[bitlug] Re: ELF??
- From: Peeyush Prasad <peeyush@xxxxxxxxxx>
- To: bitlug@xxxxxxxxxxxxx
- Date: Fri, 11 Nov 2005 15:12:04 +0530 (IST)
On Fri, 11 Nov 2005, Pavan T C wrote:
yeah, am using an ADSP blackfin processor here, got a gcc port for
that... and no shared objects...
I'm interested in creating an executable that can be loaded at an address
that I specify... looked at the --entry option in ld, that changed the
entry address, but a readelf of the executable shows the section addresses
have not changed...
For example, if you create an exe on your PC gcc and have a look and the
disassembled output, you see that the start address is always 0x8048244,
and that the other sections have addresses wrt to this start address.
Not so in my case.. I see the start address change, but not the section
addrs.
In my case, I can use the --section-start <section name> <my addr>
option to ld and specify addresses of my sections, but there has to be an
easier way... maybe I have to specify somewhere that I want absolute code
or
something...
One better way to do it is to use a mapfile for the linker.
Hi all,
This is precisely what I dont want to get into: learning Linkder Command
Language!
I dont think the relocatable objects are the defaults when you compile. On
solaris (I dont have a linux box here), i need to use ld -r to specify that I
need a relocatable object/exe. To verify that, I did a gcc -c and then used
objdump -d to check the addresses and they were all relative to zero.
On gcc, just compilation gives you relocatable code... what else could it
be?? you can check the e_type field of the ELF header to confirm that...
quoting ld :
-r
--relocatable
Generate relocatable output---i.e., generate an output file that
can in turn serve as input to ld. This is often called partial
linking.
if this option is not specified, an absolute file is produced.
which implies that what goes into ld has to be relocatable...
To see what gcc is using to build you executable, use gcc -v.
That should give you a fair idea of what you should override by passing the
linker options via gcc in the command line to get what you want.
I'll see if I can play around with the linker during the weekend.
Now that's a good idea.. I spent long hours yesterday looking into
Makefiles for that option...
Anyways , I managed to get what I wanted,here's my code:
blink.c:
--------
#include "cdefBF532.h"
void blink()
{
int i;
*pFIO_DIR = 0x2;
while(1)
{
*pFIO_FLAG_T = 0x2;
for(i=0; i<200000000; i++) continue;
}
}
main.c:
-------
int main()
{
blink();
return 0;
}
command line:
gcc -fno-builtin -ffreestanding -nostdinc -c blink.c
gcc -fno-builtin -ffreestanding -nostdinc -c main.c
ld -Bstatic -Ttext 0xffa08000 -o bld blink.o main.o -Map blink.map
...and here is my disassembled executable: (objdump -d -r bld)
(Caution: The mnemonics are for the Blackfin DSP)
bld: file format elf32-bfin
Disassembly of section .text:
ffa08000 <_blink>:
ffa08000: 00 e8 01 00 LINK 0x4;
ffa08004: 4a e1 c0 ff P2.H=ffc0 <_blink-0xff9f8040>;
ffa08008: 0a e1 30 07 P2.L=730 <_blink-0xffa078d0>;
ffa0800c: 10 60 R0=0x2(x);
ffa0800e: 10 97 W[P2]=R0;
ffa08010: 4a e1 c0 ff P2.H=ffc0 <_blink-0xff9f8040>;
ffa08014: 0a e1 0c 07 P2.L=70c <_blink-0xffa078f4>;
ffa08018: 10 60 R0=0x2(x);
ffa0801a: 10 97 W[P2]=R0;
ffa0801c: 00 60 R0=0x0(x);
ffa0801e: f0 bb [FP-4]=R0;
ffa08020: f0 b9 R0=[FP-4];
ffa08022: 41 e1 eb 0b R1.H=0xbeb;
ffa08026: 01 e1 ff c1 R1.L=-15873;
ffa0802a: 08 09 CC=R0<=R1;
ffa0802c: f2 13 IF ! CC JUMP ffa08010 <_blink+0x10>;
ffa0802e: f0 b9 R0=[FP-4];
ffa08030: 08 64 R0+=0x1;
ffa08032: f0 bb [FP-4]=R0;
ffa08034: f6 2f JUMP.S ffa08020 <_blink+0x20>;
...
ffa08038 <_main>:
ffa08038: 00 e8 03 00 LINK 0xc;
ffa0803c: ff e3 e2 ff CALL ffa08000 <_blink>;
ffa08040: 00 60 R0=0x0(x);
ffa08042: 01 e8 00 00 UNLINK;
ffa08046: 10 00 RTS;
Also, here's the 'readelf -a bld' output: (See comments inline)
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
Machine: <unknown>: 6a
Version: 0x1
Entry point address: 0xffa08000
^^^^^^^^^^^^^^^^^^^^ // I Think I need to change this to point to main
// starting (see symtab) 0xffa08038
Start of program headers: 52 (bytes into file)
Start of section headers: 4264 (bytes into file)
Flags: 0x0
Size of this header: 52 (bytes)
Size of program headers: 32 (bytes)
Number of program headers: 2
Size of section headers: 40 (bytes)
Number of section headers: 9
Section header string table index: 6
Section Headers:
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[ 0] NULL 00000000 000000 000000 00 0 0 0
[ 1] .text PROGBITS ffa08000 001000 000048 00 AX 0 0 4
[ 2] .data PROGBITS ffa09048 001048 000000 00 WA 0 0 4
[ 3] .sbss PROGBITS ffa09048 001048 000000 00 W 0 0 1
[ 4] .bss NOBITS ffa09048 001048 000000 00 WA 0 0 4
[ 5] .comment PROGBITS 00000000 001048 000024 00 0 0 1
[ 6] .shstrtab STRTAB 00000000 00106c 00003b 00 0 0 1
[ 7] .symtab SYMTAB 00000000 001210 000100 10 8 b 4
[ 8] .strtab STRTAB 00000000 001310 000035 00 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings)
I (info), L (link order), G (group), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x001000 0xffa08000 0xffa08000 0x00048 0x00048 R E 0x1000
LOAD 0x001048 0xffa09048 0xffa09048 0x00000 0x00000 RW 0x1000
^^^^^^^^^^^^^^ // What is this second load address??
Section to Segment mapping:
Segment Sections...
00 .text
01
There is no dynamic segment in this file.
There are no relocations in this file.
There are no unwind sections in this file.
Symbol table '.symtab' contains 16 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: ffa08000 0 SECTION LOCAL DEFAULT 1
2: ffa09048 0 SECTION LOCAL DEFAULT 2
3: ffa09048 0 SECTION LOCAL DEFAULT 3
4: ffa09048 0 SECTION LOCAL DEFAULT 4
5: 00000000 0 SECTION LOCAL DEFAULT 5
6: 00000000 0 SECTION LOCAL DEFAULT 6
7: 00000000 0 SECTION LOCAL DEFAULT 7
8: 00000000 0 SECTION LOCAL DEFAULT 8
9: 00000000 0 FILE LOCAL DEFAULT ABS blink.c
10: 00000000 0 FILE LOCAL DEFAULT ABS main.c
11: ffa08000 54 FUNC GLOBAL DEFAULT 1 _blink
12: ffa09048 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
13: ffa09048 0 NOTYPE GLOBAL DEFAULT ABS _edata
14: ffa09048 0 NOTYPE GLOBAL DEFAULT ABS _end
15: ffa08038 16 FUNC GLOBAL DEFAULT 1 _main
No version information found in this file.
.. or maybe I have to get position-independent-code and then just specify
the start address ?
I dont know how that will work when you have multiple objects being linked. I
feel it is better to go with the default placement and use the mapfile to
tweak what is impt for you.
I was under the impression that the linker(loader??) will change the
symbol addresses when he sees that the code is relocatable, and I have given him
an explicit start address... ie, *somebody* should take care of all the
relocatable sections and assign them addresses. Of course I can do this
with the map file, but I was hoping I could give a start addr and get away
with not writing a map file...
Another interesting thing I will try is to change the default load
address of linux executables, I mean, why 0x8048244 ?? :-)
-- P
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