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* For review: documentation of clone3() system call
@ 2019-10-25 16:59 Michael Kerrisk (man-pages)
  2019-10-25 17:07 ` Christian Brauner
                   ` (4 more replies)
  0 siblings, 5 replies; 30+ messages in thread
From: Michael Kerrisk (man-pages) @ 2019-10-25 16:59 UTC (permalink / raw)
  To: Christian Brauner
  Cc: lkml, linux-man, Kees Cook, Florian Weimer, Oleg Nesterov,
	Arnd Bergmann, David Howells, Pavel Emelyanov, Andrew Morton,
	Adrian Reber, Andrei Vagin, Linux API, Jann Horn

Hello Christian and all,

I've made a first shot at adding documentation for clone3(). You can
see the diff here:
https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/commit/?id=faa0e55ae9e490d71c826546bbdef954a1800969

In the end, I decided that the most straightforward approach was to
add the documentation as part of the existing clone(2) page. This has
the advantage of avoiding duplication of information across two pages,
and perhaps also makes it easier to see the commonality of the two
APIs.

Because the new text is integrated into the existing page, I think it
makes most sense to just show that page text for review purposes. I
welcome input on the below.

The notable changes are:
* In the first part of the page, up to and including the paragraph
with the subheading "The flags bit mask"
* Minor changes in the description of CLONE_CHILD_CLEARTID,
CLONE_CHILD_SETTID, CLONE_PARENT_SETTID, and CLONE_PIDFD, to reflect
the argument differences between clone() and clone2()

Most of the resy of page is unchanged.

I welcome fixes, suggestions for improvements, etc.

Thanks,

Michael

CLONE(2)                Linux Programmer's Manual                CLONE(2)

NAME
       clone, __clone2 - create a child process

SYNOPSIS
       /* Prototype for the glibc wrapper function */

       #define _GNU_SOURCE
       #include <sched.h>

       int clone(int (*fn)(void *), void *stack, int flags, void *arg, ...
                 /* pid_t *parent_tid, void *tls, pid_t *child_tid */ );

       /* For the prototype of the raw clone() system call, see NOTES */

       long clone3(struct  clone_args *cl_args, size_t size);

       Note: There is not yet a glibc wrapper for clone3(); see NOTES.

DESCRIPTION
       These  system  calls  create a new process, in a manner similar to
       fork(2).

       Unlike fork(2), these system calls  allow  the  child  process  to
       share  parts  of  its  execution context with the calling process,
       such as the virtual address space, the table of file  descriptors,
       and the table of signal handlers.  (Note that on this manual page,
       "calling process" normally corresponds to "parent  process".   But
       see the description of CLONE_PARENT below.)

       This page describes the following interfaces:

       *  The  glibc  clone()  wrapper function and the underlying system
          call on which it is based.  The main text describes the wrapper
          function; the differences for the raw system call are described
          toward the end of this page.

       *  The newer clone3() system call.

   The clone() wrapper function
       When the child process is created with the clone()  wrapper  func‐
       tion, it commences execution by calling the function pointed to by
       the argument fn.  (This differs from fork(2), where execution con‐
       tinues  in the child from the point of the fork(2) call.)  The arg
       argument is passed as the argument of the function fn.

       When the fn(arg) function returns, the child  process  terminates.
       The  integer  returned  by  fn  is  the  exit status for the child
       process.  The child process may also terminate explicitly by call‐
       ing exit(2) or after receiving a fatal signal.

       The stack argument specifies the location of the stack used by the
       child process.  Since the child and calling process may share mem‐
       ory,  it  is  not possible for the child process to execute in the
       same stack as the  calling  process.   The  calling  process  must
       therefore  set  up  memory  space  for  the child stack and pass a
       pointer to this space to clone().  Stacks  grow  downward  on  all
       processors  that run Linux (except the HP PA processors), so stack
       usually points to the topmost address of the memory space  set  up
       for  the  child stack.  Note that clone() does not provide a means
       whereby the caller can inform the kernel of the size of the  stack
       area.

       The remaining arguments to clone() are discussed below.

   clone3()
       The  clone3() system call provides a superset of the functionality
       of the older clone() interface.  It also provides a number of  API
       improvements,  including: space for additional flags bits; cleaner
       separation in the use of various arguments;  and  the  ability  to
       specify the size of the child's stack area.

       As  with  fork(2),  clone3()  returns  in  both the parent and the
       child.  It returns 0 in the child process and returns the  PID  of
       the child in the parent.

       The  cl_args  argument of clone3() is a structure of the following
       form:

           struct clone_args {
               u64 flags;        /* Flags bit mask */
               u64 pidfd;        /* Where to store PID file descriptor
                                    (int *) */
               u64 child_tid;    /* Where to store child TID,
                                    in child's memory (int *) */
               u64 parent_tid;   /* Where to store child TID,
                                    in parent's memory (int *) */
               u64 exit_signal;  /* Signal to deliver to parent on
                                    child termination */
               u64 stack;        /* Pointer to lowest byte of stack */
               u64 stack_size;   /* Size of stack */
               u64 tls;          /* Location of new TLS */
           };

       The size argument that is supplied to clone3() should be  initial‐
       ized  to  the  size of this structure.  (The existence of the size
       argument permits future extensions to the clone_args structure.)

       The stack for the child process is  specified  via  cl_args.stack,
       which   points   to  the  lowest  byte  of  the  stack  area,  and
       cl_args.stack_size, which specifies  the  size  of  the  stack  in
       bytes.   In the case where the CLONE_VM flag (see below) is speci‐
       fied, a stack must be explicitly allocated and specified.   Other‐
       wise,  these  two  fields  can  be  specified as NULL and 0, which
       causes the child to use the same stack area as the parent (in  the
       child's own virtual address space).

       The remaining fields in the cl_args argument are discussed below.

   Equivalence between clone() and clone3() arguments
       Unlike  the  older  clone()  interface, where arguments are passed
       individually, in the newer clone3() interface  the  arguments  are
       packaged  into  the clone_args structure shown above.  This struc‐
       ture allows for a superset  of  the  information  passed  via  the
       clone() arguments.

       The following table shows the equivalence between the arguments of
       clone() and the fields in  the  clone_args  argument  supplied  to
       clone3():

              clone()         clone(3)        Notes
                              cl_args field
              flags & ~0xff   flags
              parent_tid      pidfd           See CLONE_PIDFD
              child_tid       child_tid       See CLONE_CHILD_SETTID
              parent_tid      parent_tid      See CLONE_PARENT_SETTID
              flags & 0xff    exit_signal
              stack           stack

              ---             stack_size
              tls             tls             See CLONE_SETTLS

   The child termination signal
       When  the  child  process  terminates, a signal may be sent to the
       parent.  The termination signal is specified in the  low  byte  of
       flags  (clone())  or  in  cl_args.exit_signal (clone3()).  If this
       signal is specified as anything other than SIGCHLD, then the  par‐
       ent process must specify the __WALL or __WCLONE options when wait‐
       ing for the child with wait(2).  If  no  signal  (i.e.,  zero)  is
       specified,  then the parent process is not signaled when the child
       terminates.

   The flags bit mask
       Both clone() and clone3() allow a flags  bit  mask  that  modifies
       their  behavior  and  allows  the caller to specify what is shared
       between the calling process and the child process.  This bit  mask
       is  specified  as  a  bitwise-OR  of zero or more of the constants
       listed below.  Except as otherwise noted below,  these  flags  are
       available (and have the same effect) in both clone() and clone3().

       CLONE_CHILD_CLEARTID (since Linux 2.5.49)
              Clear (zero) the child thread ID at the location pointed to
              by child_tid (clone()) or cl_args.child_tid  (clone3())  in
              child  memory  when the child exits, and do a wakeup on the
              futex at that address.  The address involved may be changed
              by  the  set_tid_address(2)  system  call.  This is used by
              threading libraries.

       CLONE_CHILD_SETTID (since Linux 2.5.49)
              Store the child thread ID at the  location  pointed  to  by
              child_tid  (clone()) or cl_args.child_tid (clone3()) in the
              child's  memory.   The  store  operation  completes  before
              clone() returns control to user space in the child process.
              (Note that the  store  operation  may  not  have  completed
              before clone() returns in the parent process, which will be
              relevant if the CLONE_VM flag is also employed.)

       CLONE_FILES (since Linux 2.0)
              If CLONE_FILES is set, the calling process  and  the  child
              process  share  the  same  file descriptor table.  Any file
              descriptor created by the calling process or by  the  child
              process  is also valid in the other process.  Similarly, if
              one of the processes closes a file descriptor,  or  changes
              its  associated  flags  (using  the fcntl(2) F_SETFD opera‐
              tion), the other process is also affected.   If  a  process
              sharing  a  file descriptor table calls execve(2), its file
              descriptor table is duplicated (unshared).

              If CLONE_FILES is not set, the  child  process  inherits  a
              copy  of all file descriptors opened in the calling process
              at the time of clone().  Subsequent operations that open or
              close  file  descriptors,  or change file descriptor flags,
              performed by  either  the  calling  process  or  the  child
              process  do  not  affect the other process.  Note, however,
              that the duplicated file descriptors in the child refer  to
              the  same  open file descriptions as the corresponding file
              descriptors in the calling process,  and  thus  share  file
              offsets and file status flags (see open(2)).

       CLONE_FS (since Linux 2.0)
              If  CLONE_FS is set, the caller and the child process share
              the same filesystem information.  This includes the root of
              the  filesystem,  the  current  working  directory, and the
              umask.  Any call to chroot(2), chdir(2), or  umask(2)  per‐
              formed  by  the  calling  process or the child process also
              affects the other process.

              If CLONE_FS is not set, the child process works on  a  copy
              of the filesystem information of the calling process at the
              time of the clone() call.  Calls to chroot(2), chdir(2), or
              umask(2)  performed  later  by  one of the processes do not
              affect the other process.

       CLONE_IO (since Linux 2.6.25)
              If CLONE_IO is set, then the new process shares an I/O con‐
              text  with  the  calling process.  If this flag is not set,
              then (as with fork(2)) the new process has its own I/O con‐
              text.

              The  I/O  context  is  the  I/O scope of the disk scheduler
              (i.e., what the I/O scheduler uses to model scheduling of a
              process's  I/O).   If processes share the same I/O context,
              they are treated as one by the I/O scheduler.  As a  conse‐
              quence,  they  get to share disk time.  For some I/O sched‐
              ulers, if two processes share an I/O context, they will  be
              allowed  to  interleave  their  disk  access.   If  several
              threads are  doing  I/O  on  behalf  of  the  same  process
              (aio_read(3), for instance), they should employ CLONE_IO to
              get better I/O performance.

              If the kernel  is  not  configured  with  the  CONFIG_BLOCK
              option, this flag is a no-op.

       CLONE_NEWCGROUP (since Linux 4.6)
              Create the process in a new cgroup namespace.  If this flag
              is not set, then (as with fork(2)) the process  is  created
              in the same cgroup namespaces as the calling process.  This
              flag is intended for the implementation of containers.

              For  further  information   on   cgroup   namespaces,   see
              cgroup_namespaces(7).

              Only   a  privileged  process  (CAP_SYS_ADMIN)  can  employ
              CLONE_NEWCGROUP.

       CLONE_NEWIPC (since Linux 2.6.19)
              If CLONE_NEWIPC is set, then create the process  in  a  new
              IPC  namespace.   If  this  flag  is not set, then (as with
              fork(2)), the process is created in the same IPC  namespace
              as  the  calling  process.   This  flag is intended for the
              implementation of containers.

              An IPC namespace provides an isolated view of System V  IPC
              objects  (see  sysvipc(7))  and  (since Linux 2.6.30) POSIX
              message queues (see mq_overview(7)).  The common character‐
              istic of these IPC mechanisms is that IPC objects are iden‐
              tified by mechanisms other than filesystem pathnames.

              Objects created in an IPC  namespace  are  visible  to  all
              other processes that are members of that namespace, but are
              not visible to processes in other IPC namespaces.

              When an IPC namespace is destroyed  (i.e.,  when  the  last
              process  that is a member of the namespace terminates), all
              IPC objects in the namespace are automatically destroyed.

              Only  a  privileged  process  (CAP_SYS_ADMIN)  can   employ
              CLONE_NEWIPC.   This flag can't be specified in conjunction
              with CLONE_SYSVSEM.

              For further  information  on  IPC  namespaces,  see  names‐
              paces(7).

       CLONE_NEWNET (since Linux 2.6.24)
              (The  implementation  of  this  flag  was completed only by
              about kernel version 2.6.29.)

              If CLONE_NEWNET is set, then create the process  in  a  new
              network  namespace.  If this flag is not set, then (as with
              fork(2)) the process is created in the same network  names‐
              pace as the calling process.  This flag is intended for the
              implementation of containers.

              A network namespace provides an isolated view of  the  net‐
              working  stack  (network  device  interfaces, IPv4 and IPv6
              protocol stacks, IP routing  tables,  firewall  rules,  the
              /proc/net  and  /sys/class/net  directory  trees,  sockets,
              etc.).  A physical network device can live in  exactly  one
              network namespace.  A virtual network (veth(4)) device pair
              provides a pipe-like abstraction that can be used to create
              tunnels between network namespaces, and can be used to cre‐
              ate a bridge to a physical network device in another names‐
              pace.

              When  a  network  namespace  is  freed (i.e., when the last
              process in the namespace terminates), its physical  network
              devices  are  moved  back  to the initial network namespace
              (not to the parent of the process).  For  further  informa‐
              tion on network namespaces, see namespaces(7).

              Only   a  privileged  process  (CAP_SYS_ADMIN)  can  employ
              CLONE_NEWNET.

       CLONE_NEWNS (since Linux 2.4.19)
              If CLONE_NEWNS is set, the cloned child is started in a new
              mount  namespace,  initialized with a copy of the namespace
              of the parent.  If CLONE_NEWNS is not set, the child  lives
              in the same mount namespace as the parent.

              Only   a  privileged  process  (CAP_SYS_ADMIN)  can  employ
              CLONE_NEWNS.   It  is  not  permitted   to   specify   both
              CLONE_NEWNS and CLONE_FS in the same clone() call.

              For  further  information  on  mount namespaces, see names‐
              paces(7) and mount_namespaces(7).

       CLONE_NEWPID (since Linux 2.6.24)
              If CLONE_NEWPID is set, then create the process  in  a  new
              PID  namespace.   If  this  flag  is not set, then (as with
              fork(2)) the process is created in the same  PID  namespace
              as  the  calling  process.   This  flag is intended for the
              implementation of containers.

              For further  information  on  PID  namespaces,  see  names‐
              paces(7) and pid_namespaces(7).

              Only   a  privileged  process  (CAP_SYS_ADMIN)  can  employ
              CLONE_NEWPID.  This flag can't be specified in  conjunction
              with CLONE_THREAD or CLONE_PARENT.

       CLONE_NEWUSER
              (This  flag  first  became  meaningful for clone() in Linux
              2.6.23, the current clone() semantics were merged in  Linux
              3.5,  and the final pieces to make the user namespaces com‐
              pletely usable were merged in Linux 3.8.)

              If CLONE_NEWUSER is set, then create the process in  a  new
              user  namespace.   If  this  flag is not set, then (as with
              fork(2)) the process is created in the same user  namespace
              as the calling process.

              Before  Linux  3.8,  use of CLONE_NEWUSER required that the
              caller have three capabilities: CAP_SYS_ADMIN,  CAP_SETUID,
              and CAP_SETGID.  Starting with Linux 3.8, no privileges are
              needed to create a user namespace.

              This  flag  can't  be   specified   in   conjunction   with
              CLONE_THREAD   or   CLONE_PARENT.   For  security  reasons,
              CLONE_NEWUSER  cannot  be  specified  in  conjunction  with
              CLONE_FS.

              For  further  information  on  user  namespaces, see names‐
              paces(7) and user_namespaces(7).

       CLONE_NEWUTS (since Linux 2.6.19)
              If CLONE_NEWUTS is set, then create the process  in  a  new
              UTS  namespace, whose identifiers are initialized by dupli‐
              cating the identifiers from the UTS namespace of the  call‐
              ing  process.   If  this  flag  is  not  set, then (as with
              fork(2)) the process is created in the same  UTS  namespace
              as  the  calling  process.   This  flag is intended for the
              implementation of containers.

              A UTS namespace is  the  set  of  identifiers  returned  by
              uname(2); among these, the domain name and the hostname can
              be modified by setdomainname(2) and sethostname(2), respec‐
              tively.  Changes made to the identifiers in a UTS namespace
              are visible to all other processes in the  same  namespace,
              but are not visible to processes in other UTS namespaces.

              Only   a  privileged  process  (CAP_SYS_ADMIN)  can  employ
              CLONE_NEWUTS.

              For further  information  on  UTS  namespaces,  see  names‐
              paces(7).

       CLONE_PARENT (since Linux 2.3.12)
              If  CLONE_PARENT  is  set, then the parent of the new child
              (as returned by getppid(2)) will be the same as that of the
              calling process.

              If  CLONE_PARENT  is  not  set,  then (as with fork(2)) the
              child's parent is the calling process.

              Note that it is the parent process, as  returned  by  getp‐
              pid(2),  which  is  signaled  when the child terminates, so
              that if CLONE_PARENT is set, then the parent of the calling
              process,  rather  than  the calling process itself, will be
              signaled.

       CLONE_PARENT_SETTID (since Linux 2.5.49)
              Store the child thread ID at the  location  pointed  to  by
              parent_tid (clone()) or cl_args.child_tid (clone3()) in the
              parent's memory.  (In Linux 2.5.32-2.5.48 there was a  flag
              CLONE_SETTID that did this.)  The store operation completes
              before clone() returns control to user space.

       CLONE_PID (Linux 2.0 to 2.5.15)
              If CLONE_PID is set, the child process is created with  the
              same  process  ID as the calling process.  This is good for
              hacking the system, but otherwise of not  much  use.   From
              Linux  2.3.21  onward, this flag could be specified only by
              the system boot process (PID 0).  The flag disappeared com‐
              pletely  from  the  kernel  sources in Linux 2.5.16.  Since
              then, the kernel silently ignores this bit if it is  speci‐
              fied in flags.

       CLONE_PIDFD (since Linux 5.2)
              If  this flag is specified, a PID file descriptor referring
              to the child process is allocated and placed at a specified
              location in the parent's memory.  The close-on-exec flag is
              set on this new file descriptor.  PID file descriptors  can
              be used for the purposes described in pidfd_open(2).

              *  When  using  clone3(), the PID file descriptor is placed
                 at the location pointed to by cl_args.pidfd.

              *  When using clone(), the PID file descriptor is placed at
                 the  location  pointed to by parent_tid.  Since the par‐
                 ent_tid argument is used to return the PID file descrip‐
                 tor, CLONE_PIDFD cannot be used with CLONE_PARENT_SETTID
                 when calling clone().

              It is currently not possible to use this flag together with
              CLONE_THREAD.   This  means  that the process identified by
              the PID file  descriptor  will  always  be  a  thread-group
              leader.

              For  a while there was a CLONE_DETACHED flag.  This flag is
              usually ignored when passed along with other  flags.   How‐
              ever,  when  passed  alongside  CLONE_PIDFD,  an  error  is
              returned.  This ensures that this flag can  be  reused  for
              further PID file descriptor features in the future.

       CLONE_PTRACE (since Linux 2.2)
              If  CLONE_PTRACE  is  specified, and the calling process is
              being traced, then trace the child also (see ptrace(2)).

       CLONE_SETTLS (since Linux 2.5.32)
              The TLS (Thread Local Storage) descriptor is set to tls.

              The interpretation of  tls  and  the  resulting  effect  is
              architecture  dependent.   On  x86, tls is interpreted as a
              struct user_desc * (see set_thread_area(2)).  On x86-64  it
              is  the  new value to be set for the %fs base register (see
              the ARCH_SET_FS argument to arch_prctl(2)).   On  architec‐
              tures with a dedicated TLS register, it is the new value of
              that register.

       CLONE_SIGHAND (since Linux 2.0)
              If CLONE_SIGHAND is set, the calling process and the  child
              process  share  the  same table of signal handlers.  If the
              calling process or  child  process  calls  sigaction(2)  to
              change  the behavior associated with a signal, the behavior
              is changed in the other  process  as  well.   However,  the
              calling  process  and  child  processes still have distinct
              signal masks and sets of pending signals.  So, one of  them
              may  block  or unblock signals using sigprocmask(2) without
              affecting the other process.

              If CLONE_SIGHAND is not set, the child process  inherits  a
              copy  of  the signal handlers of the calling process at the
              time clone() is called.  Calls  to  sigaction(2)  performed
              later  by  one of the processes have no effect on the other
              process.

              Since Linux 2.6.0, flags  must  also  include  CLONE_VM  if
              CLONE_SIGHAND is specified

       CLONE_STOPPED (since Linux 2.6.0)
              If  CLONE_STOPPED  is  set,  then  the  child  is initially
              stopped (as though it was sent a SIGSTOP signal), and  must
              be resumed by sending it a SIGCONT signal.

              This  flag was deprecated from Linux 2.6.25 onward, and was
              removed altogether in Linux 2.6.38.  Since then, the kernel
              silently  ignores  it  without  error.  Starting with Linux
              4.6, the same bit was reused for the CLONE_NEWCGROUP flag.

       CLONE_SYSVSEM (since Linux 2.5.10)
              If CLONE_SYSVSEM is set, then the  child  and  the  calling
              process  share  a single list of System V semaphore adjust‐
              ment (semadj) values (see semop(2)).   In  this  case,  the
              shared  list accumulates semadj values across all processes
              sharing the list, and semaphore adjustments  are  performed
              only  when the last process that is sharing the list termi‐
              nates (or ceases sharing the list  using  unshare(2)).   If
              this  flag is not set, then the child has a separate semadj
              list that is initially empty.

       CLONE_THREAD (since Linux 2.4.0)
              If CLONE_THREAD is set, the child is  placed  in  the  same
              thread group as the calling process.  To make the remainder
              of the discussion of CLONE_THREAD more readable,  the  term
              "thread"  is used to refer to the processes within a thread
              group.

              Thread groups were a feature added in Linux 2.4 to  support
              the  POSIX  threads notion of a set of threads that share a
              single PID.  Internally, this shared PID is  the  so-called
              thread group identifier (TGID) for the thread group.  Since
              Linux 2.4, calls to getpid(2) return the TGID of the  call‐
              er.

              The  threads  within  a group can be distinguished by their
              (system-wide) unique thread IDs (TID).  A new thread's  TID
              is  available as the function result returned to the caller
              of clone(), and a thread can obtain its own TID using  get‐
              tid(2).

              When   a   call  is  made  to  clone()  without  specifying
              CLONE_THREAD, then the resulting thread is placed in a  new
              thread  group  whose  TGID is the same as the thread's TID.
              This thread is the leader of the new thread group.

              A new thread created with CLONE_THREAD has the same  parent
              process as the caller of clone() (i.e., like CLONE_PARENT),
              so that calls to getppid(2) return the same value  for  all
              of  the  threads  in  a  thread group.  When a CLONE_THREAD
              thread terminates, the thread that created it using clone()
              is  not  sent  a SIGCHLD (or other termination) signal; nor
              can the status of such a thread be obtained using  wait(2).
              (The thread is said to be detached.)

              After  all  of  the threads in a thread group terminate the
              parent process of the thread group is sent  a  SIGCHLD  (or
              other termination) signal.

              If  any  of  the  threads  in  a  thread  group performs an
              execve(2), then all threads other  than  the  thread  group
              leader  are  terminated, and the new program is executed in
              the thread group leader.

              If one of the threads in a thread  group  creates  a  child
              using fork(2), then any thread in the group can wait(2) for
              that child.

              Since Linux 2.5.35, flags must also  include  CLONE_SIGHAND
              if  CLONE_THREAD  is  specified (and note that, since Linux
              2.6.0,  CLONE_SIGHAND  also   requires   CLONE_VM   to   be
              included).

              Signal  dispositions  and  actions  are process-wide: if an
              unhandled signal is delivered to a  thread,  then  it  will
              affect  (terminate, stop, continue, be ignored in) all mem‐
              bers of the thread group.

              Each thread has its own signal mask,  as  set  by  sigproc‐
              mask(2).

              A  signal  may  be  process-directed or thread-directed.  A
              process-directed signal  is  targeted  at  a  thread  group
              (i.e., a TGID), and is delivered to an arbitrarily selected
              thread from among those that are not blocking  the  signal.
              A  signal  may be process-directed because it was generated
              by the kernel for reasons other than a hardware  exception,
              or  because  it  was  sent using kill(2) or sigqueue(3).  A
              thread-directed signal is targeted at (i.e., delivered  to)
              a specific thread.  A signal may be thread directed because
              it was sent  using  tgkill(2)  or  pthread_sigqueue(3),  or
              because  the thread executed a machine language instruction
              that triggered a hardware exception (e.g.,  invalid  memory
              access  triggering  SIGSEGV  or  a floating-point exception
              triggering SIGFPE).

              A call to sigpending(2) returns a signal set  that  is  the
              union  of the pending process-directed signals and the sig‐
              nals that are pending for the calling thread.

              If a process-directed  signal  is  delivered  to  a  thread
              group, and the thread group has installed a handler for the
              signal, then the handler will be invoked  in  exactly  one,
              arbitrarily  selected  member  of the thread group that has
              not blocked the signal.  If multiple threads in a group are
              waiting to accept the same signal using sigwaitinfo(2), the
              kernel will arbitrarily select  one  of  these  threads  to
              receive the signal.

       CLONE_UNTRACED (since Linux 2.5.46)
              If CLONE_UNTRACED is specified, then a tracing process can‐
              not force CLONE_PTRACE on this child process.

       CLONE_VFORK (since Linux 2.2)
              If CLONE_VFORK is set, the execution of the calling process
              is  suspended  until  the child releases its virtual memory
              resources via a call to  execve(2)  or  _exit(2)  (as  with
              vfork(2)).

              If  CLONE_VFORK  is  not set, then both the calling process
              and the child are schedulable after the call, and an appli‐
              cation  should  not rely on execution occurring in any par‐
              ticular order.

       CLONE_VM (since Linux 2.0)
              If CLONE_VM is set,  the  calling  process  and  the  child
              process  run in the same memory space.  In particular, mem‐
              ory writes performed by the calling process or by the child
              process  are  also visible in the other process.  Moreover,
              any memory mapping or unmapping performed with  mmap(2)  or
              munmap(2)  by the child or calling process also affects the
              other process.

              If CLONE_VM is not set, the child process runs in  a  sepa‐
              rate copy of the memory space of the calling process at the
              time of clone().  Memory writes or file mappings/unmappings
              performed  by one of the processes do not affect the other,
              as with fork(2).

NOTES
       One use of these systems calls is to implement  threads:  multiple
       flows  of  control  in a program that run concurrently in a shared
       address space.

       Glibc does not provide a  wrapper  for  clone(3);  call  it  using
       syscall(2).

       Note that the glibc clone() wrapper function makes some changes in
       the memory pointed to by stack (changes required to set the  stack
       up  correctly  for  the  child) before invoking the clone() system
       call.  So, in cases where clone() is used  to  recursively  create
       children, do not use the buffer employed for the parent's stack as
       the stack of the child.

   C library/kernel differences
       The raw clone() system call corresponds more closely to fork(2) in
       that  execution in the child continues from the point of the call.
       As such, the fn and arg arguments of the clone() wrapper  function
       are omitted.

       Another  difference  for  the  raw clone() system call is that the
       stack argument may be NULL, in which case the child uses a  dupli‐
       cate  of the parent's stack.  (Copy-on-write semantics ensure that
       the child gets separate copies of stack pages when either  process
       modifies  the  stack.)   In  this case, for correct operation, the
       CLONE_VM option should not be specified.  (If the child shares the
       parent's  memory  because of the use of the CLONE_VM flag, then no
       copy-on-write duplication occurs and chaos is likely to result.)

       The order of the arguments also differs in the  raw  system  call,
       and there are variations in the arguments across architectures, as
       detailed in the following paragraphs.

       The raw system call interface on x86-64 and some  other  architec‐
       tures (including sh, tile, ia-64, and alpha) is:

           long clone(unsigned long flags, void *stack,
                      int *parent_tid, int *child_tid,
                      unsigned long tls);

       On  x86-32,  and  several  other  common  architectures (including
       score, ARM, ARM 64, PA-RISC, arc, Power PC, xtensa, and MIPS), the
       order of the last two arguments is reversed:

           long clone(unsigned long flags, void *stack,
                     int *parent_tid, unsigned long tls,
                     int *child_tid);

       On  the  cris  and  s390 architectures, the order of the first two
       arguments is reversed:

           long clone(void *stack, unsigned long flags,
                      int *parent_tid, int *child_tid,
                      unsigned long tls);

       On the microblaze architecture, an  additional  argument  is  sup‐
       plied:

           long clone(unsigned long flags, void *stack,
                      int stack_size,         /* Size of stack */
                      int *parent_tid, int *child_tid,
                      unsigned long tls);

   blackfin, m68k, and sparc
       The  argument-passing conventions on blackfin, m68k, and sparc are
       different from the descriptions above.  For details, see the  ker‐
       nel (and glibc) source.

   ia64
       On ia64, a different interface is used:

           int __clone2(int (*fn)(void *),
                        void *stack_base, size_t stack_size,
                        int flags, void *arg, ...
                     /* pid_t *parent_tid, struct user_desc *tls,
                        pid_t *child_tid */ );

       The  prototype  shown above is for the glibc wrapper function; for
       the system call itself, the prototype can be described as  follows
       (it is identical to the clone() prototype on microblaze):

           long clone2(unsigned long flags, void *stack_base,
                       int stack_size,         /* Size of stack */
                       int *parent_tid, int *child_tid,
                       unsigned long tls);

       __clone2()  operates  in  the  same  way  as  clone(), except that
       stack_base points to the lowest address of the child's stack area,
       and  stack_size  specifies  the  size  of  the stack pointed to by
       stack_base.

   Linux 2.4 and earlier
       In Linux 2.4 and earlier, clone() does  not  take  arguments  par‐
       ent_tid, tls, and child_tid.

RETURN VALUE
       On  success, the thread ID of the child process is returned in the
       caller's thread of execution.  On failure, -1 is returned  in  the
       caller's context, no child process will be created, and errno will
       be set appropriately.

ERRORS
       EAGAIN Too many processes are already running; see fork(2).

       EINVAL CLONE_SIGHAND was specified, but CLONE_VM was not.   (Since
              Linux 2.6.0.)

       EINVAL CLONE_THREAD  was  specified,  but  CLONE_SIGHAND  was not.
              (Since Linux 2.5.35.)

       EINVAL CLONE_THREAD was specified, but the current process  previ‐
              ously  called unshare(2) with the CLONE_NEWPID flag or used
              setns(2) to reassociate itself with a PID namespace.

       EINVAL Both CLONE_FS and CLONE_NEWNS were specified in flags.

       EINVAL (since Linux 3.9)
              Both CLONE_NEWUSER and CLONE_FS were specified in flags.

       EINVAL Both  CLONE_NEWIPC  and  CLONE_SYSVSEM  were  specified  in
              flags.

       EINVAL One  (or both) of CLONE_NEWPID or CLONE_NEWUSER and one (or
              both) of CLONE_THREAD or  CLONE_PARENT  were  specified  in
              flags.

       EINVAL Returned  by  the glibc clone() wrapper function when fn or
              stack is specified as NULL.

       EINVAL CLONE_NEWIPC was specified in flags, but the kernel was not
              configured   with   the  CONFIG_SYSVIPC  and  CONFIG_IPC_NS
              options.

       EINVAL CLONE_NEWNET was specified in flags, but the kernel was not
              configured with the CONFIG_NET_NS option.

       EINVAL CLONE_NEWPID was specified in flags, but the kernel was not
              configured with the CONFIG_PID_NS option.

       EINVAL CLONE_NEWUSER was specified in flags, but  the  kernel  was
              not configured with the CONFIG_USER_NS option.

       EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not
              configured with the CONFIG_UTS_NS option.

       EINVAL stack is not aligned to a suitable boundary for this archi‐
              tecture.  For example, on aarch64, stack must be a multiple
              of 16.

       EINVAL CLONE_PIDFD was specified together with CLONE_DETACHED.

       EINVAL CLONE_PIDFD was specified together with CLONE_THREAD.

       EINVAL (clone() only)
              CLONE_PIDFD was specified together  with  CLONE_PARENT_SET‐
              TID.

       ENOMEM Cannot allocate sufficient memory to allocate a task struc‐
              ture for the child, or to copy those parts of the  caller's
              context that need to be copied.

       ENOSPC (since Linux 3.7)
              CLONE_NEWPID  was  specified in flags, but the limit on the
              nesting depth of PID namespaces would have  been  exceeded;
              see pid_namespaces(7).

       ENOSPC (since Linux 4.9; beforehand EUSERS)
              CLONE_NEWUSER  was  specified  in flags, and the call would
              cause the limit on the number of nested user namespaces  to
              be exceeded.  See user_namespaces(7).

              From  Linux  3.11 to Linux 4.8, the error diagnosed in this
              case was EUSERS.

       ENOSPC (since Linux 4.9)
              One of the values in flags specified the creation of a  new
              user  namespace,  but  doing so would have caused the limit
              defined by the corresponding file in /proc/sys/user  to  be
              exceeded.  For further details, see namespaces(7).

       EPERM  CLONE_NEWCGROUP,  CLONE_NEWIPC,  CLONE_NEWNET, CLONE_NEWNS,
              CLONE_NEWPID, or CLONE_NEWUTS was specified by an  unprivi‐
              leged process (process without CAP_SYS_ADMIN).

       EPERM  CLONE_PID  was specified by a process other than process 0.
              (This error occurs only on Linux 2.5.15 and earlier.)

       EPERM  CLONE_NEWUSER was specified in flags, but either the effec‐
              tive  user  ID or the effective group ID of the caller does
              not have a mapping in the parent namespace (see user_names‐
              paces(7)).

       EPERM (since Linux 3.9)
              CLONE_NEWUSER was specified in flags and the caller is in a
              chroot environment (i.e., the caller's root directory  does
              not  match  the  root  directory  of the mount namespace in
              which it resides).

       ERESTARTNOINTR (since Linux 2.6.17)
              System call  was  interrupted  by  a  signal  and  will  be
              restarted.  (This can be seen only during a trace.)

       EUSERS (Linux 3.11 to Linux 4.8)
              CLONE_NEWUSER  was specified in flags, and the limit on the
              number of nested user namespaces would  be  exceeded.   See
              the discussion of the ENOSPC error above.

VERSIONS
       The clone3() system call first appeared in Linux 5.3.

CONFORMING TO
       These  system  calls  are Linux-specific and should not be used in
       programs intended to be portable.

NOTES
       The kcmp(2) system call can be used to test whether two  processes
       share  various resources such as a file descriptor table, System V
       semaphore undo operations, or a virtual address space.

       Handlers registered using pthread_atfork(3) are not executed  dur‐
       ing a call to clone().

       In  the  Linux  2.4.x series, CLONE_THREAD generally does not make
       the parent of the new thread the same as the parent of the calling
       process.   However,  for  kernel  versions  2.4.7  to  2.4.18  the
       CLONE_THREAD flag implied the CLONE_PARENT flag (as in Linux 2.6.0
       and later).

       For  a while there was CLONE_DETACHED (introduced in 2.5.32): par‐
       ent wants no child-exit signal.  In Linux 2.6.2, the need to  give
       this  flag  together  with CLONE_THREAD disappeared.  This flag is
       still defined, but has no effect.

       On i386, clone()  should  not  be  called  through  vsyscall,  but
       directly through int $0x80.

BUGS
       GNU  C library versions 2.3.4 up to and including 2.24 contained a
       wrapper function for getpid(2) that  performed  caching  of  PIDs.
       This  caching  relied on support in the glibc wrapper for clone(),
       but limitations in the implementation meant that the cache was not
       up  to date in some circumstances.  In particular, if a signal was
       delivered to the child immediately after the clone() call, then  a
       call to getpid(2) in a handler for the signal could return the PID
       of the calling process ("the parent"), if the  clone  wrapper  had
       not  yet had a chance to update the PID cache in the child.  (This
       discussion ignores the case where  the  child  was  created  using
       CLONE_THREAD,  when  getpid(2) should return the same value in the
       child and in the process that called clone(), since the caller and
       the  child  are in the same thread group.  The stale-cache problem
       also does not occur if the flags argument includes CLONE_VM.)   To
       get  the truth, it was sometimes necessary to use code such as the
       following:

           #include <syscall.h>

           pid_t mypid;

           mypid = syscall(SYS_getpid);

       Because of the stale-cache problem,  as  well  as  other  problems
       noted  in  getpid(2), the PID caching feature was removed in glibc
       2.25.

EXAMPLE
       The following program demonstrates the use of clone() to create  a
       child  process  that  executes  in  a separate UTS namespace.  The
       child changes the hostname in its UTS namespace.  Both parent  and
       child  then display the system hostname, making it possible to see
       that the hostname differs in the UTS namespaces of the parent  and
       child.  For an example of the use of this program, see setns(2).

   Program source
       #define _GNU_SOURCE
       #include <sys/wait.h>
       #include <sys/utsname.h>
       #include <sched.h>
       #include <string.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       static int              /* Start function for cloned child */
       childFunc(void *arg)
       {
           struct utsname uts;

           /* Change hostname in UTS namespace of child */

           if (sethostname(arg, strlen(arg)) == -1)
               errExit("sethostname");

           /* Retrieve and display hostname */

           if (uname(&uts) == -1)
               errExit("uname");
           printf("uts.nodename in child:  %s\n", uts.nodename);

           /* Keep the namespace open for a while, by sleeping.
              This allows some experimentation--for example, another
              process might join the namespace. */

           sleep(200);

           return 0;           /* Child terminates now */
       }

       #define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */

       int
       main(int argc, char *argv[])
       {
           char *stack;                    /* Start of stack buffer */
           char *stackTop;                 /* End of stack buffer */
           pid_t pid;
           struct utsname uts;

           if (argc < 2) {
               fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
               exit(EXIT_SUCCESS);
           }

           /* Allocate stack for child */

           stack = malloc(STACK_SIZE);
           if (stack == NULL)
               errExit("malloc");
           stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */

           /* Create child that has its own UTS namespace;
              child commences execution in childFunc() */

           pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
           if (pid == -1)
               errExit("clone");
           printf("clone() returned %ld\n", (long) pid);

           /* Parent falls through to here */

           sleep(1);           /* Give child time to change its hostname */

           /* Display hostname in parent's UTS namespace. This will be
              different from hostname in child's UTS namespace. */

           if (uname(&uts) == -1)
               errExit("uname");
           printf("uts.nodename in parent: %s\n", uts.nodename);

           if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
               errExit("waitpid");
           printf("child has terminated\n");

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       fork(2),  futex(2),  getpid(2), gettid(2), kcmp(2), pidfd_open(2),
       set_thread_area(2),   set_tid_address(2),   setns(2),    tkill(2),
       unshare(2), wait(2), capabilities(7), namespaces(7), pthreads(7)


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

^ permalink raw reply	[flat|nested] 30+ messages in thread

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Thread overview: 30+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-10-25 16:59 For review: documentation of clone3() system call Michael Kerrisk (man-pages)
2019-10-25 17:07 ` Christian Brauner
2019-11-07 12:26   ` Michael Kerrisk (man-pages)
2019-10-26  2:28 ` G. Branden Robinson
2019-10-31  6:06   ` Michael Kerrisk (man-pages)
2019-10-28 15:12 ` Jann Horn
2019-10-28 17:21   ` Christian Brauner
2019-10-28 19:09     ` Jann Horn
2019-10-29 11:27       ` Christian Brauner
2019-10-29 14:26         ` Christian Brauner
2019-10-29 14:36           ` Florian Weimer
2019-10-29 16:04             ` Christian Brauner
2019-10-29 15:20           ` Jann Horn
2019-10-29 16:05             ` Christian Brauner
2019-11-07 15:19 ` Christian Brauner
2019-11-07 16:10   ` Florian Weimer
2019-11-09  8:09   ` Michael Kerrisk (man-pages)
2019-11-09 16:53     ` Christian Brauner
2019-11-11  9:02       ` Michael Kerrisk (man-pages)
2019-11-11 11:36         ` Christian Brauner
2019-11-11 19:56           ` Michael Kerrisk (man-pages)
2019-11-11 14:55     ` Jann Horn
2019-11-11 16:58       ` Theodore Y. Ts'o
2019-11-11 20:24         ` Jann Horn
2019-11-12 23:03           ` Kees Cook
2019-11-14 12:15       ` Michael Kerrisk (man-pages)
2019-11-14 12:29         ` Christian Brauner
2019-11-11 15:03 ` Florian Weimer
2019-11-11 15:15   ` Jann Horn
2019-11-11 15:20     ` Florian Weimer

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