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Subject[PATCH v2 0/7] CLONE_FD: Task exit notification via file descriptor
This patch series introduces a new clone flag, CLONE_FD, which lets the caller
receive 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).

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.

This patch series also introduces a clone flag CLONE_AUTOREAP, which causes the
kernel to automatically reap the child process when it exits, just as it does
for processes using SIGCHLD when the parent has SIGCHLD ignored or marked as
SA_NOCLDSTOP.

Taken together, a library can launch a process with CLONE_FD, CLONE_AUTOREAP,
and no exit signal, and completely avoid affecting either process-wide signal
handling or an existing child wait loop.

Introducing CLONE_FD and CLONE_AUTOREAP 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.

I tested clone4 and the new flags 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.

Changes in v2:
- Split out autoreaping into a separate CLONE_AUTOREAP. CLONE_FD no longer
implies autoreaping and no exit signal, and CLONE_AUTOREAP does not affect
ptracers or signal handling. Thanks to Oleg Nesterov for careful
investigation and discussion on v1.
- Accept O_CLOEXEC and O_NONBLOCK via a clonefd_flags parameter in clone4_args.
Stop overloading the low byte of the main clone flags, since CLONE_FD now
works with a non-zero signal.
- Return the file descriptor via an out parameter in clone4_args.
- Drop patch to export alloc_fd; CLONE_FD now uses the next available file
descriptor, even if that's 0-2, since clone4 no longer needs to avoid
ambiguity with the 0 return indicating the child process.
- Make poll on a CLONE_FD for an exited task also return POLLHUP, for
compatibility with FreeBSD's pdfork. Thanks to David Drysdale for calling
attention to pdfork.
- Fix typo in squelch_clone_flags.
- Pass arguments to _do_fork and copy_process as a structure.
- Construct the 64-bit flags in a separate variable, rather than inline in the
call to do_fork.
- Fix error return for copy_from_user faults.
- Add the new syscall to asm-generic.
- Add ack from Andy Lutomirski to patches 1 and 2.

I've 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:

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;
int *clonefd;
unsigned clonefd_flags;
};


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 for portability across
architectures) accepts all the same flags as clone(2), with the excep‐
tion of the obsolete CLONE_PID, CLONE_DETACHED, and CLONE_STOPPED. In
addition, flags accepts the following flags:


CLONE_AUTOREAP
When the new process exits, immediately reap it, rather than
keeping it around as a "zombie" until a call to waitpid(2) or
similar. Without this flag, the kernel will automatically reap
a process if its exit signal is set to SIGCHLD, and if the par‐
ent process has SIGCHLD set to SIG_IGN or has a SIGCHLD handler
installed with SA_NOCLDWAIT (see sigaction(2)). CLONE_AUTOREAP
allows the calling process to enable automatic reaping with an
exit signal other than SIGCHLD (including 0 to disable the exit
signal), and does not depend on the configuration of process-
wide signal handling.


CLONE_FD
Return a file descriptor associated with the new process, stor‐
ing it in location clonefd in the parent's address space. When
the new process exits, the file descriptor will become available
for reading.

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, allowing 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.

clonefd_flags may contain the following additional flags for use
with CLONE_FD:


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


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


The returned file descriptor 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 */
};


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 O_NONBLOCK set.

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.

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.

close(2)
When the file descriptor is no longer required it should
be closed.


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 new process ID
to the calling process, and the new process begins running at the spec‐
ified function.

For the raw syscall, on success, clone4() returns the new process ID to
the calling process, and returns 0 in the new 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:

EFAULT args is outside your accessible address space.

EINVAL flags contained an unknown flag.

EINVAL flags included CLONE_FD and clonefd_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 2015-03-14 CLONE4(2)


Josh Triplett and Thiago Macieira (7):
clone: Support passing tls argument via C rather than pt_regs magic
x86: Opt into HAVE_COPY_THREAD_TLS, for both 32-bit and 64-bit
Introduce a new clone4 syscall with more flag bits and extensible arguments
kernel/fork.c: Pass arguments to _do_fork and copy_process using clone4_args
clone4: Add a CLONE_AUTOREAP flag to automatically reap the child process
signal: Factor out a helper function to process task_struct exit_code
clone4: Add a CLONE_FD flag to get task exit notification via fd

arch/Kconfig | 7 ++
arch/x86/Kconfig | 1 +
arch/x86/ia32/ia32entry.S | 3 +-
arch/x86/kernel/entry_64.S | 1 +
arch/x86/kernel/process_32.c | 6 +-
arch/x86/kernel/process_64.c | 8 +--
arch/x86/syscalls/syscall_32.tbl | 1 +
arch/x86/syscalls/syscall_64.tbl | 2 +
include/linux/compat.h | 14 ++++
include/linux/sched.h | 22 ++++++
include/linux/syscalls.h | 6 +-
include/uapi/asm-generic/unistd.h | 4 +-
include/uapi/linux/sched.h | 55 ++++++++++++++-
init/Kconfig | 21 ++++++
kernel/Makefile | 1 +
kernel/clonefd.c | 121 ++++++++++++++++++++++++++++++++
kernel/clonefd.h | 32 +++++++++
kernel/exit.c | 4 ++
kernel/fork.c | 142 ++++++++++++++++++++++++++++++--------
kernel/signal.c | 26 ++++---
kernel/sys_ni.c | 1 +
21 files changed, 426 insertions(+), 52 deletions(-)
create mode 100644 kernel/clonefd.c
create mode 100644 kernel/clonefd.h

--
2.1.4



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