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flat assembler > Main > Sections/segments a) readable b) order

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jorido



Joined: 23 Jan 2017
Posts: 53
Sections/segments a) readable b) order
1)

When and when not is it needed to make a section or segment readable?
In one of the example in the "fasm" none of sections readable:


Code:

format ELF

section '.text' executable

 public _start
 _start:

 extrn writemsg

  mov esi,msg
  call  writemsg

  mov eax,1
  xor ebx,ebx
  int 0x80

section '.data' writeable

 msg db "Elves are coming!",0xA,0






2) Does the order of sections/segments matter?


Any difference between this one and the original example?



Code:


format ELF

section '.data' writeable

 msg db "Elves are coming!",0xA,0

section '.text' executable

 public _start
 _start:

 ;.........................



Post 07 Oct 2017, 01:41
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15241
Location: 1I/ʻOumuamua
1) The OS probably gives all sections the readable attribute even if you don't specify it, but this is only my guess. You will need to check the docs to see what Linux does here. Usually only executable sections make sense to be non-readable and non-writeable.

2) The order does not matter, You can put things in any order you want.
Post 07 Oct 2017, 05:15
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 6633
Location: Kraków, Poland
The sections in ELF format do not have an attribute that would mean "readable". Only the run-time segments have, but they are a different entity (defined in "Program Header").
Post 07 Oct 2017, 13:13
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_shura



Joined: 22 May 2015
Posts: 60
Actually, Linux set all segments readable and executable by default, even if the segment is neither readable, writeable nor executable, as long as your executable file does not contain a gnu-stack-segment. So if you create a executable file, please use »segment gnustack«, if you create an object-file, please link everything with »ld -z noexecstack«. Anyway, afaik everything is readable, even if you did not actually set this flag.

Afaik there where some linux-versions, where the last segment must be writeable (I guess, it had something to do with the stack, that should be, of course, writeable), but I do not know, whether this is true for section or not.
Post 07 Oct 2017, 17:47
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