How to extract only the raw contents of an ELF section?
Rather inelegant hack around objdump
and dd
:
IN_F=/bin/echo
OUT_F=./tmp1.bin
SECTION=.text
objdump -h $IN_F |
grep $SECTION |
awk '{print "dd if='$IN_F' of='$OUT_F' bs=1 count=$[0x" $3 "] skip=$[0x" $6 "]"}' |
bash
The objdump -h
produces predictable output which contains section offset in the elf file. I made the awk
to generate a dd
command for the shell, since dd
doesn't support hexadecimal numbers. And fed the command to shell.
In past I did all that manually, without making any scripts, since it is rarely needed.
Use the -O binary
output format:
objcopy -O binary --only-section=.text foobar.elf foobar.text
Just verified with avr-objcopy
and an AVR ELF image's .text
section.
Note that if, as Tim points out below, your section doesn't have the ALLOC flag, you may have to add --set-section-flags .text=alloc
to be able to extract it.
objcopy --dump-section
Introduced in Binutils 2.25, and achieves a similar effect to -O binary --only-section
.
Usage:
objcopy --dump-section .text=output.bin input.o
https://sourceware.org/binutils/docs-2.25/binutils/objcopy.html documents it as:
--dump-section sectionname=filename
Place the contents of section named sectionname into the file filename, overwriting any contents that may have been there previously. This option is the inverse of --add-section. This option is similar to the --only-section option except that it does not create a formatted file, it just dumps the contents as raw binary data, without applying any relocations. The option can be specified more than once.
Minimal runnable example
main.S
.data
.byte 0x12, 0x34, 0x56, 0x78
.text
.byte 0x9A, 0xBC, 0xDE, 0xF0
Assemble:
as -o main.o main.S
Extract data:
objcopy --dump-section .data=data.bin main.o
hd data.bin
Output:
00000000 12 34 56 78 |.4Vx|
00000004
Extract text:
objcopy --dump-section .text=text.bin main.o
hd text.bin
Output:
00000000 9a bc de f0 |....|
00000004
Tested in Ubuntu 18.04 amd64, Binutils 2.30.