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content/posts/2022-11-19-Linux-Kernel-sys_fork.md

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+---
+title: Linux Kernel, syscalls
+date: 2022-11-19T
+slug: linux-kernel-sys-fork
+description: ""
+---
+
+#### sys_fork()
+
+```asm
+.align 2
+_sys_fork:
+  call _find_empty_process
+  testl %eax,%eax
+  js 1f
+  push %gs
+  pushl %esi
+  pushl %edi
+  pushl %ebp
+  pushl %eax
+  call _copy_process
+  addl $20,%esp
+1:  ret
+```
+
+##### `FUNCTION find_empty_process()`
+
+```assembly
+A:
+  IF ++last_pid < 0
+    last_pid = 1
+  FOR i : 0 -> NR_TASKS
+    IF task[i] && task[i].pid == last_pid
+      GOTO A
+  FOR i : 1 -> NR_TASKS
+    IF NOT task[i]
+      RETURN i
+  RETURN -1
+```
+
+
+
+1. Set last_pid to 1 or more. 
+
+2. From taskid from 0 to max_tasks, if task exists and pid is last_pid then increases pid and again.
+3. From taskid from 1 to max_tasks,  if tasks not exists, return.
+
+-> It just iterates through tasks and find last taskid linearly.
+
+
+
+##### `FUNCTION copy_process`
+
+`<- nr, EBP, EDI, ESI, GS, EBX~EDX, CS~FS, EIP, EFLAGS, ESP, SS`
+`-> INT`
+
+
+```assembly
+TASK_STRUCT P
+INT I
+FILE F
+
+P = (TASK_STRUCT)(GET_FREE_PAGE())
+
+IF NOT P
+  RET ERR
+
+SET P 
+  START_TIME  = RUNNING
+  PID         = LAST_PID
+  FATHER      = CURRENT_PID
+  COUNTER     = PRIORITY
+  START_TIME  = jiffies
+  SIGNAL, ALARM, LEADER, UTIME, STIME, CUTIME, CSDTIME, BACK_LINK = 0
+
+  SET TSS
+    BACK_LINK = 0
+    ESP0      = PAGE_SIZE + ADDR(P)
+    SS0       = 0x10
+    EAX       = 0
+    ES ~ GS   = 0xFFFF
+    LDT       = _LDT(nr)
+
+    EIP, EFLAGS   = ARGUMENT 
+    ECX ~ EDI     = ARGUMENT
+
+    TRACR_BITMAP  = 0x80000000
+  
+COPY_MEM (nr, p)
+IF <-
+  FREE_PAGE ADDR(P)
+  RET ERR
+
+FOR i : 0 ~ NR_OPEN
+  IF 
+    f = P.filp[i]
+  THEN
+    INCR f.f_count 1
+
+IF 
+  CURRENT.PWD
+THEN 
+  INCR CURRENT.PWD.i_count 1
+
+IF 
+  CURRENT.ROOT
+THEN
+  INCR CURRENT.ROOT.i_count 1
+
+SET_TSS_DESC <- (GDT + nr/2 + FIRST_TSS_ENTRY) , P.TSS
+SET_LDT_DESC <- (GDT + nr/2 + FIRST_LDT_ENTRY) , P.LDT
+
+task(nr) <- p
+
+RET last_pid
+```
+
+
+
+"nr" passed is the index of the new task in the task array, where each element corresponds to a process slot.
+
+fork() copies the parent's kernel context, as well as general registers and segment selectors required for a new process to run in user space.
+
+It creates a new task structure from a new free page and sets up default values and certain parameters inherited from the parent, such as process ID, priority, and execution times.
+
+Then it copies memory regions from the parent to the new process, referred to by "nr," which is the index of the new process in the task array.
+
+Changes the parent process's working directory (PWD) and root directory reference counters, incrementing them by 1 to reflect the new child process now also using these resources.
+
+Sets the Task State Segment (TSS) and Local Descriptor Table (LDT) entries, which are x86-specific structures used for task switching and memory segmentation, respectively.
+
+TSS holds information about the task's stack for privilege level changes and also the hardware context when a task switch occurs.
+
+LDT is a segment descriptor table that stores descriptors for local segments, giving a task its own set of segment registers.
+
+Finally, it assigns the newly created task structure, p, to the task array at index "nr." This effectively makes the new task available for scheduling.
+
+
+
+"sys_fork" finds an empty process ID using the search loop in `_find_empty_process`, then it invokes `_copy_process` to clone the parent's kernel context to the new process. This sets up a complete environment for the new process to run independently from the parent, but initially as a nearly identical copy.