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diff --git a/md/writeup.md b/md/writeup.md index 96deda8..e3758f7 100644 --- a/md/writeup.md +++ b/md/writeup.md @@ -11,6 +11,7 @@ [Multiboot USB drive](writeup/multiboot_usb_drive.md) [Solving crackmes](writeup/crackme.md) [Using iptables](writeup/using_iptables.md) +[GCC inline assembly](writeup/gcc_inline_assembly.md) ## Projects diff --git a/md/writeup/gcc_inline_assembly.md b/md/writeup/gcc_inline_assembly.md new file mode 100644 index 0000000..6e9985a --- /dev/null +++ b/md/writeup/gcc_inline_assembly.md @@ -0,0 +1,183 @@ +# GCC inline assembly + +Here is attempt to make best notes about gcc inline asm. As this is just small +operator that is for long time, i dont thing there is some full guide how to use +it lets do it here for wisdom of internet. + +## Inline assembly syntax + +``` +asm [volatile] ( + AssemblerTemplate + : OutputOperands + [ : InputOperands + [ : Clobbers ] ] +) +``` + +### Output operands + +| Option | Note | +|---|---| +| "=r" | Write output to register | + +### Input operands + +| Option | Note | +|---|---| +| "r" | Input to register | + +### Clobbers + +Clobbers used to tell GCC that registers are used. + +| Clobber option | Note | +|---|---| +| "0" | map to register "0" same register | + +### Modifiers + +|Modifier|Specifies| +|---|---| +| = | Write-only operand, usually used for all output operands | +| + | Read-write operand, must be listed as an output operand | +| & | A register that should be used for output only | + +### Table of register names + +Table of AMD64 register names + +```text ++---+--------------------+ +| r | Register(s) | ++---+--------------------+ +| a | %eax, %ax, %al | +| b | %ebx, %bx, %bl | +| c | %ecx, %cx, %cl | +| d | %edx, %dx, %dl | +| S | %esi, %si | +| D | %edi, %di | ++---+--------------------+ +``` + +|Register|Name| +|---|---| +| rax | %%rax | +| rbx | %%rbx | +| rcx | %%rcx | +| rdx | %%rdx | +| rdi | %%rdi | +| rsi | %%rsi | +| rbp | %%rbp | +| rsp | %%rsp | +| r8 | %%r8 | +| r9 | %%r9 | +| r10 | %%r10 | +| r11 | %%r11 | +| r12 | %%r12 | +| r13 | %%r13 | +| r14 | %%r14 | +| r15 | %%r15 | + +## Syscall calling convention + +<!--### Intel i386 Linux--> + +### Intel AMD64 Linux + +| Param | Register | +| --- | --- | +| 1th | __rdi__ | +| 2th | __rsi__ | +| 3th | __rdx__ | +| 4th | __rcx__ | +| 5th | __r8__ | +| 6th | __r9__ | +| syscall number | __rax__ | + +## Example + +### AMD64 Add two numbers + +``` +int32_t a=1,b=2,c=-1; +asm( + "movl %1, %0\n\t" + "addl %2, %0\n\t" + :"=r"(c) + :"r"(a),"r"(b) + :"0"); +``` +__a__,__b__ - use regisers and save result __c__ to register, +make to use for __c__ same register by mentioning "0" in clobber register + +__Output__ +``` + movl %edx, %edx + addl %ecx, %edx +``` + +``` +int32_t a=1,b=2,c=-1; +asm( + "movl %1, %0\n\t" + "addl %2, %0\n\t" + :"=r"(c) + :"g"(a),"g"(b) + :"0"); +``` + +__Output__ +``` + movl -4(%rbp), %edx + addl -8(%rbp), %edx +``` + +### AMD64 Call linux syscall mmap with inline asm + +``` +long sys_mmap(unsigned long addr, unsigned long len, unsigned long prot, unsigned long flags, unsigned long fd, unsigned long off) +{ + long ret; + asm( + "mov %p6, %%r9\n\t" + "mov %p5, %%r8\n\t" + "mov %p4, %%r10\n\t" + "mov %p3, %%rdx\n\t" + "mov %p2, %%rsi\n\t" + "mov %p1, %%rdi\n\t" + "mov $9, %%rax\n\t" + "syscall\n\t" + :"=a"(ret) + :[p1]"m"(addr),[p2]"m"(len),[p3]"m"(prot),[p4]"m"(flags),[p5]"m"(fd),[p6]"m"(off)); + return ret; +} +``` +Put result of execution to __ret__, all paramters in memory + +<!-- + +### Intel i386 BSD +### Intel AMD64 BSD +### ARM +### MIPS + +--> + +## Switching on of intel/att syntax + +## Asm goto + + + +## Links + +1. https://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html +2. https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html +3. https://en.wikipedia.org/wiki/Inline_assembler +4. http://www.ethernut.de/en/documents/arm-inline-asm.html +5. http://wiki.osdev.org/Inline_Assembly +6. http://asm.sourceforge.net/articles/rmiyagi-inline-asm.txt +7. http://wiki.osdev.org/Inline_Assembly/Examples +8. http://wiki.osdev.org/Calling_Conventions + |