fastcall and RSP usage

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system error

Joined: 01 Sep 2013
Posts: 671

system error

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: 17716
Location: In your JS exploiting you and your system

revolution

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: 671

system error

hmmm... okay. Thanks mom.

03 Nov 2016, 02:40

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 17716
Location: In your JS exploiting you and your system

revolution

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

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