Questions about Kernel Memory that I didn't find answers in Google - Please Help

Lucas Tanure tanure at linux.com
Mon Aug 4 11:22:07 EDT 2014


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
>>
>> --
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>
>
>
>
> --
> Regards,
> Peter Teoh



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