fasm 1.72: empty relocation section bug

flat assembler > Compiler Internals > fasm 1.72: empty relocation section bug

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ShabbyBumblebutt

Joined: 02 Jan 2018
Posts: 5

W7 SP1 x64 rejects binary (as non win32 program) with empty relocation section:

Code:

include 'win32ax.inc'

section '.reloc' fixups data readable discardable

.code

  start:
        ret

.end start

Code:

Number  Name   VirtSize   RVA    PhysSize  Offset    Flag
    1 .reloc   00000000 00001000 00000000 00000200 42000040
    2 .text    00000001 00001000 00000200 00000200 60000020
    3 .idata   00000014 00002000 00000200 00000400 C0000040

Removing '.reloc' section or adding 'lea eax, dword ptr start' to fill that section helps.

02 Jan 2018, 00:53

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16390
Location: Hippocamp

Actually it is a Windows bug in the loader refusing to accept empty relocs sections.

You can "fix" it by placing a dummy "dq 0" in the relocs section at the end. Or if you are worried about the extra bloat of 8 bytes you can check if the section is empty and only place the dq 0 if there is nothing else there.

Last edited by revolution on 02 Jan 2018, 16:31; edited 1 time in total

02 Jan 2018, 05:10

ShabbyBumblebutt

Joined: 02 Jan 2018
Posts: 5

I'm sure that Fasm must show a warning in that case. How i'm supposed to know why my binary not works?

02 Jan 2018, 05:17

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16390
Location: Hippocamp

There are systems other than Windows that use the PE format. It is unfortunate that Windows has such a bug, but for most programs you can run without a relocs section perfectly fine. And for those non-trivial programs that want to have things like ASLR available then the relocs section would never be empty anyway. The impact is too low for MS to care about fixing it.

02 Jan 2018, 05:32

ShabbyBumblebutt

Joined: 02 Jan 2018
Posts: 5

I make some trivial stub and it not even loading - how i'm supposed to know the reason without such warning?

02 Jan 2018, 05:46

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16390
Location: Hippocamp

I'm not sure what you are suggesting. Are you expecting fasm to tell of problems with Windows? It's actually Windows job to tell you why it fails. But Windows is not very helpful in these cases. It would be possible to make fasm always output a non-empty relocs section but then other systems that aren't Windows might fail. I don't know all the details of it, but empty sections are a problem for Windows, not just the relocs sections. It affects all sections.

02 Jan 2018, 06:29

ShabbyBumblebutt

Joined: 02 Jan 2018
Posts: 5

Fasm can check subsystem (gui/console/native) for detecting windows program.

02 Jan 2018, 06:54

Tomasz Grysztar
Assembly Artist

Joined: 16 Jun 2003
Posts: 7190
Location: Kraków, Poland

revolution wrote:

I don't know all the details of it, but empty sections are a problem for Windows, not just the relocs sections. It affects all sections.

It was even worse than that. Windows 9x accepted empty sections, but frowned on empty fixups, while NT-based Windows had no problem with empty fixups but did not allow empty sections. There was no consistent Win32 behavior for these cases.

02 Jan 2018, 09:16

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