Assembly Language for Beginners

1.20. ARRAYS

aMarch: .ascii "March"<0>
aApril: .ascii "April"<0>
aMay: .ascii "May"<0>
aJune: .ascii "June"<0>
aJuly: .ascii "July"<0>
aAugust: .ascii "August"<0>
aSeptember: .ascii "September"<0>
aOctober: .ascii "October"<0>
aNovember: .ascii "November"<0>
aDecember: .ascii "December"<0>

Array overflow

Our function accepts values in the range of 0..11, but what if 12 is passed? There is no element in table
at this place.

So the function will load some value which happens to be there, and return it.

Soon after, some other function can try to get a text string from this address and may crash.

Let’s compile the example in MSVC for win64 and open it inIDAto see what the linker has placed after
the table:

Listing 1.239: Executable file in IDA

off_140011000 dq offset aJanuary_1 ; DATA XREF: .text:0000000140001003
; "January"
dq offset aFebruary_1 ; "February"
dq offset aMarch_1 ; "March"
dq offset aApril_1 ; "April"
dq offset aMay_1 ; "May"
dq offset aJune_1 ; "June"
dq offset aJuly_1 ; "July"
dq offset aAugust_1 ; "August"
dq offset aSeptember_1 ; "September"
dq offset aOctober_1 ; "October"
dq offset aNovember_1 ; "November"
dq offset aDecember_1 ; "December"
aJanuary_1 db 'January',0 ; DATA XREF: sub_140001020+4
; .data:off_140011000
aFebruary_1 db 'February',0 ; DATA XREF: .data:0000000140011008
align 4
aMarch_1 db 'March',0 ; DATA XREF: .data:0000000140011010
align 4
aApril_1 db 'April',0 ; DATA XREF: .data:0000000140011018

Month names are came right after.

Our program is tiny, so there isn’t much data to pack in the data segment, so it just the month names.
But it has to be noted that there might be reallyanythingthat linker has decided to put by chance.

So what if 12 is passed to the function? The 13th element will be returned.

Let’s see how the CPU treats the bytes there as a 64-bit value:

Listing 1.240: Executable file in IDA

off_140011000 dq offset qword_140011060
; DATA XREF: .text:0000000140001003
dq offset aFebruary_1 ; "February"
dq offset aMarch_1 ; "March"
dq offset aApril_1 ; "April"
dq offset aMay_1 ; "May"
dq offset aJune_1 ; "June"
dq offset aJuly_1 ; "July"
dq offset aAugust_1 ; "August"
dq offset aSeptember_1 ; "September"
dq offset aOctober_1 ; "October"
dq offset aNovember_1 ; "November"
dq offset aDecember_1 ; "December"
qword_140011060 dq 797261756E614Ah ; DATA XREF: sub_140001020+4