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ASM compare against multiple values

 
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Birdi
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PostPosted: Wed Sep 15, 2021 2:38 am    Post subject: ASM compare against multiple values Reply with quote
Just something I've been wondering about recently, is there a simpler way to check multiple specific values in ASM than actually checking individually?

ex:
Code:

cmp eax,1
je @f
cmp eax,2
je @f
cmp eax,3
je @f
cmp eax,4
jne code
@@:
//something
jmp return
code:
...


or loop (not sure if this is formatted correctly, just typing from phone)

Code:

mov ebx,0
@@:
add ebx,1
cmp eax,ebx
je @f
cmp ebx,5
jl @b
@@:
...


..but if a loop is not applicable, is there an easier/better way than checking specific values something may be?
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ParkourPenguin
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Posts: 4289
PostPosted: Wed Sep 15, 2021 12:25 pm    Post subject: Reply with quote
If you know any system programming language (C, C++, Rust...), throw code into a compiler and see what comes out.

e.g. this C++ code:
Code:
void foo();
void bar();

void injection(int a) {
    if (a == 1 || a == 2 || a == 3 || a == 4)
        foo();
    else
        bar();
}
Using x86-64 gcc 11.2, unoptimized (-O0), it produces more or less the same code you wrote. Optimized (-O2), it generates this:
Code:
injection(int):
  sub  edi, 1
  cmp  edi, 3
  ja   .L2
  jmp  foo()
.L2:
  jmp  bar()
This sub/cmp/jcc pattern is generally what you'll see for comparing whether or not an integer is in a certain range- in this case, 1 to 4 inclusive.
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Birdi
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PostPosted: Wed Sep 15, 2021 12:45 pm    Post subject: Reply with quote
Honestly I never thought about doing something like that, thanks for the suggestion Penguin!
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