diff options
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86_64.html | 62 |
1 files changed, 55 insertions, 7 deletions
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 <blockquote> <p>See <a href="https://github.com/johannst/mini-kvm-rs/blob/main/guest/guest64-msr.S">guest64-msr.S</a> as an example.</p> </blockquote> +<h4 id="some-interesting-msrs"><a class="header" href="#some-interesting-msrs">Some interesting MSRs</a></h4> +<ul> +<li><code>C000_0082: IA32_LSTAR</code> target address for <a href="https://www.felixcloutier.com/x86/syscall"><code>syscall</code></a> instruction +in <strong>IA-32e</strong> (64 bit) mode.</li> +<li><code>C000_0100: IA32_FS_BASE</code> storage for <strong>%fs</strong> segment base address.</li> +<li><code>C000_0101: IA32_GS_BASE</code> storage for <strong>%gs</strong> segment base address.</li> +<li><code>C000_0102: IA32_KERNEL_GS_BASE</code> additional register, <a href="https://www.felixcloutier.com/x86/swapgs"><code>swapgs</code></a> +swaps <strong>GS_BASE</strong> and <strong>KERNEL_GS_BASE</strong>, without altering any register state. +Can be used to swap in a pointer to a kernel data structure on syscall entry, +as for example in <a href="https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/entry/entry_64.S?h=v6.13#n87"><code>entry_SYSCALL_64</code></a>.</li> +</ul> +<h2 id="current-privilege-level"><a class="header" href="#current-privilege-level">Current privilege level</a></h2> +<p>The current privilege level can be found at any time in the last two bits of the +code segment selector <code>cs</code>. The following shows an example debugging an entry +and exit of a syscall in x86_64-linux.</p> +<pre><code>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) +</code></pre> <h2 id="size-directives"><a class="header" href="#size-directives">Size directives</a></h2> <p>Explicitly specify size of the operation.</p> <pre><code class="language-x86asm">mov byte ptr [rax], 0xff // save 1 byte(s) at [rax] @@ -282,6 +317,18 @@ mov al, 0xaa mov cx, 0x10 rep stosb </code></pre> +<h2 id="att-syntax-for-intel-syntax-users"><a class="header" href="#att-syntax-for-intel-syntax-users">AT&T syntax for intel syntax users</a></h2> +<pre><code class="language-x86asm">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 +</code></pre> <h2 id="time-stamp-counter---rdtsc"><a class="header" href="#time-stamp-counter---rdtsc">Time stamp counter - <code>rdtsc</code></a></h2> <pre><code class="language-c">static inline uint64_t rdtsc() { uint32_t eax, edx; @@ -475,34 +522,35 @@ must must save these registers in case they are used.</p> <li>gnu assembler <a href="https://sourceware.org/binutils/docs/as"><code>gas</code></a></li> <li>intel syntax</li> </ul> -<pre><code class="language-x86asm"># file: greet.s +<pre><code class="language-x86asm">// 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 </code></pre> <blockquote> -<p>Syscall numbers are defined in <code>/usr/include/asm/unistd.h</code>.</p> +<p>Files with <code>.S</code> suffix are pre-processed, while files with <code>.s</code> suffix are not.</p> </blockquote> <p>To compile and run:</p> -<pre><code class="language-bash">> gcc -o greet greet.s -nostartfiles -nostdlib && ./greet +<pre><code class="language-bash">> gcc -o greet greet.S -nostartfiles -nostdlib && ./greet Hi ASM-World! </code></pre> <h2 id="mbr-boot-sectors-example"><a class="header" href="#mbr-boot-sectors-example">MBR boot sectors example</a></h2> |