3.5 KiB
3.5 KiB
title | date | slug | description |
---|---|---|---|
Linux Kernel, syscalls | 2023-11-11T01:22:30Z | linux-kernel-sys-fork |
sys_fork()
.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()
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
-
Set last_pid to 1 or more.
-
From taskid from 0 to max_tasks, if task exists and pid is last_pid then increases pid and again.
-
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
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()
"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.