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

From: Josh Triplett
Date: Thu Mar 12 2015 - 21:40:28 EST

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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).

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.

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.

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:

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 encomâ
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 simâ
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 folâ
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.

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.

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 */
};

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 addiâ
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. 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.

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 archiâ
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)

Josh Triplett and Thiago Macieira (6):
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
signal: Factor out a helper function to process task_struct exit_code
fs: Make alloc_fd non-private
clone4: Introduce new 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 +
fs/file.c | 2 +-
include/linux/compat.h | 12 ++++
include/linux/file.h | 1 +
include/linux/sched.h | 20 ++++++
include/linux/syscalls.h | 6 +-
include/uapi/linux/sched.h | 54 ++++++++++++++-
init/Kconfig | 21 ++++++
kernel/Makefile | 1 +
kernel/clonefd.c | 123 +++++++++++++++++++++++++++++++++
kernel/clonefd.h | 27 ++++++++
kernel/exit.c | 10 ++-
kernel/fork.c | 143 ++++++++++++++++++++++++++++++++-------
kernel/signal.c | 24 ++++---
kernel/sys_ni.c | 1 +
22 files changed, 425 insertions(+), 49 deletions(-)
create mode 100644 kernel/clonefd.c
create mode 100644 kernel/clonefd.h

--
2.1.4

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Next message: Josh Triplett: "[PATCH 1/6] clone: Support passing tls argument via C rather than pt_regs magic"
Previous message: Lai Jiangshan: "Re: [PATCH 4/4] workqueue: Allow modifying low level unbound workqueue cpumask"
Next in thread: Josh Triplett: "[PATCH 1/6] clone: Support passing tls argument via C rather than pt_regs magic"
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