Questions about Kernel Memory that I didn't find answers in Google - Please Help
Lucas Tanure
tanure at linux.com
Tue Aug 12 20:32:42 EDT 2014
I`m still reading all the resources, but I found a few more.
http://www.ualberta.ca/CNS/RESEARCH/LinuxClusters/mem.html
I didn`t find a few answers yet, but how a module is loaded ?
When you do a insmod the kernel take some pages and load all the module's
sections to these pages ?
How this work ? Where I can find more information about memory allocation
for a new module.
If I don`t find, I will read the source code for that part, but I try to
find a better way to learn. I know that source code is the best and
updated, but takes more time too.
And when I do a int inside my module I suppose that my int will be inside
that pages that kernel allocated for my new module. Right ??
But if I do this inside kernel code ? It's compiler task do allocate this
memory in sections ? How the kernel runs ? Like my module is driven by
kernel, but how kernel driven it's self ???
Thanks
--
Lucas Tanure
+55 (19) 988176559
On Tue, Aug 5, 2014 at 12:31 AM, Peter Teoh <htmldeveloper at gmail.com> wrote:
> And Q2:
>
> Just want to comment that the load address has to be fixed initially,
> because unlike normal ELF, after loading ELF, there is a relocation tasks
> done by the linker. In vmlinuz we cannot have relocation, before
> executing the kernel is the BIOS / uboot / bootloader etc. One possible
> answer. Others:
>
> https://groups.google.com/forum/#!topic/comp.os.linux.embedded/0-SAzCqQKFM
>
> And perhaps some of the links below may help you:
>
> http://jianggmulab.blogspot.sg/2010_01_01_archive.html
>
>
> http://stackoverflow.com/questions/5647279/why-does-the-module-start-from-address-0xbf000000
>
> http://www.arm.linux.org.uk/developer/memory.txt
>
> http://en.wikipedia.org/wiki/High_memory
>
> bottomline: keep googling.
>
> Q6 and 7 makes no sense to me....sorry.
>
>
>
> On Mon, Aug 4, 2014 at 11:22 PM, Lucas Tanure <tanure at linux.com> wrote:
>
>> Thanks!
>>
>> A quick look in all of that show me that there a lot of information
>> about how kernel manage memory.
>> But, I will find the answer for question 2, 6 and 7 in it ?
>>
>> Thanks!
>> --
>> Lucas Tanure
>> +55 (19) 988176559
>>
>>
>> On Sun, Aug 3, 2014 at 8:58 PM, Peter Teoh <htmldeveloper at gmail.com>
>> wrote:
>> > I like your curiosities and interests in Linux
>> > kernel.
>> http://virtuallyhyper.com/2013/07/rhcsa-and-rhce-chapter-10-the-kernel/
>> >
>> > Instead of answering one by one, I think I will just identify the
>> knowledge
>> > you are lacking:
>> >
>> > Memory management (from both x86/intel and linux kernel perspective).
>> >
>> > There are many many resources out there for you in these area, eg:
>> >
>> > http://en.wikipedia.org/wiki/Page_table
>> > http://en.wikipedia.org/wiki/X86-64
>> >
>> > (both boring, but just understand it well enough)
>> >
>> > http://wiki.osdev.org/Paging (good explanation....understand it very
>> very
>> > well).
>> >
>> > The ultimate classic ebook:
>> >
>> > https://www.kernel.org/doc/gorman/pdf/understand.pdf
>> >
>> > And this blog site has tons of good info on intel/memory etc:
>> >
>> >
>> http://duartes.org/gustavo/blog/post/cpu-rings-privilege-and-protection/
>> > http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory/
>> >
>> > http://virtuallyhyper.com/2013/07/rhcsa-and-rhce-chapter-10-the-kernel/
>> >
>> > http://www.cse.psu.edu/~anand/spring01/linux/memory.ppt
>> >
>> > One more thing:
>> >
>> > "readelf -S -W vmlinux" shows u the sections and the address where the
>> > different sections are supposed to be loaded in memory. If u replace
>> the
>> > vmlinux with the kernel module, eg: ip_tables.ko, then it says:
>> >
>> > starting at offset 0x328c blah blah....
>> >
>> > so the loaded address is with respect to ZERO, but then the actual
>> module
>> > address is:
>> >
>> > sudo cat /proc/modules |grep ip_table
>> >
>> > ip_tables 18106 1 iptable_filter, Live 0xf8bf5000
>> >
>> > So all the output from your readelf, just add 0xf8bf5000 to it and you
>> will
>> > get the actual virtual address of that section IN MEMORY.
>> >
>> > Just only in memory. In file, the file offset of the section is
>> different.
>> > And many parts inside the ELF is also different from memory too: you
>> will
>> > need to add the virtual load address (above) to the offset as specified
>> > inside the relocation tables (objdump -r), and for each section there
>> is a
>> > separate relocation table (all independent from another, meaning that
>> the
>> > different section CAN BE loaded to different parts in memory).
>> >
>> > Thanks.
>> >
>> >
>> > On Sun, Aug 3, 2014 at 11:59 PM, Lucas Tanure <tanure at linux.com> wrote:
>> >>
>> >> Hi,
>> >>
>> >> I'm looking for some site, pdf, book etc, that can answer this
>> questions.
>> >> For now I have :
>> >>
>> >>
>> http://unix.stackexchange.com/questions/5124/what-does-the-virtual-kernel-memory-layout-in-dmesg-imply
>> >>
>> >>
>> >> I want to understand a few things about the memory and the execution
>> >> of Linux kernel.
>> >> Taking from a X86 and grub I have:
>> >>
>> >> 1) Grub loads kernel and root file system in memory, and the vmlinux
>> >> has the code to decompress it self, right ? linux
>> >>
>> >> 2) The address of load kernel is always the same ? And It's at
>> >> compilation time that is chosen ?
>> >>
>> >> 2a) The kernel takes places in 3g-4g memory place, and user space from
>> 0
>> >> to 3gb.
>> >> But if the pc has only 256mb of memory ?
>> >> And when pc has 16gb of memory, the user space will be split in two ?
>> >>
>> >> 2b) And if kernel has soo many modules that needs more than 1gb to run
>> ?
>> >>
>> >> 2c) How we configure all of that memory configs ? make menuconfig and
>> >> friends ?
>> >>
>> >> 3) The function A will call functon B. B is at 0xGGGGGG in .text
>> >> section, but kernel was loaded in address 0xJJJJJJJJJJ, how A will
>> >> find B ?
>> >>
>> >> 4) Please consider this:
>> >> $ readelf -S -W vmlinux
>> >> There are 37 section headers, starting at offset 0xe05718:
>> >>
>> >> Section Headers:
>> >> [Nr] Name Type Address
>> >> Off Size ES Flg Lk Inf Al
>> >> [ 0] NULL
>> >> 0000000000000000 000000 000000 00 0 0 0
>> >> [ 1] .text PROGBITS
>> >> ffffffff81000000 200000 53129a 00 AX 0 0 4096
>> >> [ 2] .notes NOTE
>> >> ffffffff8153129c 73129c 0001d8 00 AX 0 0 4
>> >> [ 3] __ex_table PROGBITS ffffffff81531480
>> >> 731480 002018 00 A 0 0 8
>> >> [ 4] .rodata PROGBITS
>> >> ffffffff81600000 800000 1655ee 00 A 0 0 64
>> >> [ 5] __bug_table PROGBITS ffffffff817655f0
>> >> 9655f0 005424 00 A 0 0 1
>> >> [ 6] .pci_fixup PROGBITS ffffffff8176aa18
>> >> 96aa18 002f88 00 A 0 0 8
>> >> [ 7] .tracedata PROGBITS ffffffff8176d9a0
>> >> 96d9a0 00003c 00 A 0 0 1
>> >> [ 8] __ksymtab PROGBITS ffffffff8176d9e0
>> >> 96d9e0 00e710 00 A 0 0 16
>> >> [ 9] __ksymtab_gpl PROGBITS ffffffff8177c0f0
>> >> 97c0f0 00a150 00 A 0 0 16
>> >> [10] __kcrctab PROGBITS ffffffff81786240
>> >> 986240 007388 00 A 0 0 8
>> >> [11] __kcrctab_gpl PROGBITS ffffffff8178d5c8
>> >> 98d5c8 0050a8 00 A 0 0 8
>> >> [12] __ksymtab_strings PROGBITS ffffffff81792670
>> >> 992670 01cb42 00 A 0 0 1
>> >> [13] __init_rodata PROGBITS ffffffff817af1c0
>> >> 9af1c0 0000e8 00 A 0 0 32
>> >> [14] __param PROGBITS ffffffff817af2a8
>> >> 9af2a8 000b00 00 A 0 0 8
>> >> [15] __modver PROGBITS ffffffff817afda8
>> >> 9afda8 000258 00 A 0 0 8
>> >> [16] .data PROGBITS
>> >> ffffffff81800000 a00000 0e1180 00 WA 0 0 4096
>> >> [17] .vvar PROGBITS
>> >> ffffffff818e2000 ae2000 001000 00 WA 0 0 16
>> >> [18] .data..percpu PROGBITS 0000000000000000
>> >> c00000 015300 00 WA 0 0 4096
>> >> [19] .init.text PROGBITS
>> >> ffffffff818f9000 cf9000 0503ea 00 AX 0 0 16
>> >> [20] .init.data PROGBITS
>> >> ffffffff8194a000 d4a000 09e4c8 00 WA 0 0 4096
>> >> [21] .x86_cpu_dev.init PROGBITS ffffffff819e84c8
>> >> de84c8 000018 00 A 0 0 8
>> >> [22] .parainstructions PROGBITS ffffffff819e84e0
>> >> de84e0 00bd3c 00 A 0 0 8
>> >> [23] .altinstructions PROGBITS ffffffff819f4220
>> >> df4220 005f40 00 A 0 0 1
>> >> [24] .altinstr_replacement PROGBITS ffffffff819fa160
>> >> dfa160 001a69 00 AX 0 0 1
>> >> [25] .iommu_table PROGBITS ffffffff819fbbd0
>> >> dfbbd0 0000f0 00 A 0 0 8
>> >> [26] .apicdrivers PROGBITS ffffffff819fbcc0
>> >> dfbcc0 000020 00 WA 0 0 8
>> >> [27] .exit.text PROGBITS ffffffff819fbce0
>> >> dfbce0 0009bc 00 AX 0 0 1
>> >> [28] .smp_locks PROGBITS ffffffff819fd000
>> >> dfd000 005000 00 A 0 0 4
>> >> [29] .data_nosave PROGBITS ffffffff81a02000
>> >> e02000 001000 00 WA 0 0 4
>> >> [30] .bss NOBITS
>> >> ffffffff81a03000 e03000 122000 00 WA 0 0 4096
>> >> [31] .brk NOBITS
>> >> ffffffff81b25000 e03000 425000 00 WA 0 0 1
>> >> [32] .comment PROGBITS 0000000000000000
>> >> e03000 000027 01 MS 0 0 1
>> >> [33] .debug_frame PROGBITS 0000000000000000
>> >> e03028 002560 00 0 0 8
>> >> [34] .shstrtab STRTAB
>> >> 0000000000000000 e05588 00018a 00 0 0 1
>> >> [35] .symtab SYMTAB 0000000000000000
>> >> e06058 1a29f8 18 36 43659 8
>> >> [36] .strtab STRTAB
>> >> 0000000000000000 fa8a50 180d92 00 0 0 1
>> >> Key to Flags:
>> >> W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
>> >> I (info), L (link order), G (group), T (TLS), E (exclude), x
>> (unknown)
>> >> O (extra OS processing required) o (OS specific), p (processor
>> specific)
>> >>
>> >> So the vmlinux is loaded in memory like a dd ?
>> >>
>> >> 5) In my function A, inside the module that I wrote, a non-initialized
>> >> variable will take place in non-initialized section that was loaded in
>> >> memory ?
>> >> Or my modules has a new sections for it's own use, and my module is
>> >> loaded my memory like a process, with all his sections?
>> >> So how another module or kernel code will fin my exported
>> >> variable/function ?
>> >>
>> >>
>> >> 6) Let's suppose:
>> >> I have a int variable, with 17 as content, and the address is 0xGGGGGG.
>> >> If I stop the linux in this time, read my memory at address 0xGGGGGG I
>> >> will got 17, right ?
>> >> 0xGGGGGGG will be bigger than 0xc0000000 always, right ?
>> >>
>> >>
>> >> 7) Now take int from question and change for:
>> >> struct mystruct * foo = (struct mystruct* ) kmalloc(sizeof(struct
>> >> mystruct));
>> >>
>> >> I will be able to read at address 0xGGGGGG the struct that created,
>> >> and it address will be greater than 0xc0000000, right ?
>> >> But for this struct, the memory will be allocated for ever, until I
>> >> free the pointer, right ?
>> >>
>> >>
>> >>
>> >> Well, this just a start. I really want to understand how kernel is
>> >> run, loaded etc. Any help is appreciate, answering my questions, links
>> >> to read, books to read.
>> >> Actually, I didn't find any book with that kind of information .
>> >>
>> >>
>> >> --
>> >> Lucas Tanure
>> >> +55 (19) 988176559
>> >>
>> >> --
>> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> >> the body to majordomo at kvack.org. For more info on Linux MM,
>> >> see: http://www.linux-mm.org/ .
>> >> Don't email: <a href=mailto:"dont at kvack.org"> email at kvack.org </a>
>> >
>> >
>> >
>> >
>> > --
>> > Regards,
>> > Peter Teoh
>>
>
>
>
> --
> Regards,
> Peter Teoh
>
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