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Reminder

RTFM : https://nasm.us/doc/nasmdoc0.html

https://www.nekosecurity.com/x86-64-assembly/part-2-cpu-process-register-eflags-section-and-segment
https://www.ipgirl.com/59255/ou-puis-je-trouver-une-liste-complete-des-instructions-dassemblage-x86_64.html
https://sevanspowell.net/posts/learning-nasm-on-macos.html#sec-1-5
https://cs.lmu.edu/~ray/notes/nasmtutorial/

Data size

Size in bytes Size in bits prefixe Initialized (.data) Uninitialized (.bss)
1 8 byte db resb
2 16 word dw resw
4 32 dword dd resd
8 64 qword dq resq
10 80 tword dt rest
16 128 oword do / ddq reso / resdq
32 256 yword dy resy
64 512 zword dz resz

NASM sections

Segment / Section Meaning c equivalent example
.text Code printf("%d", i);
.data Initialized Data int i = 1;
.bss Uninitialized Data int i;

More about section directive: elf macho win

.data (Initialized Data) :

unit Size in bytes (octet) c equivalent example
Byte db 1 char c = 'a';
Word dw 2 short int i = 4;
Double Word dd 4 int i = 42; float f = 1.4f;
Quad Word dq 8 long int i = 13; double d = 1.4;
10 bytes dt 10 long double d = 4.42;

d stand for defined.

.bss (Uninitialized Data) :

unit Size in bytes (octet) c equivalent example
Byte resb 1 char c;
Word resw 2 short int i;
Double Word resd 4 int i; float f;
Quad Word resq 8 long int i; double d;
10 bytes rest 10 long double d;

res stand for reserved.

Registers :

00000000000000000000000000000000 00000000000000000000000000000000
al
ax
eax
rax
8-bit 16-bit 32-bit 64-bit Purpose Preserved
al ax eax rax accumulator No
bl bx ebx rbx base, addressing Yes
cl cx ecx rcx counter, iterations No
dl dx edx rdx data No
sil si esi rsi - No
dil di edi rdi - No
bpl bp ebp rbp frame pointer Yes
spl sp esp rsp Stack pointer Yes
r8b r8w r8d r8 - No
r9b r9w r9d r9 - No
r10b r10w r10d r10 - No
r11b r11w r11d r11 - No
r12b r12w r12d r12 - Yes
r13b r13w r13d r13 - Yes
r14b r14w r14d r14 - Yes
r15b r15w r15d r15 - Yes

syscall input by register (Fastcall convention):

syscall MacOS Id (%rax) Linux ID (%rax) ARG_1 (%rdi) ARG_2 (%rsi) ARG_3 (%rdx) ARG_4 (if syscall %r10 else %rcx ) ARG_5 (%r8) ARG_6 (%r9)
sys_read 0x2000003 0 fd & buffer count
sys_write 0x2000004 1 fd & buffer count
sys_open 0x2000005 2 filename Flags mode
sys_close 0x2000006 3 fd
sys_exit 0x2000001 60 error_code ... ... ... ... ...
... ... ... ... ... ... ... ... ...
pwritev2 - 328 ... ... ... ... ... ...

Also, the return value goes into %rax.

Flags

Flag symbol Type Flag Description
CF Carry It contains the carry of 0 or 1 from a high-order bit (leftmost) after an arithmetic operation. It also stores the contents of last bit of a shift or rotate operation.
PF Parity It indicates the total number of 1-bits in the result obtained from an arithmetic operation. An even number of 1-bits clears the parity flag to 0 and an odd number of 1-bits sets the parity flag to 1.
ZF Zero It indicates the result of an arithmetic or comparison operation. A nonzero result clears the zero flag to 0, and a zero result sets it to 1.
SF Sign It shows the sign of the result of an arithmetic operation. This flag is set according to the sign of a data item following the arithmetic operation. The sign is indicated by the high-order of leftmost bit. A positive result clears the value of SF to 0 and negative result sets it to 1.
OF Overflow It indicates the overflow of a high-order bit (leftmost bit) of data after a signed arithmetic operation.
AF Auxiliary Carry It contains the carry from bit 3 to bit 4 following an arithmetic operation; used for specialized arithmetic. The AF is set when a 1-byte arithmetic operation causes a carry from bit 3 into bit 4.
IF Interrupt Enabed It determines whether the external interrupts like keyboard entry, etc., are to be ignored or processed. It disables the external interrupt when the value is 0 and enables interrupts when set to 1.
TF Trap It allows setting the operation of the processor in single-step mode. The DEBUG program we used sets the trap flag, so we could step through the execution one instruction at a time.
DF Direction It determines left or right direction for moving or comparing string data. When the DF value is 0, the string operation takes left-to-right direction and when the value is set to 1, the string operation takes right-to-left direction.

Pointers

Pointer Name Meaning Description
rip (eip, ip) Index pointer Points to next address to be executed in the control flow.
rsp (esp, sp) Stack pointer Points to the top address of the stack.
rbp (ebp, bp) Stack base pointer Points to the bottom of the stack.
... ... ...

jump symbols

Jump symbol (signed) Jump symbol (unsigned) result of cmp a, b
je - a = b
jne - a != b
jg ja a > b
jge jae a >= b
jl jb a < b
jle jbe a <= b
jz - a = 0
jnz - a != 0
jo - Overflow occured
jno - Overlow did not occur
js - Jump if signed
jns - Jump if not signed

Registers as pointers

The default registers can be treated as pointers. To treat a register as a pointer, surround the register name with square brackets. Such as, "rax" becomes "[rax]".

mov rax, rbx # loads the value of rbx into rax
mov rax, [rbx] # loads the value the rbx register is pointing to, into rax. Like dereferencing pointer in c

Math operations list

Operation Name Operation Name (signed) Description
add a, b - a = a + b
sub a, b - a = a - b
mul reg imul reg rax = rax * reg
div reg idiv reg rax = rax / reg
neg reg - reg = -reg
inc reg - reg = reg + 1
dec reg - reg = reg - 1
adc a, b - a = a + b + CF
sbb a, b - a = a - b - CF

%rdx hold the remainder of division. Also, if %rdx is not zero, %rax and %rdx will act as a 128bits registers. To avoid unexpected results. Set %rdx to 0.

Stack operations

Operation Description
push reg/value Pushes a value onto the stack
pop reg Pops a value off the stack and stores it in reg
mov reg, [rsp] Stores the peek value in reg

Note : Usually in places where you can use registers, you can also use pointers. Such as, instead of "pop reg", you can use "pop [label]" to pop a value off the stack directly into a position in memory.

Instructions

Purpose
mov dest, src Move data between registers, load immediate data into registers, move data between registers and memory.
push src Insert a value onto the stack. Useful for passing arguments, saving registers, etc.
pop dest Remove topmost value from the stack. Equivalent to "mov dest, [rsp]; add 8,rsp".
call label Push the address of the next instruction and start executing func.
ret Pop the return program counter, and jump there. Ends a subroutine.
add dest, src dest = dest + src
mul src Multiply rax and src as unsigned integers, and put the result in rax. High 64 bits of product (usually zero) go into rdx
div src Divide rax by src, and put the ratio into rax, and the remainder into rdx. Bizarrely, on input rdx must be zero, or you get a SIGFPE.
shr val,bits Bitshift a value right by a constant, or the low 8 bits of rcx ("cl"). Shift count MUST go in rcx, no other register will do!
jmp label Goto the instruction label:. Skips anything else in the way.
cmp a,b Compare two values. Sets flags that are used by the conditional jumps (below).
jl label Goto label if previous comparison came out as less-than. Other conditionals available are: jle (<=), je (==), jge (>=), jg (>), jne (!=), and many others. Also available in unsigned comparisons: jb (<), jbe (<=), ja (>), jae (>=).
loop target The execution of the Loop instruction involves two steps: 1. First, it subtracts 1 from ECX. 2. Next, it compares ECX to zero. If ECX is not equal to zero; a jump is taken to the label identified by destination. Otherwise, if ECX equals zero, no jump takes place and control passes to the instruction following the loop.

String instructions (more here)

Purpose
MOVS / MOVSB Move string / Move byte string.
MOVS / MOVSW Move string / Move word string.
MOVS / MOVSD Move string / Move doubleword string.
CMPS / CMPSB Compare string / Compare byte string.
CMPS / CMPSW Compare string / Compare word string.
CMPS / CMPSD Compare string / Compare doubleword string.
SCAS / SCASB Scan string / Scan byte string
SCAS / SCASW Scan string / Scan word string.
SCAS / SCASD Scan string / Scan doubleword string.
LODS / LODSB Load string / Load byte string.
LODS / LODSW Load string / Load word string.
LODS / LODSD Load string / Load doubleword string.
STOS / STOSB Store string / Store byte string.
STOS / STOSW Store string / Store word string.
STOS / STOSD Store string / Store doubleword string.
REP Repeat while ECX not zero.
REPE / REPZ Repeat while equal / Repeat while zero.
REPNE / REPNZ Repeat while not equal / Repeat while not zero.

more

loops in nasm

search for local labels ;) https://nasm.us/doc/nasmdoc3.html#section-3.9 https://nasm.us/doc/nasmdoc3.html

Defining macro

  • name : Name of macro.
  • argc : NUmber of arguments the macro will take. Within the macro body, these inputs are referenced using "%n". "%1" == first input, "%2" == the second...
  • macro body : the definition of the macro
%macro <name> <argc>
  <macro body>
%endmacro

; Example :

%macro exit 0
  mov rax, 60
  mov rdi, 0
  syscall
%endmacro

Defining constants

STDIN equ 0
STDOUT equ 1

SYS_READ equ 0
SYS_WRITE equ 1
SYS_EXIT equ 60

section .data
  text db "Hello, world !", 10, 0

section .text
  global _start
  
_start:
  mov rax, SYS_WRITE
  mov rdi, STDOUT
  mov rsi, text
  mov rdx, 14
  syscall
  
  mov rax, SYS_EXIT
  mov rdi, 0
  syscall

Including external files

%include "filename.inc"

Debugging using lldb

Examples :
// Flow
thread step-inst
thread step-inst-over
n

// man 
help thread
help thread step-inst
help thread step-inst-over

// Show expression based on registers
print $rax
p $rax
p/s $rax
p/s $rax
p  (int)$rax + 1
p  $rdi + $rsi

// SHow register values
register read
reg read
reg r
re r
re r/s
re r rax
re r rax rdi sil
// Show memory values
memory read $rax
mem rea $rax
mem rea/s $rax

x $rsi
x -c5 $rsi // -c : the number of total items to display (here 5 octets / bytes)

// man
help x
help memory
help memory read
help register
help register read
// etc....

// help about format
register read -f 42

https://www.nesono.com/sites/default/files/lldb%20cheat%20sheet.pdf https://nickdesaulniers.github.io/blog/2016/01/20/debugging-x86-64-assembly-with-lldb-and-dtrace/ https://stackoverflow.com/questions/30512452/lldb-read-memory-pointer lldb - switch from at&t to intel syntax (you can put this line in ~/.lldbinit to make it permanent):
settings set target.x86-disassembly-flavor intel

debugging reverse engineering

https://mirrors.ocf.berkeley.edu/parrot/misc/openbooks/programming/ReverseEngineeringForBeginners.en.pdf

Errno and ___error

https://pubs.opengroup.org/onlinepubs/009695399/functions/errno.html https://pubs.opengroup.org/onlinepubs/9699919799/ https://stackoverflow.com/questions/29047592/accessing-errno-h-in-assembly-language https://stackoverflow.com/questions/44319762/how-to-use-errno-from-asm https://www.thegeekstuff.com/2010/10/linux-error-codes/ man errno man error

Are the Prologue and Epilogue mandatory ?

Not really...But usefull still...

errors

undefined reference to ... (linux and macos):

When making a label global, Macos want leading underscore (_ft_strlen), and linux don't want it...

https://stackoverflow.com/questions/19101815/how-can-c-code-call-an-external-function-without-a-leading-underscore https://en.wikipedia.org/wiki/Name_mangling https://clang.llvm.org/docs/UsersManual.html#differences-between-various-standard-modes https://stackoverflow.com/questions/5313536/whats-the-difference-between-gnu99-and-c99-clang https://stackoverflow.com/questions/1034852/adding-leading-underscores-to-assembly-symbols-with-gcc-on-win32/1035937#1035937 https://stackoverflow.com/questions/1703670/is-there-anything-to-change-the-exports-name-mangling-scheme-in-gcc https://stackoverflow.com/questions/50763624/cant-link-an-assembly-file-s-with-gcc-on-macos-sierra https://stackoverflow.com/questions/44358292/how-to-override-prefix-option-in-nasm-per-symbol

To make it work under linux and osx, you can use the attibute asm('label_name').
For example int ft_strlen(char *s) asm('_ft_strlen'); https://clang.llvm.org/docs/AttributeReference.html#asm
https://gcc.gnu.org/onlinedocs/gcc/Asm-Labels.html

Another solution is to use pragma gprefix to add _ when is osx : %pragma macho gprefix _ (will add '_' prefix if format is macho32|macho64) To check if the prefix is added to global and extern symbols, use the nm command : nm libasm.a

Mach-O 64-bit format does not support 32-bit absolute addresses :

Using macho64 format produce this error. Instead of mov rax, [var], do mov rax, [rel var] OR use DEFAULT REL at the top of your souce code...

https://nasm.us/doc/nasmdoc7.html#section-7.2.1
https://nasm.us/doc/nasmdoc3.html#section-3.3 https://stackoverflow.com/questions/26394359/mach-o-64-bit-format-does-not-support-32-bit-absolute-addresses-nasm
https://forum.nasm.us/index.php?topic=1970.0

Linux: GCC and PIE : relocation R_X86_64_PC32 against symbol `malloc' can not be used when making a PIE object; recompile with -fPIE

While calling like that :
call malloc The Linker is lost trying to make a PIE. So use wrt ..plt: call malloc wrt ..plt

Referring to a procedure name using wrt ..plt causes the linker to build a procedure linkage table entry for the symbol, and the reference gives the address of the PLT entry. You can only use this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for CALL or JMP), since ELF contains no relocation type to refer to PLT entries absolutely.

Of course in order to make a position independent executable, you should address all in relative. (except data section maybe ??) call [rel malloc wrt ..plt] or DEFAULT REL.....

Segfault (stack_not_16_byte_aligned_error) :

https://sevanspowell.net/posts/learning-nasm-on-macos.html

Ressources

NASM documentation

https://www.nasm.us/xdoc/2.14.03rc2/nasmdoc.pdf

call convention

https://www.agner.org/optimize/calling_conventions.pdf https://wiki.osdev.org/Calling_Conventions https://beta.hackndo.com/conventions-d-appel https://h-deb.clg.qc.ca/Sujets/AuSecours/Conventions-appel.html https://docs.microsoft.com/fr-fr/cpp/build/x64-calling-convention?view=vs-2019 https://www.raywenderlich.com/615-assembly-register-calling-convention-tutorial https://stackoverflow.com/questions/9268586/what-are-callee-and-caller-saved-registers

call tables

https://filippo.io/linux-syscall-table/ https://blog.rchapman.org/posts/Linux_System_Call_Table_for_x86_64/ https://blog.packagecloud.io/eng/2016/04/05/the-definitive-guide-to-linux-system-calls/

Man

https://software.intel.com/sites/default/files/managed/39/c5/325462-sdm-vol-1-2abcd-3abcd.pdf https://planetpdf.com/codecuts/pdfs/aoa.pdf https://www.egr.unlv.edu/~ed/assembly64.pdf https://sites.google.com/site/microprocessorsbits/string-instructions/cmps-cmpsb-cmpsw https://blog.packagecloud.io/eng/2016/04/05/the-definitive-guide-to-linux-system-calls/
https://stackoverflow.com/tags/x86/info

Instructions

https://fr.wikipedia.org/wiki/Jeu_d%27instructions_x86 https://fr.wikipedia.org/wiki/Jeu_d%27instructions#%C2%AB_%C3%A0_accumulateur_%C2%BB

other

string instructions

repn scasb end conditions
https://www.conradk.com/codebase/2017/06/06/x86-64-assembly-from-scratch/
https://www.csc.depauw.edu/~bhoward/asmtut/asmtut7.html

Best practice

https://www.codeproject.com/Articles/1116188/40-Basic-Practices-in-Assembly-Language-Programmin#Introduction

cld and std direction flag

https://stackoverflow.com/questions/23860583/x86-abi-should-df-flag-be-always-0
https://c9x.me/x86/html/file_module_x86_id_29.html https://en.wikipedia.org/wiki/Direction_flag https://www.agner.org/optimize/calling_conventions.pdf

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