OpenBSD Kernel Internals — Creation of process from user-space to kernel space.

12 min readMay 29, 2018

Hello readers,

I know this time it is a little late, but I am also busy with some other professional things. :)

This time let’s discuss about the process creation in OpenBSD operating system from user-space level to kernel space.

We will take an example of the user-space process that will be launched from the Command Line Interface (console), for example, “ls”, and then what happens in kernel-space as a result of it.

I will divide this series into 3 parts, like creation, execution, exit, because the creation of process itself took some amount of time for me to learn, and analyzing or tracking from user-space to kernel-space had to be done line by line.

I have used gdb to debug the process and analyze it line by line.

Now, I will not waste your time too much.

Let’s dive into the user-space to kernel-space and learn and see the beauty of puffer.

I have divided the full process and functions that are used in the kernel into the points, so, I think it will be easy to read and learn.

Now, suppose you have launched “ls” command from CLI (xterm):

Here, the parent process is “ksh”, that is, default shell in OpenBSD which invokes “ls” command or any other command.

Every process is created by sys_fork() , that is, fork system call which is indirectly (internally) calls fork1()

fork1() creates a new process out of p1, which should be the current thread. This function is used primarily to implement the fork(2) and vfork(2) system calls, as well as the kthread_create(9) function.

Life cycle of a process (in brief):

“ls” → fork(2) → sys_fork() → fork1() → sys_execve() → sys_exit() → exit1()

Under the hood working of fork1()

After “ls” from user-space it goes to fork() (libc) then from there to sys_fork().

FORK_FORK: It is a macro which defines that the call is done by the fork(2) system call. Used only for statistics.

#define FORK_FORK 0x00000001

So, the value of flags variable is set to 1 , because the call is done by fork(2).
check for PTRACING then update the flags with PTRACE_FORK else leave it and return to the fork1()

Now, fork1()

The above code includes, curp->p_p->ps_comm is “ksh”, that is, parent process which will fork “ls” (user-space).
Initially some process structures, then, setting
uid = curp->p_ucred->cr_ruid , it means setting the uid as real user id.
Then, the structure for process address space information.
Then, some variables and ptrace_state structure and then the condition checking using KASSERT.
fork_check_maxthread(uid) → it is used to the check or track the number of threads invoked by the specific uid .
It checks the number of threads invoked by specific uid shouldn’t be greater than the number of maximum threads allowed or also for maxthread —5 . Because the last 5 process from the maxthread is reserved for the root.
If it is greater than defined maxthread or maxthread — 5, it will print the messagetablefullonce every 10 seconds. Else, it will increment the number of threads.

Now, after fork_check_thread, again, the same implementation happens for tracking process. If you want you can have a look in our fork1 code screen-shot.

Now, we will proceed further,

It is changing the count of threads for a specific user via chgproccnt(uid,1).

uidinfo structure maintains every uid resource consumption counts, including the process count and socket buffer space usage.
uid_find function looks up and returns the uidinfo structure for uid. If no uidinfo structure exists for uid, a new structure will be allocated and initialized.

Then, it increments the ui_proccnt , that is, number of processes by diff and then returns count.

After, that, it is checks for the non-privileged uid and also that the number of process is greater than the soft limit of resources, that is, 9223372036854775807, from what I have found in gdb.

Have a look in the below screen-shot for the proper view of values:

If non-privileged is allowed and the count is increased by the maximum resource limit, it will decrease the count via chgproccnt() by passing -1 as diff parameter and also decrease the number of processes and threads.

Next, the uvm_uarea_alloc() function allocates a thread's ‘uarea’, the memory where its kernel stack and PCB are stored.

Now, it checks if the uaddr variable doesn’t contain any thread’s address, if it is zero, then it decrements the count of the number of process and thread.

Now, there are the some important functions:

→ thread_new(struct proc *parent, vaddr_t uaddr)

→ process_new(struct proc *p, struct process *parent, int flags)

Here, in the thread_new function, we will get our user-space process, that is, in our case “ls”. The process gets retrieved from the pool of process, that is, proc_pool via pool_get() function.

Then, we set the state of the thread to be SIDL , which means that the process/thread is being created by fork . We then setp →p_flag = 0.

Now, they are zeroing the section of proc . See, the below code snippet from sys/proc.h

code snippet for members that will be zeroed upon creation in fork, via memset

In above code snippet, all the variables will be zeroed via memset upon creation in the fork.

Then, they are copying the section from parent→p_startcopy to
p→p_startcopyvia memcpy. Have a look below in the screen-shot to know which of the field members will be copied.

code snippet for the members those will be copied upon in fork

The, crhold(p->p_ucred) means it will increment the reference count in struct ucred structure, that is, p->p_ucred->cr_ref++ .
Now, typecast the thread’s addr, that is, (struct user *)uaddr and save it in kernel’s virtual addr of u-area.
Now, it will initialize the timeout.

dummy function to show the timeout_set function working.

timeout_set(timeout, b, argument)

It means initialize the timeout struture and call the function b with argument .

void
timeout_set(struct timeout *new, void (*fn)(void *), void *arg)
{
        new->to_func = fn;
        new->to_arg = arg;
        new->to_flags = TIMEOUT_INITIALIZED;
}

scheduler_fork_hook(parent, p): It is a macro which will update the p_estcpu of child from parent’s p_estcpu.

p_estcpu holds an estimate of the amount of CPU that the process has used recently

/* Inherit the parent’s scheduler history */
#define scheduler_fork_hook(parent, child) do {    \
 (child)->p_estcpu = (parent)->p_estcpu;           \
} while (0)

Then, return the newly created thread p .

Now, another important function is process_new() which will create the process in a similar fashion to what we have seen above in the thread_new func.

process_new(struct proc *p, struct process *parent, int flags)

In above code snippet, the same thing is happening again like select process from process_pool via pool_get then zeroing using memset and copying using memcpy.

So, for the detailed explanation, please go through the thread_new() function first.

Next is initialization of process using process_initialize function.

ps_mainproc : It is the original and main thread in the process. It’s only special for the handling of p_xstat and some signal and ptrace behaviours that need to be fixed.

→Copy initial thread, that is, p to pr->mainproc .

→Initialize the queue with referenced by head. Here, head is pr→ps_threads. Then, Insert elm at the TAIL of the queue. Here, elm is p .

→set the number of references to 1, that is, pr->ps_refcnt = 1

→copy the process pr to the process of initial thread.

→set the same creds for process as the initial thread.

→condition check for the new thread and the new process via KASSERT.

→Initialize the List referenced by head. Here, head is pr->ps_children

→Again, initialize timeout. (for detail, see thead_new)

Now, after the process initialization, pid allocation takes place.

ps→ps_pid = allocpid(); allocpid() returns unused pid

allocpid() internally calls the arc4random_uniform() which again calls the arc4random() then via arc4random() a fully randomized number is returned which is used as pid.

Then, for the availability of pid, or in other words, for unused pid, it verifies that whether the new pid is already taken or not by any process. It verifies this one by one in the process, process groups, and zombie process by using function ispidtaken(pid_t pid) which internally calls these functions:

prfind(pid_t pid) : Locate a process by number
pgfind(pid_t pgid) : Locate a process group by number
zombiefind(pid_t pid :Locate a zombie process by number

code snippet for **allocpid** and **ispidtaken**

Now, store the pointer to parent process in pr→ps_pptr .

Increment the number of references count in process limit structure, that is, struct plimit .

Store the vnode of executable of parent into pr→ps_textvp ,that is, pr→ps_textvp = parent→ps_textvp; .

if (pr→ps_textvp)
        vref(pr→ps_textvp); /* vref --> vnode reference */

Above code snippet means, if valid vnode found then increment the v_usecount++ variable inside the struct vnode structure of the executable.

Now, the calculation for setting up process flags:

pr→ps_flags = parent →ps_flags & (PS_SUGID | PS_SUGIDEXEC | PS_PLEDGE | PS_EXECPLEDGE | PS_WXNEEDED);pr →ps_flags = parent →ps_flags & (0x10 | 0x20 | 0x100000 | 0x400000 | 0x200000)if (vnode of controlling terminal != NULL)
        pr→ps_flags |= parent→ps_flags & PS_CONTROLT;

process_new continued…

Checks:

* if child_able_to_share_file_descriptor_table_with_parent:
         pr->ps_fd = fdshare(parent)      /* share the table */
  else
         pr->ps_fd = fdcopy(parent)       /* copy the table */* if child_able_to_share_the_parent's_signal_actions:
         pr->ps_sigacts = sigactsshare(parent) /* share */
  else
         pr->ps_sigacts = sigactsinit(parent)  /* copy */* if child_able_to_share_the_parent's addr space:
         pr->ps_vmspace = uvmspace_share(parent)
  else
         pr->ps_vmspace = uvmspace_fork(parent)* if process_able_to_start_profiling:
         smartprofclock(pr);    /* start profiling on a process */* if check_child_able_to_start_ptracing:
         pr->ps_flags |= parent->ps_flags & PS_PTRACED* if check_no_signal_or_zombie_at_exit:
         pr->ps_flags |= PS_NOZOMBIE /*No signal or zombie at exit* if check_signals_stat_swaping:
         pr->ps_flags |= PS_SYSTEM

update the pr→ps_flags with PS_EMBRYO by ORing it, that is,
pr→ps_flags |= PS_EMBRYO /* New process, not yet fledged */

membar_producer() → Force visibility of all of the above changes.

— All stores preceding the memory barrier will reach global visibility before any stores after the memory barrier reach global visibility.

In short, I think it is used to forcefully make visible changes globally.

Now, Insert the new elm, that is, pr at the head of the list. Here, head is allprocess .

return pr

fork1() continued…

Substructures
p→p_fd and p→p_vmspace directly copy of pr→ps_fd and pr→ps_vmspace.

checks,

** if (process_has_no_signals_stats_or_swapping) then atomically set bits.

atomic_setbits_int(pr →ps_flags, PS_SYSTEM);

** if (child_is_suspending_the_parent_process_until_the_child_is terminated (by calling _exit(2) or abnormally), or makes a call to execve(2)) then atomically set bits,

atomic_setbits_int(pr →ps_flags, PS_PPWAIT);
atomic_setbits_int(pr →ps_flags, PS_ISPWAIT);

#ifdef KTRACE
/* Some KTRACE related things */
#endif

cpu_fork(curp, p, NULL, NULL, func, arg ?arg: p)

— To create or Update PCB and make child ready to RUN.

/*
 * Finish creating the child thread. cpu_fork() will copy
 * and update the pcb and make the child ready to run. The
 * child will exit directly to user mode via child_return()
 * on its first time slice and will not return here.
 */

Address space,
vm = pr→ps_vmspace

if (call is done by fork syscall); then
increment the number of fork() system calls.
update the vm_pages affected by fork() syscall with addition of data page and stack page. else if (call is done by vfork() syscall); then
do as same as if it was fork syscall but for vfork system call. (see above if {for fork})else
increment the number of kernel threads created.

Check,

If (process is being traced && created by fork system call);then
{
        The malloc() function allocates the uninitialized memory in the kernel address space for an object whose size is specified by size, that is, here, sizeof(*newptstat). And, struct ptrace_state *newptstat}

allocate thread ID, that is, p→p_tid = alloctid();
This is also the same calling arc4random directly and using tfind function for finding the thread ID by number.

* inserts the new element p at the head	of the allprocess list.
* insert the new element p at the head of the thread hash list.
* insert the new element pr at the head of the process hash list.
* insert the new element pr after the curpr element.
* insert the new element pr at the head of the children process  list.

fork1() continued…

Again,
If (isProcessPTRACED())
{
then save the parent process id during ptracing, that is,
pr→ps_oppid = curpr→ps_pid .
If (pointer to parent process_of_child != pointer to parent process_of_current_process)
{
proc_reparent(pr, curpr→ps_pptr); /* Make current process the new parent of process child, that is, pr*/

Now, check whether newptstat contains some address, in our case, newptstat contains a kernel virtual address returned by malloc(9.
If above condition is True, that is, newptstat != NULL . Then, set the ptrace status:
Set newptstat point to the ptrace state structure. Then, make the newptstat point to NULL .

→Update the ptrace status to the curpr process and also the pr process.

curpr->ps_ptstat->pe_report_event = PTRACE_FORK;
pr->ps_ptstat->pe_report_event = PTRACE_FORK;
curpr->ps_ptstat->pe_other_pid = pr->ps_pid;
pr->ps_ptstat->pe_other_pid = curpr->ps_pid;

Now, for the new process set accounting bits and mark it as complete.

get the nano time to start the process.
Set accounting flags to AFORK which means forked but not execed.
atomically clear the bits.
Then, check for the new child is in the IDLE state or not, if yes then make it runnable and add it to the run queue by fork_thread_start function.
If it is not in the IDLE state then put arg to the current CPU, running on.

Freeing the memory or kernel virtual address that is allocated by malloc for newptstat via free .

Notify any interested parties about the new process via KNOTE .

Now, update the stats counter for successfully forked.

uvmexp.forks++; /* -->For forks */if (flags & FORK_PPWAIT)
        uvmexp.forks_ppwait++; /* --> counter for forks where parent waits */if (flags & FORK_SHAREVM)
        uvmexp.forks_sharevm++; /* --> counter for forks where vmspace is shared */

Now, pass pointer to the new process to the caller.

if (rnewprocp != NULL)
        *rnewprocp = p;

setting the PPWAIT on child and the PS_ISPWAIT on ourselves, that is, the parent and then go to the sleep on our process via tsleep .
Check, If the child is started with tracing enables && the current process is being traced then alert the parent by using SIGTRAP signal.
Now, return the child pid to the parent process.
return (0)

Then, finally, I have seen in the debugger that after the fork1, it jumps to sys/arch/amd64/amd64/trap.c file for system call handling and for the setting frame.

Some of the machine independent (MI) functions defined in sys/sys/syscall_mi.h file, like, mi_syscall(), mi_syscall_return() and mi_child_return().

Then, after handling the system calls from trap.c then, control pass to the sys_execve system call, which I will explain later (in the second part) and also I will explain more about the trap.c code in upcoming posts. It has already become a long post.

References:

I have tried to cover most of what I have learned. In case I have forgotten or missed something, please feel free to update me.
If I have explained something wrongly or incorrectly then please feel free to update or correct me, I am also a learner and beginner so that may happen. :)

Happy Kernel Hacking :D

OpenBSD Kernel Internals — Creation of process from user-space to kernel space.

Written by Neeraj Pal