | Code: | | objdump -h new-bzImage |
is:
| Code: |
Sections:
Idx Name Size VMA LMA File off Algn
0 .setup 00003be0 0000000000000200 0000000000000200 00000250 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .reloc 00000020 0000000000003de0 0000000000003de0 00003e30 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .text 0071d230 0000000000003e00 0000000000003e00 00003e50 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
3 .bss 00c5afd0 0000000000721030 0000000000721030 00000000 2**4
ALLOC
4 .initrd 00000200 0000000003000000 0000000003000000 00721090 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
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I also get this warning from the "objcopy" part:
| Code: |
Warning: Ignoring section flag IMAGE_SCN_MEM_NOT_PAGED in section .bss
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The second place I'm looking at is here. There they manage to load the initrd not from a file, but from a specific location in the memory. So I see that something like this is possible, although their case is very different from mine.
Does anyone have a clue how could I load an "external" initrd from the same file that already contains an internal initramfs? Really banging my head against the wall here...  |
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blochl
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Joined: 28 Aug 2018
Posts: 16
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| Posted: Wed Oct 24, 2018 12:54 pm Post subject: |
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Alright, marking this as solved, because I've realized that this problem is unsolvable:
* The externally linked section is good only for systemd-boot, and I can't use this, because I want my kernel to be bootable in legacy mode as well as in UEFI, but systemd-boot "EFI Unified Kernel Images" are working only with UEFI boot.
* Re-linking the initramfs inside the kernel is impossible to do in a reliable way: first of all, a lot of work needs to be done to re-link the binary, and even when done and automated, there is no guarantee that it will work on another version of the kernel, or even on the same version that was compiled with a different config.
Therefore I'm going for the really messy solution of recompiling the kernel each time I want to change the initramfs. It's very messy and not elegant, but it's the only way to get what I want in a reliable manner.
I'm amazed of the asymmetry - how easy it is to extract the built-in initramfs, and how difficult it is to insert a new one in its place! |
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khayyam
Watchman
Joined: 07 Jun 2012
Posts: 6227
Location: Room 101
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| Posted: Wed Oct 24, 2018 8:46 pm Post subject: |
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| blochl wrote: | | Therefore I'm going for the really messy solution of recompiling the kernel each time I want to change the initramfs. It's very messy and not elegant, but it's the only way to get what I want in a reliable manner. |
blochl ... why not build an initramfs that doesn't require updating? ... there's no reason to really (it's just that some think it a good idea to include kernel modules in the initramfs). Here's an example, or the prebuilt version. Note, these will be built against uClibc (and with the 'devel' branch, musl), and so the size is considerably smaller.
Back to packing!!! & best ... khay |
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