lectures.alex.balgavy.eu

Lecture notes from university.
git clone git://git.alex.balgavy.eu/lectures.alex.balgavy.eu.git
Log | Files | Refs | Submodules

index.md (2687B)

      1 +++
      2 title = 'System calls'
      3 +++
      4 # System calls
      5 
      6 - every process starts with 3 files open: stdin, stdout, stderr
      7 - steps:
      8 
      9 ![](3008f702882cb6bfcab5abd16d53719a.png)
     10 
     11 - what has to happen to print hello world to stdout?
     12     - build process:
     13 
     14         ![](5b26d2fb6045ae318661888ee0febe89.png)
     15 
     16     - iteration 1
     17 
     18         ```c
     19         #include <stdio.h>
     20             int main(int argc, char **argv) {
     21             printf("Hello World!\n");
     22                 return 0;
     23             }
     24         ```
     25 
     26     - iteration 2
     27 
     28         ```c
     29         #include <unistd.h>
     30         #define STDOUT 1
     31         int main(int argc, char **argv) {
     32             char msg[] = "Hello World!\n";
     33             write(STDOUT, msg, sizeof(msg));
     34             return 0;
     35         }
     36         ```
     37 
     38     - iteration 3
     39         ```c
     40         #define _GNU_SOURCE
     41         #include <sys/syscall.h>
     42         #define STDOUT 1
     43         int main(int argc, char **argv) {
     44             char msg[] = "Hello World!\n”;
     45             int nr = SYS_write;
     46             syscall(nr, STDOUT, msg, sizeof(msg));
     47             return 0;
     48         }
     49         ```
     50 - syscall diagram
     51 
     52     ![](7398350cc1064e43b0aa996f2fa2b199.png)
     53 
     54 - syscall (x86 Linux) is triggered by instruction (like 0x80):
     55     - privilege level changed to kernel mode
     56     - program counter set to specific location
     57     - arguments passed in registers:
     58         - rax <- syscall number
     59         - ebx, ecdx, edx, esi, edi, ebp <- arguments
     60         - stack <- more arguments
     61     - x86-64 supports legacy int 0x80, new instruction syscall
     62         - rax <- syscall number (different from 32bit)
     63         - rdi, rsi, rdx, r10, r8, r9 <- arguments
     64 - hello world without glibc -- manual system calls, in-line assembly:
     65 
     66     ```c
     67     ssize_t write(int fd, const void *buf, size_t nbytes) {
     68             ssize_t ret;
     69             asm volatile
     70                 (
     71                     /* request syscall to OS (can also be ‘int $0x80’) */
     72                     “syscall”
     73 
     74                     /* return result in %eax */
     75                     : "=a" (ret)
     76 
     77                     /* __NR_write (1) into same place as operand 0, fd into %rdi, buffer into %rsi, length into %rdx */
     78 
     79                     : "0" (__NR_write), "D"(fd), "S"(buf), "d"(nbytes)
     80 
     81                     /*  modified cc, registers %rcx and %r11, and memory */
     82                     : "cc", "rcx", "r11", "memory"
     83                 );
     84             return ret;
     85         }
     86     ```
     87     - actual objdump of this program
     88 
     89         ![](d9c7a810130c9f8fd2152d4c2c1e48b5.png)