Is it possible to execute 80h system function in assembly insert? [duplicate]

Question

I'm trying to print single character on the screen using assembly inserts with this code:

char z[]="x"; int result; asm( "mov 4, %%eax" : ); asm( "mov 1, %%ebx" : ); asm( "mov %%edx, %%ecx" : : "d"(&z) ); asm( "mov 2, %%edx" : ); //int 80h

But I don't known how to insert 80h function call into this c++ code.

asm("int 80h") and asm("int 80h" : ) did not work. I am using clang++

OK, I actually solved this problem with this code which compiles succesfully:

char z[]="x"; int result; asm( "mov 4, %%eax" : : : "eax" ); asm( "mov 1, %%ebx" : : : "ebx" ); asm( "mov %%edx, %%ecx" : : "d"(&z) : "ecx" ); asm( "mov 2, %%edx" : : : "edx" ); asm( "int $0x80" : );

But after executing I received segmentation fault error. I'm also receiving this message with -fsanitize=undefined,address flag:

AddressSanitizer:DEADLYSIGNAL ================================================================= ==6068==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000004 (pc 0x558c9691bb24 bp 0x7ffde7b2e180 sp 0x7ffde7b2e0c0 T0) ==6068==The signal is caused by a READ memory access. ==6068==Hint: address points to the zero page. #0 0x558c9691bb23 (/notroot/Documents/asmcheckcpp.e+0x137b23) #1 0x7faa9eb49ce2 (/usr/lib/libc.so.6+0x23ce2) #2 0x558c9680311d (/notroot/Documents/asmcheckcpp.e+0x1f11d) AddressSanitizer can not provide additional info. SUMMARY: AddressSanitizer: SEGV (/notroot/Documents/asmcheckcpp.e+0x137b23) ==6068==ABORTING

I also debugged it with gdb so I knew that this program is crushing on this line:

asm( "mov 4, %%eax" : : : "eax" );

but I do not known why. - I already solved this, so this code:

cout << 'a' << endl; char z[]="x"; asm( "mov $4, %%eax \n" "mov $1, %%ebx \n" "mov $2, %%edx \n" "int $0x80" : : "ecx"(&z) : "eax", "ebx", "edx" ); cout << 'a' << endl;

works but displays

a a

not

a xa

on the screen

Is there a reason you didn’t try asm(“int 80h”)? Or if you did, then why didn’t you include that in the question, along with why it doesn’t work? — prl
– prl, Commented May 26, 2019 at 13:17
Guidelines for using inline assembly: 1. Don’t. 2 Don’t split a single sequence of assembly instructions into multiple asm statements. 3. Don’t use registers without telling the compiler about it. 4. Don’t load registers within the inline assembly; use operand constraints. — prl
– prl, Commented May 26, 2019 at 13:21
@prl: GAS doesn't support trailing h suffix. You have to use int $0x80. Presumably clang's built-in assembler is the same. But yeah, this code is hilariously broken. I found a duplicate that shows how to use it mostly correctly. — Peter Cordes
– Peter Cordes, Commented May 26, 2019 at 15:06
@Roman: like prl said, you can't use separate asm statements for each instruction; those all need to be part of the same asm statement, like in the linked duplicate. (Or better, use input and output constraints so the only actual inline asm is "int $0x80") — Peter Cordes
– Peter Cordes, Commented May 26, 2019 at 16:46
To actually debug if you're curious why it crashed, you need to switch your debugger to disassembly, not source, to see how your inline asm mixed with compiler-generated asm. Oh, NVM, no you don't. mov 4, %%eax is a load from absolute address 4. You wanted $4 to have 4 as an immediate, not a memory address. — Peter Cordes
– Peter Cordes, Commented May 26, 2019 at 16:56

Collectives™ on Stack Overflow

Is it possible to execute 80h system function in assembly insert? [duplicate]

0

Linked

Hot Network Questions