How to extract only the raw contents of an ELF sec

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

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.

Answer 3

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

a.S:

.data
    .byte 0x12, 0x34, 0x56, 0x78
.text
    .byte 0x9A, 0xBC, 0xDE, 0xF0

Assemble and extract:

as -o a.o a.S
objcopy --dump-section .data=data.bin a.o
objcopy --dump-section .text=text.bin a.o
hd data.bin
hd text.bin

Output:

00000000  12 34 56 78                                       |.4Vx|
00000004
00000000  9a bc de f0                                       |....|
00000004

Tested in Ubuntu 18.04 amd64, Binutils 2.30.