Re: [PATCH 0/6] CLONE_FD: Task exit notification via file descriptor

On Fri, Mar 13, 2015 at 1:40 AM, Josh Triplett [off-list ref] wrote:

This patch series introduces a new clone flag, CLONE_FD, which lets the caller
handle child process exit notification via a file descriptor rather than
SIGCHLD.  CLONE_FD makes it possible for libraries to safely launch and manage
child processes on behalf of their caller, *without* taking over process-wide
SIGCHLD handling (either via signal handler or signalfd).

Hi Josh,

From the overall description (i.e. I haven't looked at the code yet)
this looks very interesting.  However, it seems to cover a lot of the
same ground as the process descriptor feature that was added to FreeBSD
in 9.x/10.x:
  https://www.freebsd.org/cgi/man.cgi?query=pdfork&sektion=2

I think it would ideally be nice for a userspace library developer to be
able to do subprocess management (without SIGCHLD) in a similar way
across both platforms, without lots of complicated autoconf shenanigans.

So could we look at the overlap and seeing if we can come up with
something that covers your requirements and also allows for something
that looks like FreeBSD's process descriptors?

(I've actually got some rough patches to add process descriptor
functionality on Linux, so I can look at how the two approaches compare
and contrast.)

Note that signalfd for SIGCHLD does not suffice here, because that still
receives notification for all child processes, and interferes with process-wide
signal handling.

The CLONE_FD file descriptor uniquely identifies a process on the system in a
race-free way, by holding a reference to the task_struct.  In the future, we
may introduce APIs that support using process file descriptors instead of PIDs.

FreeBSD has pdkill(2) and (theoretically) pdwait4(2) along these lines.
I suspect we need either need pdkill(2) or a way to retrieve a PID from
a process file descriptor, so that there's a way to send signals to the
child.

Introducing CLONE_FD required two additional bits of yak shaving: Since clone
has no more usable flags (with the three currently unused flags unusable
because old kernels ignore them without EINVAL), also introduce a new clone4
system call with more flag bits and an extensible argument structure.  And
since the magic pt_regs-based syscall argument processing for clone's tls
argument would otherwise prevent introducing a sane clone4 system call, fix
that too.

I tested the CLONE_SETTLS changes with a thread-local storage test program (two
threads independently reading and writing a __thread variable), on both 32-bit
and 64-bit, and I observed no issues there.

Worth preserving in tools/testing/selftests/ ?

I tested clone4 and the new CLONE_FD call with several additional test
programs, launching either a process or thread (in the former case using
syscall(), in the latter case by calling clone4 via assembly and returning to
C), sleeping in parent and child to test the case of either exiting first, and
then printing the received clone4_info structure.  Thiago also tested clone4
with CLONE_FD with a modified version of libqt's process handling, which
includes a test suite.

I've also included the manpages patch at the end of this series.  (Note that
the manpage documents the behavior of the future glibc wrapper as well as the
raw syscall.)  Here's a formatted plain-text version of the manpage for
reference:

FYI, I've added some comparisons with the FreeBSD equivalents below.

CLONE4(2)                  Linux Programmer's Manual                 CLONE4(2)



NAME
       clone4 - create a child process

SYNOPSIS
       /* Prototype for the glibc wrapper function */

       #define _GNU_SOURCE
       #include <sched.h>

       int clone4(uint64_t flags,
                  size_t args_size,
                  struct clone4_args *args,
                  int (*fn)(void *), void *arg);

       /* Prototype for the raw system call */

       int clone4(unsigned flags_high, unsigned flags_low,
                  unsigned long args_size,
                  struct clone4_args *args);

       struct clone4_args {
           pid_t *ptid;
           pid_t *ctid;
           unsigned long stack_start;
           unsigned long stack_size;
           unsigned long tls;
       };


DESCRIPTION
       clone4()  creates  a  new  process,  similar  to  clone(2) and fork(2).
       clone4() supports additional flags that clone(2) does not, and  accepts
       arguments via an extensible structure.

       args  points to a clone4_args structure, and args_size must contain the
       size of that structure, as understood by the  caller.   If  the  caller
       passes  a  shorter  structure  than  the  kernel expects, the remaining
       fields will default to 0.  If the caller passes a larger structure than
       the  kernel  expects  (such  as one from a newer kernel), clone4() will
       return EINVAL.  The clone4_args structure may gain additional fields at
       the  end  in  the future, and callers must only pass a size that encom‐
       passes the number of fields they understand.  If the  caller  passes  0
       for args_size, args is ignored and may be NULL.

       In  the clone4_args structure, ptid, ctid, stack_start, stack_size, and
       tls have the same semantics as they do with clone(2) and clone2(2).

       In the glibc wrapper, fn and arg have the same  semantics  as  they  do
       with clone(2).  As with clone(2), the underlying system call works more
       like fork(2), returning 0 in the child process; the glibc wrapper  sim‐
       plifies  thread execution by calling fn(arg) and exiting the child when
       that function exits.

       The 64-bit  flags  argument  (split  into  the  32-bit  flags_high  and
       flags_low arguments in the kernel interface) accepts all the same flags
       as  clone(2),  with  the   exception   of   the   obsolete   CLONE_PID,
       CLONE_DETACHED, and CLONE_STOPPED.  In addition, flags accepts the fol‐
       lowing flags:


       CLONE_FD
              Instead of returning a process ID, clone4()  with  the  CLONE_FD
              flag  returns a file descriptor associated with the new process.
              When the new process exits, the kernel will not send a signal to
              the  parent process, and will not keep the new process around as
              a "zombie" process  until  a  call  to  waitpid(2)  or  similar.
              Instead,  the file descriptor will become available for reading,
              and the new process will be immediately reaped.

Just to confirm: presumably a waitpid(-1,...) call that's already in
progress won't return when one of these child processes exits?

              Unlike using  signalfd(2)  for  the  SIGCHLD  signal,  the  file
              descriptor  returned  by  clone4()  with the CLONE_FD flag works
              even with SIGCHLD unblocked in one or more threads of the parent
              process,  and  allows the process to have different handlers for
              different child processes, such as those created by  a  library,
              without  introducing  race conditions around process-wide signal
              handling.

              clone4() will never return a file descriptor in the range 0-2 to
              the caller, to avoid ambiguity with the return of 0 in the child
              process.  Only the  calling  process  will  have  the  new  file
              descriptor open; the child process will not.

FreeBSD's pdfork(2) returns a PID but also takes an int *fdp argument to
return the file descriptor separately, which avoids the need for special
case processing for low FD values (and means that POSIX's "lowest file
descriptor not currently open" behaviour can be preserved if desired).

              Since the kernel does not send a termination signal when a child
              process created with CLONE_FD exits, the low byte of flags  does
              not contain a signal number.  Instead, the low byte of flags can
              contain the following additional flags for use with CLONE_FD:


              CLONEFD_CLOEXEC
                     Set the O_CLOEXEC flag on the new open  file  descriptor.
                     See  the description of the O_CLOEXEC flag in open(2) for
                     reasons why this may be useful.


              CLONEFD_NONBLOCK
                     Set the O_NONBLOCK flag on the new open file  descriptor.
                     Using  this flag saves extra calls to fcntl(2) to achieve
                     the same result.


              clone4() with the CLONE_FD flag returns a file  descriptor  that
              supports the following operations:

              read(2) (and similar)
                     When  the  new  process  exits,  reading  from  the  file
                     descriptor produces a single clonefd_info structure:

                     struct clonefd_info {
                         uint32_t code;   /* Signal code */
                         uint32_t status; /* Exit status or signal */
                         uint64_t utime;  /* User CPU time */
                         uint64_t stime;  /* System CPU time */
                     };

Presumably there is no way to get full rusage information for the exited
process?

[FreeBSD theoretically has pdwait4(2) to do wait4-like operations on a
process descriptor, including rusage retrieval.  However, I don't think
they actually implemented it:
  http://fxr.watson.org/fxr/source/kern/syscalls.master#L928]

                     If the new process has not  yet  exited,  read(2)  either
                     blocks  until  it does, or fails with the error EAGAIN if
                     the file descriptor has been made nonblocking.

                     Future kernels may extend clonefd_info by appending addi‐
                     tional  fields  to  the end.  Callers should read as many
                     bytes as they understand; unread data will be  discarded,
                     and  subsequent  reads  after  the first will return 0 to
                     indicate end-of-file.  Callers requesting more bytes than
                     the  kernel  provides  (such as callers expecting a newer
                     clonefd_info structure) will receive a shorter  structure
                     from older kernels.

FreeBSD also implements fstat(2) for its process descriptors, although
only a few of the fields get filled in.

              poll(2), select(2), epoll(7) (and similar)
                     The  file  descriptor  is readable (the select(2) readfds
                     argument; the poll(2) POLLIN flag) if the new process has
                     exited.

FreeBSD uses POLLHUP here.

              close(2)
                     When  the file descriptor is no longer required it should
                     be closed.  If no process has a file descriptor open  for
                     the new process, no process will receive any notification
                     when the new process exits.  The new process  will  still
                     be immediately reaped.

FreeBSD has two different behaviours for close(2), depending on a flag
value (PD_DAEMON).  With the flag set it's roughly like this, but
without PD_DAEMON a close(2) operation on the (last open) file
descriptor terminates the child process.

This can be quite useful, particularly for the use case where some
userspace library has an FD-controlled subprocess -- if the application
using the library terminates, the process descriptor is closed and so
the subprocess is automatically terminated.

   C library/kernel ABI differences
       As with clone(2), the raw clone4() system call corresponds more closely
       to fork(2) in that execution in the child continues from the  point  of
       the call.

       Unlike  clone(2),  the  raw  system call interface for clone4() accepts
       arguments in the same order on all architectures.

       The raw system call accepts flags as two 32-bit  arguments,  flags_high
       and  flags_low, to simplify portability across 32-bit and 64-bit archi‐
       tectures and calling conventions.  The glibc wrapper accepts flags as a
       single 64-bit argument for convenience.


RETURN VALUE
       For the glibc wrapper, on success, clone4() returns the file descriptor
       (with CLONE_FD) or new process ID (without  CLONE_FD),  and  the  child
       process begins running at the specified function.

       For  the  raw syscall, on success, clone4() returns the file descriptor
       or new process ID to the calling process, and  returns  0  in  the  new
       child process.

       On failure, clone4() returns -1 and sets errno accordingly.


ERRORS
       clone4()  can  return any error from clone(2), as well as the following
       additional errors:

       EINVAL flags contained an unknown flag.

       EINVAL flags included CLONE_FD, but the kernel configuration  does  not
              have the CONFIG_CLONEFD option enabled.

       EMFILE flags  included  CLONE_FD,  but  the  new  file descriptor would
              exceed the process limit on open file descriptors.

       ENFILE flags included CLONE_FD,  but  the  new  file  descriptor  would
              exceed the system-wide limit on open file descriptors.

       ENODEV flags  included  CLONE_FD,  but  clone4()  could  not  mount the
              (internal) anonymous inode device.


CONFORMING TO
       clone4() is Linux-specific and should not be used in programs  intended
       to be portable.


SEE ALSO
       clone(2), epoll(7), poll(2), pthreads(7), read(2), select(2)



Linux                             2015-03-01                         CLONE4(2)