Thread: Need problem points of custom elf highlighted

I finally got my elf to execute but it immediately segfaulted when I was expecting it to return 0 straight away, the idea was simply to ensure I was handling a basic call to main correctly which would in turn allow me to understand the bare mininum for

Code:

int main() { return 0; }

to output as, from there I plan to start writing my own addon supporting compiler under MIT license starting with just integer handling & a simple version of puts that was helpfully provided by flp1969 (didn't ask for it but very greatful for it)
Output of readelf -all test.elf

Code:

ELF Header:
  Magic:   7f 45 4c 46 02 01 01 03 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - GNU
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x40
  Start of program headers:          95 (bytes into file)
  Start of section headers:          0 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         1
  Size of section headers:           0 (bytes)
  Number of section headers:         0
  Section header string table index: 0

There are no sections in this file.

There are no sections to group in this file.

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000         0x0

There is no dynamic section in this file.

There are no relocations in this file.

The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.

Dynamic symbol information is not available for displaying symbols.

No version information found in this file.

test.elf - Google Drive

It's very hard to run something from an empty ELF file...

My post seems to have been deleted, anyway the intent behind uploading it was for a hex editor to be used on it to see anything not seen with readelf, as for it being empty it might seem that way if you're seeing it in kb or higher, only a couple hundred bytes since I needed to see how a function should be called and returned before trying to write any compiler for it

Originally Posted by awsdert

My post seems to have been deleted, anyway the intent behind uploading it was for a hex editor to be used on it to see anything not seen with readelf, as for it being empty it might seem that way if you're seeing it in kb or higher, only a couple hundred bytes since I needed to see how a function should be called and returned before trying to write any compiler for it

Well... Your file have some data, alright. But the ELF header says it is empty:

For the meaning of these fields, look at SysV ABI documents...

You got a link? all the docs I find lead me to the conclusion that the elf header is correct (8 byte addresses remember) and the point of suspicion is either the program header PT_LOAD (which I mostly 0'd out, only entry and after main()) or main() itself (which should be just 3 bytes near the end - look around C3)

osdev wiki has alll the links you'll need:

System V ABI - OSDev Wiki

Not understood the cause of the segfault yet but I did manage to learn how to use the integer instructions, didn't know I needed to set a flag holder before using the instructions

Okay, no knowing WHERE the segfault was coming from was definitly hindering me (kept trying to modify _start & main bytes) so I removed all but the essential syscall from _start() and still got a segfault, tried adding in a .text segment and still got the error, I've removed the old upload and here is a new upload, got gospel soon so just gonna hope someone can pin point where I'm going wrong there. By the way readelf is for some reason saying the segment size reported is less than the size it thinks they are, if you can figure out why that is despite me using sizeof(elsh64_t) then please be my gest, here's the typedef along with the enum I used:

Code:

enum {
	ELSH_SHT_NULL = 0,
	ELSH_SHT_PROGBITS,
	ELSH_SHT_SYMTAB,
	ELSH_SHT_STRTAB,
	ELSH_SHT_RELA,
	ELSH_SHT_HASH, // Used this one, haven'y found the note that states which one to use just yet
	ELSH_SHT_DYNAMIC,
	ELSH_SHT_NOTE,
	ELSH_SHT_NOBITS,
	ELSH_SHT_SHLIB,
	ELSH_SHT_DYNSYM,
	ELSH_SHT_INIT_ARRAY = 0xE,
	ELSH_SHT_FINI_ARRAY,
	ELSH_SHT_PREINIT_ARRAY,
	ELSH_SHT_GROUP,
	ELSH_SHT_SYMTAB_SHNDX,
	ELSH_SHT_NUM,
	ELSH_SHT_LOOS = 0x60000000,
	ELSH_SHT_HIOS = 0x6FFFFFFF,
	ELSH_SHT_LOPROC = 0x70000000,
	ELSH_SHT_HIPROC = 0x7FFFFFFF
};
typedef struct __attribute__((packed)) {
	d32 sh_name; // stored directly before the entrypoint after both the program header/s and segment header/s
	u32 sh_type;
	u64 sh_flags;
	u64 sh_addr;
	d64 sh_offset;
	u32 sh_link;
	u32 sh_info;
	u64 sh_adralign;
	u64 sh_entsize;
} elsh64_t;