Re: ld: Simple Linker Script Example

[Barry Rountree <rountree at uga dot edu>]

On Thursday 06 November 2003 10:23 pm, Ian Lance Taylor wrote:


> What kind of problems are you seeing?

What I'm trying to do is put all of the sections coming from ext.c into one 
place in virtual memory.

Given two files:

/*
 * main.c
 */
#include <stdio.h>
extern int ext(int x);
int main(){
        return printf("%d\n",ext(1));
}

and 

/*
 * ext.c
 */
int ext(int x){
        return ++x;
}
	gcc -Wall -c main.c ext.c 
	gcc -Wall -Wl,--verbose main.o ext.o > script.ld 2>&1

Edit script.ld to remove everything that before or after the script proper.

	gcc -Wall -Wl,-T script.ld main.c ext.c

Executable works -- great, I've got a baseline.  Now change the script as 
follows:

--- script.ld.orig      2003-11-06 22:45:57.000000000 -0500
+++ script.ld   2003-11-06 22:52:08.000000000 -0500
@@ -9,7 +9,24 @@
 SECTIONS
 {
   /* Read-only sections, merged into text segment: */
-  . = 0x08048000 + SIZEOF_HEADERS;
+  . = 0x08048000 + 0x200;
+
+  . += 0x1000;
+  . = ALIGN(0x1000);
+
+  .sandbox_text   : { ext.o ( .text ) }
+  . += 0x1000;
+  . = ALIGN(0x1000);
+
+  .sandbox_data   : { ext.o ( .data ) }
+  . += 0x1000;
+  . = ALIGN(0x1000);
+
+  .sandbox_bss   : { ext.o ( .bss  ) }
+  . += 0x1000;
+  . = ALIGN(0x1000);
+
+
   .interp         : { *(.interp) }
   .hash           : { *(.hash) }
   .dynsym         : { *(.dynsym) }

	
Finally, 
	gcc -Wall -Wl,-T script.ld main.o ext.o

No complaints from gcc, but the binary segfaults.

This is using dynamic linking -- I hadn't thought about going to static until 
you mentioned it.  Replacing SIZEOF_HEADERS with 0x200 is just a guess.  

readelf -a looks reasonable as far as the addresses go:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf 
Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  
0
  [ 1] .sandbox_text     PROGBITS        0804a000 001000 00000b 00  AX  0   0  
4
  [ 2] .sandbox_data     PROGBITS        0804c000 002000 000000 00  WA  0   0  
4
  [ 3] .sandbox_bss      NOBITS          0804d000 002000 000000 00  WA  0   0  
4
  [ 4] .interp           PROGBITS        0804e000 002000 000013 00   A  0   0  
1
  [ 5] .note.ABI-tag     NOTE            0804e014 002014 000020 00   A  0   0  
4
  [ 6] .hash             HASH            0804e034 002034 000028 04   A  7   0  
4
	.
	.
	.


Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x00001034 0x00000000 0x00120 0x00120 R E 0x4
  INTERP         0x002000 0x0804e000 0x0804e000 0x00013 0x00013 R   0x1
      [Requesting program interpreter: /lib/ld-linux.so.2]
  LOAD           0x001000 0x0804a000 0x0804a000 0x0000b 0x0000b R E 0x1000
  LOAD           0x002000 0x0804c000 0x0804c000 0x00000 0x00000 RW  0x1000
  LOAD           0x002000 0x0804d000 0x0804d000 0x00000 0x00000 RW  0x1000
  LOAD           0x002000 0x0804e000 0x0804e000 0x00340 0x00340 R E 0x1000
  LOAD           0x002340 0x0804f340 0x0804f340 0x00100 0x00104 RW  0x1000
  DYNAMIC        0x00234c 0x0804f34c 0x0804f34c 0x000c8 0x000c8 RW  0x4
  NOTE           0x002014 0x0804e014 0x0804e014 0x00020 0x00020 R   0x4


This is really obvious, right?  I can feel it...  ;-)

>
> Ian

Barry