133 lines
3.5 KiB
Markdown
133 lines
3.5 KiB
Markdown
---
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title: Linux Kernel, syscalls
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date: 2023-11-11T01:22:30Z
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slug: linux-kernel-sys-fork
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description: ""
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---
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### sys_fork()
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```asm
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.align 2
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_sys_fork:
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call _find_empty_process
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testl %eax,%eax
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js 1f
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push %gs
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pushl %esi
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pushl %edi
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pushl %ebp
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pushl %eax
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call _copy_process
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addl $20,%esp
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1: ret
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```
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#### `FUNCTION find_empty_process()`
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```assembly
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A:
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IF ++last_pid < 0
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last_pid = 1
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FOR i : 0 -> NR_TASKS
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IF task[i] && task[i].pid == last_pid
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GOTO A
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FOR i : 1 -> NR_TASKS
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IF NOT task[i]
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RETURN i
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RETURN -1
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```
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1. Set last_pid to 1 or more.
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2. From taskid from 0 to max_tasks, if task exists and pid is last_pid then increases pid and again.
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3. From taskid from 1 to max_tasks, if tasks not exists, return.
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-> It just iterates through tasks and find last taskid linearly.
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#### `FUNCTION copy_process`
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```
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<- nr, EBP, EDI, ESI, GS, EBX~EDX, CS~FS, EIP, EFLAGS, ESP, SS
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-> INT
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```
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```assembly
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TASK_STRUCT P
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INT I
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FILE F
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P = (TASK_STRUCT)(GET_FREE_PAGE())
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IF NOT P
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RET ERR
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SET P
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START_TIME = RUNNING
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PID = LAST_PID
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FATHER = CURRENT_PID
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COUNTER = PRIORITY
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START_TIME = jiffies
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SIGNAL, ALARM, LEADER, UTIME, STIME, CUTIME, CSDTIME, BACK_LINK = 0
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SET TSS
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BACK_LINK = 0
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ESP0 = PAGE_SIZE + ADDR(P)
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SS0 = 0x10
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EAX = 0
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ES ~ GS = 0xFFFF
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LDT = _LDT(nr)
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EIP, EFLAGS = ARGUMENT
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ECX ~ EDI = ARGUMENT
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TRACR_BITMAP = 0x80000000
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COPY_MEM (nr, p)
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IF <-
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FREE_PAGE ADDR(P)
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RET ERR
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FOR i : 0 ~ NR_OPEN
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IF
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f = P.filp[i]
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THEN
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INCR f.f_count 1
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IF
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CURRENT.PWD
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THEN
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INCR CURRENT.PWD.i_count 1
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IF
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CURRENT.ROOT
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THEN
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INCR CURRENT.ROOT.i_count 1
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SET_TSS_DESC <- (GDT + nr/2 + FIRST_TSS_ENTRY) , P.TSS
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SET_LDT_DESC <- (GDT + nr/2 + FIRST_LDT_ENTRY) , P.LDT
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task(nr) <- p
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RET last_pid
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```
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- "nr" passed is the index of the new task in the task array, where each element corresponds to a process slot.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- TSS holds information about the task's stack for privilege level changes and also the hardware context when a task switch occurs.
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- LDT is a segment descriptor table that stores descriptors for local segments, giving a task its own set of segment registers.
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- 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.
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### `_sys_fork()`
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"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.
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