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cadwal_se
n00b
Joined: 16 Feb 2018
Posts: 5
Location: Sweden
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 Posted: Fri Feb 16, 2018 4:58 pm Post subject: bzImage doubled in size |
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Somewhere around when binutils and gcc 6.4.0 turned up the size of bzImage doubled in size for me.
Kernel 4.12.5 had previously been built, bzImage was/is about 5.5 MB with my options, but any new kernel compiles for the same version with the same config yields bzImage that is 9.5 MB.
(Later kernels is just as big, including the the new 4.9.76-r1 after I switched to the 17.0 profile.)
At the time I tried switching back to previous binutils and gcc (numbers lost since I later removed the old versions in my experimentation) but that did not change anything.
I left it alone for a while on the assumption that "later versions would fix it" ...
Things are still the same though so this time I wanted to look deeper:
So more info:
arch/x86/boot/bzImage is big because it is based on arch/x86/boot/compressed/vmlinux which is also big.
arch/x86/boot/compressed/vmlinux is created using this command during kernel build (this was for 4.14.8, pgtable_64.o does not exist in older kernels):
| Code: |
ld -m elf_x86_64 -z noreloc-overflow -pie --no-dynamic-linker -T arch/x86/boot/compressed/vmlinux.lds arch/x86/boot/compressed/head_64.o arch/x86/boot/compressed/misc.o arch/x86/boot/compressed/string.o arch/x86/boot/compressed/cmdline.o arch/x86/boot/compressed/error.o arch/x86/boot/compressed/piggy.o arch/x86/boot/compressed/cpuflags.o arch/x86/boot/compressed/early_serial_console.o arch/x86/boot/compressed/pgtable_64.o -o arch/x86/boot/compressed/vmlinux |
looking at the files involved we have this:
| Code: | ecce /usr/src/linux/arch/x86/boot/compressed # ls -lrt vmlinux.lds head_64.o misc.o string.o cmdline.o error.o piggy.o cpuflags.o early_serial_console.o pgtable_64.o vmlinux
-rw-r--r-- 1 root root 775 29 dec 13.48 vmlinux.lds
-rw-r--r-- 1 root root 6480 29 dec 13.48 head_64.o
-rw-r--r-- 1 root root 11976 29 dec 13.48 misc.o
-rw-r--r-- 1 root root 3480 29 dec 13.48 string.o
-rw-r--r-- 1 root root 2640 29 dec 13.48 cmdline.o
-rw-r--r-- 1 root root 1864 29 dec 13.48 error.o
-rw-r--r-- 1 root root 5905888 29 dec 13.48 piggy.o
-rw-r--r-- 1 root root 928 29 dec 13.48 cpuflags.o
-rw-r--r-- 1 root root 3440 29 dec 13.48 early_serial_console.o
-rw-r--r-- 1 root root 1312 29 dec 13.48 pgtable_64.o
-rwxr-xr-x 1 root root 12256280 29 dec 15.15 vmlinux
ecce /usr/src/linux/arch/x86/boot/compressed #
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Now, piggy.o is the actual kernel (compressed) and has the same size it did before any of this happened. (there is a slight increase with every kernel as expected).
Can anyone tell me how to find out how vmlinux ends up being more than twice the size of the files it is based on? Some odd hidden default flags to ld since reading the docs on the flags used does not tell me this is a likely outcome?
Note: I have not actually tried using any of the kernels after the size increase. |
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frostschutz
Advocate
Joined: 22 Feb 2005
Posts: 2977
Location: Germany
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| Posted: Fri Feb 16, 2018 6:27 pm Post subject: |
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You could binwalk --extract those bzImages (and again the extracted contents) just to see if there is anything obvious. Like a built-in initramfs that could be responsible for the size.
I also had a size increase, 4.12.5 is 9.4M, 4.13.4 9.5M, but bzImage-4.15.2 is 11M. This is a static kernel (no modules, required drivers builtin) and also a built-in initramfs that didn't change in any way worth mentioning.
I assumed it's related to whatever that meltdown/spectre nonsense. |
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R0b0t1
Apprentice
Joined: 05 Jun 2008
Posts: 264
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| Posted: Fri Feb 16, 2018 10:00 pm Post subject: |
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When bringing my original configuration forward, I noticed a lot of options defaulted to on, yet there was no noticeable size increase:
| Code: | # du -h `find /usr/src/ -name bzImage`
6.9M /usr/src/linux-4.14.8-gentoo-r1/arch/x86/boot/bzImage
6.8M /usr/src/linux-4.12.12-gentoo/arch/x86/boot/bzImage
7.0M /usr/src/linux-4.8.17-hardened-r2/arch/x86/boot/bzImage |
How did you get your kernel down to 5M in the first place, if you don't mind? |
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josephg
l33t
Joined: 10 Jan 2016
Posts: 783
Location: usually offline
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| Posted: Fri Feb 16, 2018 11:08 pm Post subject: Re: bzImage doubled in size |
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| cadwal_se wrote: | Somewhere around when binutils and gcc 6.4.0 turned up the size of bzImage doubled in size for me.
Kernel 4.12.5 had previously been built, bzImage was/is about 5.5 MB with my options, but any new kernel compiles for the same version with the same config yields bzImage that is 9.5 MB.
(Later kernels is just as big, including the the new 4.9.76-r1 after I switched to the 17.0 profile.) |
my gcc is 6.4.0-r1, kernel is 4.9.76-r1, and bzImage is 2.5M. previous kernels have been smaller. i do have initramfs and also build all modules out of kernel.
| Code: | -rw-r--r-- 1 root root 16154 2018-01-16 17:40 vmlinux.lds
-rw-r--r-- 1 root root 2980 2018-01-16 17:40 string.o
-rw-r--r-- 1 root root 644 2018-01-16 17:40 cpuflags.o
-rw-r--r-- 1 root root 640 2018-01-18 19:24 cmdline.o
-rw-r--r-- 1 root root 1572 2018-01-18 19:24 error.o
-rw-r--r-- 1 root root 19052 2018-02-11 19:48 misc.o
-rw-r--r-- 1 root root 2585952 2018-02-11 19:48 piggy.o
-rwxr-xr-x 1 root root 2608916 2018-02-11 19:48 vmlinux |
_________________
"Growth for the sake of growth is the ideology of the cancer cell." Edward Abbey |
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cadwal_se
n00b
Joined: 16 Feb 2018
Posts: 5
Location: Sweden
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| Posted: Sat Feb 17, 2018 11:59 am Post subject: |
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| R0b0t1 wrote: | | How did you get your kernel down to 5M in the first place, if you don't mind? |
I have the absolute minimum of drivers compiled in and have most of the "odd" networking and crypto stuff as modules. |
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bunder
Bodhisattva
Joined: 10 Apr 2004
Posts: 5934
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| Posted: Sat Feb 17, 2018 1:01 pm Post subject: |
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| frostschutz wrote: | | Like a built-in initramfs that could be responsible for the size. |
or changing compression... xz should yield the smallest file. |
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cadwal_se
n00b
Joined: 16 Feb 2018
Posts: 5
Location: Sweden
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| Posted: Sat Feb 17, 2018 5:20 pm Post subject: Re: bzImage doubled in size |
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| josephg wrote: |
my gcc is 6.4.0-r1, kernel is 4.9.76-r1, and bzImage is 2.5M. previous kernels have been smaller. i do have initramfs and also build all modules out of kernel.
| Code: | -rw-r--r-- 1 root root 16154 2018-01-16 17:40 vmlinux.lds
-rw-r--r-- 1 root root 2980 2018-01-16 17:40 string.o
-rw-r--r-- 1 root root 644 2018-01-16 17:40 cpuflags.o
-rw-r--r-- 1 root root 640 2018-01-18 19:24 cmdline.o
-rw-r--r-- 1 root root 1572 2018-01-18 19:24 error.o
-rw-r--r-- 1 root root 19052 2018-02-11 19:48 misc.o
-rw-r--r-- 1 root root 2585952 2018-02-11 19:48 piggy.o
-rwxr-xr-x 1 root root 2608916 2018-02-11 19:48 vmlinux |
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That is the way I would expect the file sizes to end up.
What versions of ld (and binutils) do you have? |
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cadwal_se
n00b
Joined: 16 Feb 2018
Posts: 5
Location: Sweden
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| Posted: Sat Feb 17, 2018 8:33 pm Post subject: |
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Ok, I have done some more checking and there is something odd going on with ld in this case...
If I remove the -pie option in the aforementioned ld command line and run it manually, the size of the intermediate vmlinux file goes down from twice the size of piggy.o to about 1.5 times the size.
And if I then let the regular make turn that into bzImage its back to its "normal" size:
| Code: |
-rw-r--r-- 1 root root 5547296 17 feb 21.28 ../bzImage
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Mmm...
And if I go the other way with binwalk:
| Code: |
ecce /tmp/t # ls -lrt
totalt 15172
-rw-r--r-- 1 root root 5720624 17 feb 18.17 kernel-4.12.5-gentoo
-rw-r--r-- 1 root root 9785328 17 feb 18.26 kernel-4.9.76-gentoo-r1
ecce /tmp/t # binwalk --extract kernel-4.12.5-gentoo
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
17014 0x4276 bzip2 compressed data, block size = 900k
ecce /tmp/t # binwalk --extract kernel-4.9.76-gentoo-r1
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
16502 0x4076 bzip2 compressed data, block size = 900k
ecce /tmp/t # ls -l
totalt 15180
-rw-r--r-- 1 root root 5720624 17 feb 18.17 kernel-4.12.5-gentoo
drwxr-xr-x 2 root root 4096 17 feb 18.43 _kernel-4.12.5-gentoo.extracted
-rw-r--r-- 1 root root 9785328 17 feb 18.26 kernel-4.9.76-gentoo-r1
drwxr-xr-x 2 root root 4096 17 feb 18.43 _kernel-4.9.76-gentoo-r1.extracted
ecce /tmp/t # ls -l _kernel-4.12.5-gentoo.extracted/
totalt 18628
-rw------- 1 root root 19048560 17 feb 18.43 4276
ecce /tmp/t # ls -l _kernel-4.9.76-gentoo-r1.extracted/
totalt 18516
-rw------- 1 root root 18933872 17 feb 18.43 4076
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And if I go one step further (skipping the long output from binwalk which appears fairly identical in both cases)
| Code: |
ecce /tmp/t # ls -l _kernel-4.12.5-gentoo.extracted/_4276.extracted/
totalt 51844
-rw-r--r-- 1 root root 19048560 17 feb 21.23 0.elf
-rw-r--r-- 1 root root 8523088 17 feb 21.23 A09B20.elf
-rw-r--r-- 1 root root 101139 17 feb 21.23 A13128
-rw-r--r-- 1 root root 6695765 17 feb 21.23 BC7D1B.xz
-rw-r--r-- 1 root root 6211696 17 feb 21.23 C3E000.elf
-rw-r--r-- 1 root root 6203504 17 feb 21.23 C40000.elf
-rw-r--r-- 1 root root 6199408 17 feb 21.23 C41000.elf
ecce /tmp/t # ls -l _kernel-4.9.76-gentoo-r1.extracted/_4076.extracted/
totalt 51364
-rw-r--r-- 1 root root 18933872 17 feb 21.23 0.elf
-rw-r--r-- 1 root root 8406992 17 feb 21.23 A0A0A0.elf
-rw-r--r-- 1 root root 97242 17 feb 21.23 A135E8
-rw-r--r-- 1 root root 6635801 17 feb 21.23 BBA757.xz
-rw-r--r-- 1 root root 6146160 17 feb 21.23 C32000.elf
-rw-r--r-- 1 root root 6137968 17 feb 21.23 C34000.elf
-rw-r--r-- 1 root root 6133872 17 feb 21.23 C35000.elf
ecce /tmp/t #
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So the contents of the newly compiled kernel are structurally similar and "logically" slightly smaller than the previous one, but does not compress as well.
I don't know how to interpret that result... |
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cadwal_se
n00b
Joined: 16 Feb 2018
Posts: 5
Location: Sweden
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| Posted: Sat Feb 17, 2018 8:47 pm Post subject: |
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| cadwal_se wrote: | Ok, I have done some more checking and there is something odd going on with ld in this case...
If I remove the -pie option in the aforementioned ld command line and run it manually, the size of the intermediate vmlinux file goes down from twice the size of piggy.o to about 1.5 times the size.
And if I then let the regular make turn that into bzImage its back to its "normal" size:
| Code: |
-rw-r--r-- 1 root root 5547296 17 feb 21.28 ../bzImage
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Mmm... |
And I checked the history now, that -pie option has been in the kernel since late March 2016 but I don't know if an ld (with a config) that triggers that code path has been present for that long in my gentoo installation. |
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josephg
l33t
Joined: 10 Jan 2016
Posts: 783
Location: usually offline
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| Posted: Sat Mar 03, 2018 9:35 am Post subject: Re: bzImage doubled in size |
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| cadwal_se wrote: | | What versions of ld (and binutils) do you have? |
everything on stable: sys-devel/binutils-2.29.1-r1 |
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