fasm silently overwrites output file with debug info

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dunkaist

Joined: 31 Jul 2015
Posts: 24

dunkaist 08 Nov 2019, 23:42

Hi Tomasz,

Due to a mistake in my makefile I got the following listing:

Code:

$ cat main.asm 
use32
xor eax, eax
ret

$ fasm main.asm main.bin -s main.fas
flat assembler  version 1.73.16  (16384 kilobytes memory, x64)
1 passes, 3 bytes.

$ fasm main.asm main.fas -s main.fas
flat assembler  version 1.73.16  (16384 kilobytes memory, x64)
1 passes, 3 bytes.

$ listing main.fas main.lst

$ ls
main.asm  main.bin  main.fas  main.lst

$ cat main.lst 
                                                                use32
00000000: 66 61                                                 xor eax, eax
00000002: 73                                                    ret

Imagine how I was surprised to see these 66 61 73 Shocked

Please, consider adding a check for this case to fasm.
Thank you.

08 Nov 2019, 23:42

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 09 Nov 2019, 07:38

You can also try this:

Code:

fasm main.asm main.asm -s main.asm

09 Nov 2019, 07:38

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 09 Nov 2019, 08:58

This has the same problems that we discussed with "includeonce". The OS abstraction layer that fasm uses has no ability to deal with them in a reliable way. In fact, fasm is not even aware whether the file system uses case-sensitive or case-insensitive names.

A practical solution could be to write some shell script around fasm, handling things in a way appropriate for a given OS.

09 Nov 2019, 08:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 09 Nov 2019, 14:27

dunkaist:

One way to detect this problem would be to temporarily mark each file as read-only.

Fasm could do this itself. Once the input has been read, mark it read-only. Once the binary has been written, mark it as read-only. Then before exiting restore the permissions to the original. If you get a permission error when writing then you know something is wrong. You would have to modify your own copy of fasm to support it.

But that wouldn't protect against output redirection:

Code:

fasm main.asm > main.asm

Last edited by revolution on 09 Nov 2019, 16:28; edited 1 time in total

09 Nov 2019, 14:27

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 09 Nov 2019, 16:23

revolution wrote:

Once the input has been, read mark it read-only.

Note that there are legitimate use cases when one might want to actually replace the source file with its output. I do it frequently with generated test sources, for example - but you could also have something like self-patching, etc.

Things like the attributes may also be highly OS-dependent. Good news is that you can implement such tricks entirely within the OS abstraction layer (fasm's "interface") without having to touch the fasm's core. You can make even a highly customized interface which should be able to be assembled with various versions of fasm's core with no problems.

09 Nov 2019, 16:23

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 09 Nov 2019, 16:34

Another possibility is to write the output to a temporary filename and then rename it when it is closed.

All OSes that I have encountered will fail the rename if another file of the same name already exists.

09 Nov 2019, 16:34

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8359
Location: Kraków, Poland

Tomasz Grysztar 09 Nov 2019, 16:39

revolution wrote:

Another possibility is to write the output to a temporary filename and then rename it when it is closed.

All OSes that I have encountered will fail the rename if another file of the same name already exists.

Then you would not be able to re-assemble the same thing without deleting it first.

09 Nov 2019, 16:39

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