Place .text section at the very end of ELF

Question

For a specific reason i need to place .text section at the very end of my ELF file.

I've tried to achieve this in this way:

I took default large linker script and moved .text section to the very end of SECTIONS { ... } part.

$ readelf -S beronew

[ #] Name               Type            Address            Offset
     Size              Size.Ent         Flags     -     -  Alignment
[ 0]                   NULL             0000000000000000  00000000
     0000000000000000  0000000000000000           0     0     0
[ 1] .data             PROGBITS         00000000006000b0  000000b0
     000000000000003b  0000000000000000  WA       0     0     1
[ 2] .text             PROGBITS         0000000000a000f0  000000f0
     00000000000003e9  0000000000000000  AX       0     0     1
[ 3] .shstrtab         STRTAB           0000000000000000  000004d9
     0000000000000027  0000000000000000           0     0     1
[ 4] .symtab           SYMTAB           0000000000000000  00000680
     0000000000000438  0000000000000018           5    41     8
[ 5] .strtab           STRTAB           0000000000000000  00000ab8
     0000000000000258  0000000000000000           0     0     1

What i see is that ld added extra sections after my "ending" section. To replace them i used -nostdlib -s linker option (to not use stdlib (just in case) and omit all symbol information).

Run $ readelf -S beronew one more time:

[ #] Name               Type            Address            Offset
     Size              Size.Ent         Flags     -     -  Alignment
[ 0]                   NULL             0000000000000000  00000000
     0000000000000000  0000000000000000           0     0     0
[ 1] .data             PROGBITS         00000000006000b0  000000b0
     000000000000003b  0000000000000000  WA       0     0     1
[ 2] .text             PROGBITS         0000000000a000f0  000000f0
     00000000000003e9  0000000000000000  AX       0     0     1
[ 3] .shstrtab         STRTAB           0000000000000000  000004d9
     0000000000000017  0000000000000000           0     0     1

Section header string table is still there. I've tried $strip -R .shstrtab beronew. It had no effect, section is still there.

This section is only 0x17 bytes long but i couldn't achieve my goal. Then i looked at hexdump of my file:

$ hexdump beronew

...
00004d0 0060 c748 bfc6 6000 0000 732e 7368 7274
00004e0 6174 0062 642e 7461 0061 742e 7865 0074
00004f0 0000 0000 0000 0000 0000 0000 0000 0000
*
0000530 000b 0000 0001 0000 0003 0000 0000 0000
0000540 00b0 0060 0000 0000 00b0 0000 0000 0000
0000550 003b 0000 0000 0000 0000 0000 0000 0000
...

What i see is that there is another code after sections part. According to ELF structure it is Section header table at the end of file. So even if i remove somehow .shstrtab section, there still will be this header at the end.

So my question is how can i place my .text section at the very end of file? I don't really need to remove all the sections and headers so if You know a (better) way to achieve this, it will be highly appreciated.

.

.

P.S. For those who wonder why do i need this:

This ELF file (beronew) contains rutime library. It will be used as "header" for another file that generates asm instructions with some logic in opcode form. This opcode will be added to the very end of beronew. Then i'm gonna patch sh_size field in .text's section header to be able to run my recently added 'code'.

(One more question: Is this all that i need to patch in case 'text' section is the last one in file?)

P.P.S. I know that this is a bad architecture but it is my course project - porting an app that was built this way in Win32 to Linux64, and now I'm stuck at the point where i merge runtime library "header" file and "logic" part because i can't place .text section at the end of ELF.

Thanks one more time!

.

UPD:

Based on fuz's comment i've tried to add PHDRS to simple linker script as this:

PHDRS
{
   headers PT_PHDR PHDRS ;
   data PT_LOAD ;
   bss PT_LOAD ;
   text PT_LOAD ;
}

SECTIONS 
{ 
    . = 0x200000;
    .data : { *(.data) *(COMMON) }  :data
    .bss  : { *(.bss) }  :bss
    .text : { *(.text) } :text
} 

but it doesn't seem to work now.

Answer 1

For someone who wonder how after all I've managed to get this working there is an answer:

1. I've made a linker script that placed text section after the data section (here is the script). But there were some debugging sections at the end of file like Shstrtab section and so on (picture below). So I've converted this file byte-by-byte into string.

2. After this I got an elf on a picture below

in a string-of-bytes form. Then I've just read some headers and found out where the section of code ends (right before the Shstrtab section), so I could split this string into 2 pieces. First one contained data for loader. Second one - for linker.

3. Then I've converted my 'extra' code into opcode form which is also an array (string) of bytes and concatenated it to the original .text section. Next I've concatenated the ending part to it. So I've got a single file with my extra code in it.

4. To get this working I've edited values from picture below:

First column is a name of field that needs to be edited (it corresponds to picture of elf structure).

Second column is an offset from the beginning of file to the beginning of the field. Let function s(xxx) be the the size_of(some_header_structure) and injSize be the size of my injected extra code. And values like 0x18, 0x20, 0x28 are the offsets of fields inside their strucures (section_headers, program_headers, elf_headers).

Third one represents the value that should replace that original one.

*note that represented elf is and ELF64 so widths of some fields differ from ELF32's ones.

After I've made all this I've managed to just run this new Elf file and it worked perfectly! Maybe (for sure) it's not the best solution but it works and it was a nice meterial for my research work.