How to extract only the raw contents of an ELF section?

Question

I've tried the following, but the resulting file is still an ELF and not purely the section content.

$ objcopy --only-section=<name> <infile> <outfile>

I just want the contents of the section. Is there any utility that can do this? Any ideas?

Answer 1

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.

Answer 2

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.

Answer 3

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.