Foundations of Linux ARM64: Debug, Disassemble, and Reverse/

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A Disassembly Example with Function and Stack

A Disassembly Example with Function and Stack

Learn how function parameters are passed via stack in the disassembly code and how parameters mismatch can result in defects.

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Example of disassembled code with comments

Here is the commented code disassembly of the main function with memory addresses removed for visual clarity:

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stp     x29, x30, [sp,#-48]!       // establishing stack frame for
mov     x29, sp                    // parameters and local variables
str     w0, [x29,#28]              // saving the first main parameter
str     x1, [x29,#16]              // saving the second main parameter
mov     w0, #0x1                   // setting the first parameter
                                   //   for arithmetic function
mov     w1, #0x1                   // setting the second parameter
                                   //   for arithmetic function
bl      0x4005e4 <_Z10arithmeticii> 
str     w0, [x29,#44]              // setting the result local variable
mov     w0, #0x0                   // main should return 0
ldp     x29, x30, [sp],#48         // restoring the previous stack frame,
                                   // frame and link registers
ret                                // return from main

Here is the commented disassembly of the arithmetic function, with memory addresses removed for visual clarity:

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sub     sp, sp, #0x10              // establishing stack frame for
                                   // parameters and local variables
str     w0, [sp,#12]               // saving the first arithmetic parameter (a)
str     w1, [sp,#8]                // saving the second arithmetic parameter (b)
ldr     w1, [sp,#8]                // w1 <- [b]
ldr     w0, [sp,#12]               // w0 <- [a]
add     w0, w1, w0                 // w0 <-w1 + w0
str     w0, [sp,#8]                // [b] <- w0
ldr     w0, [sp,#12]               // w0 <- [a]
add     w0, w0, #0x1               // w0 <- w0 + 1
str     w0, [sp,#12]               // [a] <- w0
ldr     w1, [sp,#8]                // w1 <- [b]
ldr     w0, [sp,#12]               // w0 <- [a]
mul     w0, w1, w0                 // w0 <- w1 * w0 
str     w0, [sp,#8]                //[b] <- w0
ldr     w0, [sp,#8]                // w0 <- [b]
                                   // return result
add     sp, sp, #0x10              // restoring the previous stack frame
ret                                // return from arithmetic

We can put a breakpoint on the first arithmetic calculations address and examine raw stack data pointed to by the sp register:

gcc FunctionParameters.cpp Arithmetic.cpp -o FunctionParameters
gdb ./FunctionParameters

After executing and loading the program, we get into the GDB container and see the following output:

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Loading object code into GDB
Loading object code into GDB

We create the breakpoint of the programs with the break main command:

break main

The breakpoint is shown below:

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Breakpoint 1 at 0x734
(gdb)

Now, we run the program until the GDB breaks in:

set disable-randomization off
run

After running the program, it gives the breakpoint and starts the program’s execution:

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