Japheth

[[e-mail]]

My HX (2.17) crashes on first run, and I had lost hope
of ever seeing Japheth again, but it looks like he's
back:

https://github.com/Baron-von-Riedesel

I'm actually interested in 32-bit relocatable ELF
modules at the moment, so looking for the first
jwlink that supports that.

BFN. Paul.

[[e-mail]]

Post by ***@gmail.com
I'm actually interested in 32-bit relocatable ELF
modules at the moment, so looking for the first
jwlink that supports that.

It looks like even wlink has that. I just didn't understand
the format. I still don't. Can someone tell me at what
offset in the below executable I can find structures like this:

typedef struct {
Elf32_Addr r_offset;
Elf32_Word r_info; /* Top byte is symbol index, bottom is rel. type. */
Elf32_Sword r_addend;
} Elf32_Rela;

Thanks. Paul.


000000 7F454C46 01010100 00000000 00000000 .ELF............
000010 02000300 01000000 74800408 34000000 ........t...4...
000020 FC010000 00000000 34002000 02002800 ........4. ...(.
000030 09000100 01000000 00000000 00800408 ................
000040 00000000 E1000000 E1000000 05000000 ................
000050 00100000 01000000 E4000000 E4900408 ................
000060 00000000 2E000000 38000000 06000000 ........8.......
000070 00100000 8B442404 A3189104 088B4424 .....D$.......D$
000080 08A31491 0408E904 00000031 C0C3C368 ...........1...h
000090 E4900408 6A006A00 E8280000 0083C40C ....j.j..(......
0000A0 68FF9004 086A006A 00E81700 000083C4 h....j.j........
0000B0 0C680F91 04086A00 6A01E806 00000083 .h....j.j.......
0000C0 C40C31C0 C3FF7424 0CFF7424 0CFF7424 ..1...t$..t$..t$
0000D0 0CFF3514 910408FF 15189104 0883C410 ..5.............
0000E0 C3000000 68656C6C 6F2C2077 6F726C64 ....hello, world
0000F0 2066726F 6D206F73 776F726C 640A006A from osworld..j
000100 75737420 63686563 6B696E67 210A003A ust checking!..:
000110 3A000500 00000102 00003400 00000E00 :.........4.....
000120 00000102 00003000 00001C00 00000102 ......0.........
000130 00000000 00002D00 00000102 00000000 ......-.........
000140 00003E00 00000102 00000000 00005F00 ..>..........._.
000150 00000102 00003000 00006500 00000102 ......0...e.....
000160 00003400 00000000 00000000 00000000 ..4.............
000170 00000000 00000100 00007480 04080000 ..........t.....
000180 00000300 02000600 0000E490 04080000 ................
000190 00000300 03000200 00000300 00000200 ................
0001A0 00000100 00000000 00000000 00000000 ................
0001B0 00000041 55544F00 4447524F 55500000 ...AUTO.DGROUP..
0001C0 2E73796D 74616200 2E746578 74002E64 .symtab..text..d
0001D0 61746100 2E627373 002E7265 6C612E74 ata..bss..rela.t
0001E0 65787400 2E686173 68002E73 74727461 ext..hash..strta
0001F0 62002E73 68737472 74616200 00000000 b..shstrtab.....
000200 00000000 00000000 00000000 00000000 ................
000210 00000000 00000000 00000000 00000000 ................
000220 00000000 33000000 03000000 00000000 ....3...........
000230 00000000 BF010000 3D000000 00000000 ........=.......
000240 00000000 00000000 00000000 09000000 ................
000250 01000000 06000000 74800408 74000000 ........t...t...
000260 6D000000 00000000 00000000 01000000 m...............
000270 00000000 0F000000 01000000 03000000 ................
000280 E4900408 E4000000 2E000000 00000000 ................
000290 00000000 04000000 00000000 15000000 ................
0002A0 08000000 03000000 14910408 12010000 ................
0002B0 08000000 00000000 00000000 04000000 ................
0002C0 00000000 1A000000 04000000 02000000 ................
0002D0 00000000 12010000 54000000 06000000 ........T.......
0002E0 02000000 04000000 0C000000 01000000 ................
0002F0 02000000 00000000 00000000 66010000 ............f...
000300 30000000 07000000 00000000 00000000 0...............
000310 10000000 2B000000 03000000 00000000 ....+...........
000320 00000000 B2010000 0D000000 00000000 ................
000330 00000000 00000000 00000000 25000000 ............%...
000340 05000000 00000000 00000000 96010000 ................
000350 1C000000 06000000 00000000 00000000 ................
000360 00000000 ....

[Scott Lurndal]

Post by ***@gmail.com

Post by ***@gmail.com
I'm actually interested in 32-bit relocatable ELF
modules at the moment, so looking for the first
jwlink that supports that.

It looks like even wlink has that. I just didn't understand
the format. I still don't. Can someone tell me at what
typedef struct {
Elf32_Addr r_offset;
Elf32_Word r_info; /* Top byte is symbol index, bottom is rel. type. */
Elf32_Sword r_addend;
} Elf32_Rela;
Thanks. Paul.

Start by reading the header (Elf32_Ehdr). Then read the section table (Elf32_Shdr).
Identify the section table entry that contains the relocation
entries. Process that table entry.

$ man objdump
$ man readelf

Check the psABI for x86 for x86-specifics.

[[e-mail]]

Post by Scott Lurndal
$ man readelf

Thanks! That's what I needed.

readelf -a osworld.exe

showed:

Relocation section '.rela.text' at offset 0x112 contains 7 entries:
Offset Info Type Sym.Value Sym. Name + Addend
00000005 00000201 R_386_32 080490e4 DGROUP + 34
0000000e 00000201 R_386_32 080490e4 DGROUP + 30
0000001c 00000201 R_386_32 080490e4 DGROUP + 0
0000002d 00000201 R_386_32 080490e4 DGROUP + 0
0000003e 00000201 R_386_32 080490e4 DGROUP + 0
0000005f 00000201 R_386_32 080490e4 DGROUP + 30
00000065 00000201 R_386_32 080490e4 DGROUP + 34

And I can see that data at offset 0x112.

000110 3A000500 00000102 00003400 00000E00 :.........4.....
000120 00000102 00003000 00001C00 00000102 ......0.........
000130 00000000 00002D00 00000102 00000000 ......-.........
000140 00003E00 00000102 00000000 00005F00 ..>..........._.
000150 00000102 00003000 00006500 00000102 ......0...e.....
000160 00003400 00000000 00000000 00000000 ..4.............

One of the things that fooled me was that I was expecting
to see a relocation for x'13' here:

0000 8B 44 24 04 mov eax,dword ptr 0x4[esp]
0004 A3 00 00 00 00 mov dword ptr __exposfunc,eax
0009 8B 44 24 08 mov eax,dword ptr 0x8[esp]
000D A3 00 00 00 00 mov dword ptr __cbdata,eax
0012 E9 00 00 00 00 jmp main

(and couldn't find such a thing)

But now I see the executable actually has:

000080 08A31491 0408E904 00000031 C0C3C368

So it has filled in "4" there, which would appear to be a
relative jump to get to that x'68' which is correct.

0000 main:
0000 68 00 00 00 00 push offset L$1
0005 6A 00 push 0x00000000
0007 6A 00 push 0x00000000
0009 E8 00 00 00 00 call osfunc

Now that I know where the data is, I'll try to find the
structure that points to it.

It's not Elf32_Addr sh_addr, which is 0.

BFN. Paul.

[[e-mail]]

Post by ***@gmail.com
Now that I know where the data is, I'll try to find the
structure that points to it.

My small executable is now able to be loaded by this
ELF loader code:

https://sourceforge.net/p/pdos/gitcode/ci/master/tree/bios/exeload.c

Next I'll try the new hx and then on to jwasm 16-bit
offsets.

BFN. Paul.

[Scott Lurndal]

Post by ***@gmail.com

Post by Scott Lurndal
$ man readelf

Thanks! That's what I needed.
readelf -a osworld.exe

readelf is open source. Feel free to download it and
examine the source to determine how it finds the relocation
entries.