gmane.linux.assembly

Re: how does ld.so call ELF's entry?

Sofiane Akermoun — 2013-04-07T07:47:59

I don't know what you are doing. if you are writing your proper
assembler, or whatever. But the standard tools are gdb, printing on
screen information like addresses. if it is kernel related then kdb
and an emulator like bochs, qemu or virtualbox to avoid kernel
freezing in performing remote debugging.

Although it is interesting, without asm code not sure it is the best
place to talk about inner operating system mechanism.

regards.

Sofiane Akermoun
akersof< at >gmail.com

2013/4/7 ishare <june.tune.sea< at >gmail.com>:

Re: how does ld.so call ELF's entry?

ishare — 2013-04-07T06:56:58

  yeah!

  I have found the work is done through create_elf_tables() function.

  at early time kernel copy args to bprm structure , then copy it to user stack.

  I am dealing with one problem .
  
  When kernel execute "/bin/sh" , it load ld.so successfully , but without jumping 
  to the elf's entry point ,it exit ,which cause  kernel panic . 

  how can I debug for this problem ? how to catch more information for resolving it?
  
  thanks! 
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Re: how does ld.so call ELF's entry?

Sofiane Akermoun — 2013-04-07T06:26:14

The address is passed by the elf loader in something called "auxillary
vectors" structure.
In elf.h you can investigate on a_type/entry type and find that there
is defininition of AT_ENTRY.
As i wrote previsouly if you check in
http://lxr.linux.no/#linux+v3.8.6/fs/binfmt_elf.c you will find how
the elf loader prepare the execution of the interpreter.
To describe the step 4) i mentioned previously, with more accuracy...
the interpreter is executed
by the elf loader as a program loader with known parameters passed on the
stack through Elf32_auxv_t structure. For an easy to understand
explanation you can read an article from phrack 58:
http://www.phrack.org/issues.html?issue=58&id=5#article

regards,

Sofiane Akermoun
alersof< at >gmail.com

2013/4/7 ishare <june.tune.sea< at >gmail.com>:

Re: how does ld.so call ELF's entry?

ishare — 2013-04-07T04:52:07

  how does the interpreter know this address?

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Re: how does ld.so call ELF's entry?

Sofiane Akermoun — 2013-04-07T04:45:58

EntryPoint is set in the ELF Header at assembling/linking time.

2013/4/7 ishare <june.tune.sea< at >gmail.com>:

Re: how does ld.so call ELF's entry?

Sofiane Akermoun — 2013-04-07T04:40:30

from 'man exeve':
if the executable is a dynamically linked ELF  executable,  the
interpreter named in the PT_INTERP segment is used to load the needed
shared libraries.  This interpreter is typically /lib/ld-linux.so.1
for  binaries  linked  with the Linux libc 5, or /lib/ld-linux.so.2
for binaries linked with the glibc 2.
If you really want to know what really happens then go to dig the
exec.c source of linux:
http://lxr.linux.no/#linux+v3.8.6/fs/exec.c and binfmt*.c. If you
follow the process you will find how the loader find, parse, and
handle the interpretor.

regards,

Sofiane Akermoun

2013/4/7 ishare <june.tune.sea< at >gmail.com>:

Re: how does ld.so call ELF's entry?

ishare — 2013-04-07T03:45:54

  How does it know the entry point's address?

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Re: how does ld.so call ELF's entry?

Chris Evans — 2013-04-07T03:43:16

With a far jump to entry offset 


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On Apr 6, 2013, at 8:39 PM, ishare <june.tune.sea< at >gmail.com> wrote:

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Re: how does ld.so call ELF's entry?

ishare — 2013-04-07T03:39:13



  How does  the step 4 been  done ?
  
  thanks!
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Re: how does ld.so call ELF's entry?

Sofiane Akermoun — 2013-04-07T03:13:37

Hello,


The interpreter is specified during linking process and the program
header of your binary is filled with good values.
Then when the operating system load your binary, he finds next the
interpreter to use.
The linker ld sets the good values by default but you can overwrite it
or specify other values if you want.
The steps are:
1)The operating system loads your binary
2)The program loader system execute the Interpreter specifiy in the binary
3)The interpreter gather all the dynamic libraries needed in memory
4)The Control is passed to the entry point of your program

The entry point is specified in your code source as a "global". And
could be find in your object file by the linker.
In theory there are some defaults tag to specify entry point, like
"..start" for nasm, but you can also passed yours to the linker (ld
-e).

regards,

Sofiane Akermoun
akersof< at >gmail.com

2013/4/7 ishare <june.tune.sea< at >gmail.com>:

how does ld.so call ELF's entry?

ishare — 2013-04-07T02:19:21

 
  For an ELF ,which needs a interpreter , how is it  been called by the interpreter ?

  As I know the interpreter is loaded first and do something essential ,then call the main routine of ELF .
  How is this procedure implemented ?

  Thanks!
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Re: how to disassembly a specified section

Sofiane Akermoun — 2013-03-18T15:31:58

Hello,

"readelf" is a tool for reading elf structure.
use "man readelf" or google it for full options.. there are a lots
(section headers, etc..)
For disassembling a specific section you can use objdump with the "-d"
for disassemble and "-j nameofthesection"
Else use a GUI tools like IDA.

regards,

Sofiane Akermoun
akersof< at >gmail.com


2013/3/18 ishare <june.tune.sea< at >gmail.com>:

how to disassembly a specified section

ishare — 2013-03-18T14:52:23

  I need to look up the disassembly  content of a section in an elf ?
  how to type that command ?

  thanks!
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Re: applicazione.

Stansie — 2013-03-14T19:35:30

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Re: [NASM] ROR -> error: invalid combination of opcode and operands

Sofiane Akermoun — 2013-03-11T22:40:05

As mentioned in your error the combination of opcode 'ror' and
operands 'al' and 'bl" is bad.
If you read the documentation you will see that ror just support the
following expressions:

ror r/m8,1
ror r/m8,CL
ror r/m8,imm8
ror r/m16,1
ror r/m16,CL
ror r/m16,imm8
ror r/m32,1 D3  /1
ror r/m32,CL
ror r/m32,imm8

It means that unless your second operand is the CL register, you need
an immediate value for your second operand.

kinds regards,

Sofiane Akermoun

2013/3/11 Meher Assel <asselmeher< at >gmail.com>:

Re: [NASM] ROR -> error: invalid combination of opcode and operands

Chris Evans — 2013-03-11T22:22:30

You may need to use a immediate value for src or dest argument 

-Chris
Http://nxdos.com/ 


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On Mar 11, 2013, at 3:09 PM, Meher Assel <asselmeher< at >gmail.com> wrote:

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[NASM] ROR -> error: invalid combination of opcode and operands

Meher Assel — 2013-03-11T22:09:13

Hello all,
why i can't do ' ror al,bl' ?
i'am using nasm in 16bits mode ( [BITS 16] )
i get this error when compiling:
error: invalid combination of opcode and operands
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RE: how to use .section command in asm code?

Robert Plantz — 2013-03-04T15:31:35

Actually, the terminology is "assembler directive", "assembly directive", "pseudo opcode", "pseudo-operation", or "pseudo-op". They direct the assembler to do things as it assembles the source file. For example, as Sofiane points out, ".section .text" tells the assembler to place what follows in the .text segment of the object code. I recommend reading http://en.wikipedia.org/wiki/Assembly_language

(I recognize that computer programming language terminology is largely English-centric, and that English is not the native language for (probably) most programmers in the world.)

--Bob Plantz

________________________________________
From: linux-assembly-owner< at >vger.kernel.org [linux-assembly-owner< at >vger.kernel.org] on behalf of Sofiane Akermoun [akersof< at >gmail.com]
Sent: Sunday, March 03, 2013 10:06 PM
To: ishare
Cc: linux-assembly< at >vger.kernel.org
Subject: Re: how to use .section command in asm code?

This is not a command.
.section is a keyword related to your assembler syntax, probably gas
in your case.
A binary