fastcall and RSP usage

flat assembler
Message board for the users of flat assembler.

Index > Windows > fastcall and RSP usage

Author

Thread

system error

Joined: 01 Sep 2013
Posts: 670

system error 03 Nov 2016, 02:32

Not an expert on macro but I took it that FASM's fastcall allocating shadow space before populating the arguments instead of after? No?

Code:

fastcall [printf],fmt,rsp

vs

mov rdx,rsp
mov rcx,fmt
sub rsp,0x20
call [printf]
add rsp,0x20
...
...
fmt db '%016llX",0

The fastcall version is off by 32 bytes. Not saying this is wrong or not, just want to know if this is the intended behaviour / design.

03 Nov 2016, 02:32

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20623
Location: In your JS exploiting you and your system

revolution 03 Nov 2016, 02:36

Any usage of RSP is not guaranteed to work. Basically if you use RSP then it is your own responsibility to make sure you get what you intend. This caveat the same for any call method; ccall, stdcall, etc.

03 Nov 2016, 02:36

system error

Joined: 01 Sep 2013
Posts: 670

system error 03 Nov 2016, 02:40

hmmm... okay. Thanks mom.

03 Nov 2016, 02:40

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20623
Location: In your JS exploiting you and your system

revolution 03 Nov 2016, 05:59

If you still want to use RSP directly and take advantage of the fastcall macro you could do this:

Code:

mov rdx,rsp
fastcall [printf],fmt,rdx

But it would still be prudent to check the disassembly to make sure it does what you expect.

03 Nov 2016, 05:59

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum