From 2cad8341019659a65fc6e94992165b3d7b7a37db Mon Sep 17 00:00:00 2001 From: johannst Date: Sun, 23 Mar 2025 23:51:01 +0000 Subject: deploy: 916b73bee95494c205ba67e4a50e6a525afc3a3c --- arch/x86_64.html | 62 +++++++++++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 55 insertions(+), 7 deletions(-) (limited to 'arch/x86_64.html') diff --git a/arch/x86_64.html b/arch/x86_64.html index 6f0e435..19ae728 100644 --- a/arch/x86_64.html +++ b/arch/x86_64.html @@ -216,6 +216,41 @@ wrmsr // Write MSR register, effectively does MSR[ECX] <- EDX:EAX

See guest64-msr.S as an example.

+

Some interesting MSRs

+ +

Current privilege level

+

The current privilege level can be found at any time in the last two bits of the +code segment selector cs. The following shows an example debugging an entry +and exit of a syscall in x86_64-linux.

+
Breakpoint 1, entry_SYSCALL_64 () at arch/x86/entry/entry_64.S:90
+90		swapgs
+(gdb) info r rax rcx cs
+rax            0x0                 0                ; syscall nr
+rcx            0x7feb16399e56      140647666916950  ; ret addr
+cs             0x10                16               ; cs & 0x3 -> 0 (ring0,kernel)
+
+(gdb) c
+Breakpoint 2, entry_SYSCALL_64 () at arch/x86/entry/entry_64.S:217
+217		sysretq
+(gdb) info r rcx cs
+rcx            0x7feb16399e56      140647666916950  ; ret addr
+cs             0x10                16               ; cs & 0x3 -> 0 (ring0,kernel)
+
+(gdb) b *$rcx
+(gdb) s
+Breakpoint 3, 0x00007feb16399e56 in ?? ()
+(gdb) info r cs
+cs             0x33                51  ; cs & 0x3 -> 3 (ring3,user)
+

Size directives

Explicitly specify size of the operation.

mov  byte ptr [rax], 0xff    // save 1 byte(s) at [rax]
@@ -282,6 +317,18 @@ mov al, 0xaa
 mov cx, 0x10
 rep stosb
+

AT&T syntax for intel syntax users

+
mov %rax, %rbx           // mov rbx, rax
+mov $12, %rax            // mov rax, 12
+
+mov (%rsp), %rax         // mov rax, [rsp]
+mov 8(%rsp), %rax        // mov rax, [rsp + 8]
+mov (%rsp,%rcx,4), %rax  // mov rax, [rsp + 8 * rcx]
+mov 0x100, %rax          // mov rax, [0x100]
+mov (0x100), %rax        // mov rax, [0x100]
+
+mov %gs:8, %rax          // mov rax, gs:8
+

Time stamp counter - rdtsc

static inline uint64_t rdtsc() {
   uint32_t eax, edx;
@@ -475,34 +522,35 @@ must must save these registers in case they are used.

 
  • gnu assembler gas
    • 
 
  • intel syntax
    • 
 
-
    # file: greet.s
+
    // file: greet.S
+#include <asm/unistd.h>
 
     .intel_syntax noprefix
 
     .section .text, "ax", @progbits
     .global _start
 _start:
-    mov rdi, 1                      # fd
+    mov rdi, 1                      # fd (stdout)
     lea rsi, [rip + greeting]       # buf
     mov rdx, [rip + greeting_len]   # count
-    mov rax, 1                      # write(2) syscall nr
+    mov rax, __NR_write             # write(2) syscall nr
     syscall
 
-    mov rdi, 0                      # exit code
+    mov rdi, __NR_exit              # exit code
     mov rax, 60                     # exit(2) syscall nr
     syscall
 
     .section .rdonly, "a", @progbits
 greeting:
-    .asciz "Hi ASM-World!\n"
+    .ascii "Hi ASM-World!\n"
 greeting_len:
     .int .-greeting
 
    
 
    
-
    Syscall numbers are defined in /usr/include/asm/unistd.h.
    
+
    Files with .S suffix are pre-processed, while files with .s suffix are not.
    
 
    
 
    To compile and run:
    
-
    > gcc -o greet greet.s -nostartfiles -nostdlib && ./greet
+
    > gcc -o greet greet.S -nostartfiles -nostdlib && ./greet
 Hi ASM-World!
 
    
 
    MBR boot sectors example
    
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