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Eric W. Biederman 821cc7b0b2
waitid: Add support for waiting for the current process group
It was recently discovered that the linux version of waitid is not a
superset of the other wait functions because it does not include support
for waiting for the current process group. This has two downsides:
1. An extra system call is needed to get the current process group.
2. After the current process group is received and before it is passed
   to waitid a signal could arrive causing the current process group to change.
   Inherent race-conditions as these make it impossible for userspace to
   emulate this functionaly and thus violate async-signal safety
   requirements for waitpid.

Arguments can be made for using a different choice of idtype and id
for this case but the BSDs already use this P_PGID and 0 to indicate
waiting for the current process's process group.  So be nice to user
space programmers and don't introduce an unnecessary incompatibility.

Some people have noted that the posix description is that
waitpid will wait for the current process group, and that in
the presence of pthreads that process group can change.  To get
clarity on this issue I looked at XNU, FreeBSD, and Luminos.  All of
those flavors of unix waited for the current process group at the
time of call and as written could not adapt to the process group
changing after the call.

At one point Linux did adapt to the current process group changing but
that stopped in 161550d74c ("pid: sys_wait... fixes").  It has been
over 11 years since Linux has that behavior, no programs that fail
with the change in behavior have been reported, and I could not
find any other unix that does this.  So I think it is safe to clarify
the definition of current process group, to current process group
at the time of the wait function.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Rich Felker <dalias@libc.org>
Cc: Alistair Francis <alistair23@gmail.com>
Cc: Zong Li <zongbox@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: GNU C Library <libc-alpha@sourceware.org>
Link: https://lore.kernel.org/r/20190814154400.6371-2-christian.brauner@ubuntu.com
2019-09-10 17:05:46 +02:00
arch hexagon: switch to generic version of pte allocation 2019-07-21 09:53:00 -07:00
block docs conversion for v5.3-rc1 2019-07-16 12:21:41 -07:00
certs Revert "Merge tag 'keys-acl-20190703' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs" 2019-07-10 18:43:43 -07:00
crypto USB / PHY patches for 5.3-rc1 2019-07-11 15:40:06 -07:00
Documentation Devicetree fixes for 5.3: 2019-07-21 10:28:39 -07:00
drivers iommu/amd: fix a crash in iova_magazine_free_pfns 2019-07-21 09:57:13 -07:00
fs SMB3 fixes: two fixes for stable, one that had dependency on earlier patch in this merge window and can now go in, and a perf improvement in SMB3 open 2019-07-21 10:01:17 -07:00
include pidfd: add P_PIDFD to waitid() 2019-08-01 21:49:46 +02:00
init Merge branch 'work.mount0' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-07-19 10:42:02 -07:00
ipc Merge branch 'work.mount0' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-07-19 10:42:02 -07:00
kernel waitid: Add support for waiting for the current process group 2019-09-10 17:05:46 +02:00
lib Kbuild updates for v5.3 (2nd) 2019-07-20 09:34:55 -07:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm Merge branch 'work.mount0' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-07-19 10:42:02 -07:00
net Merge branch 'work.mount0' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-07-19 10:42:02 -07:00
samples Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2019-07-19 10:06:06 -07:00
scripts Kbuild updates for v5.3 (2nd) 2019-07-20 09:34:55 -07:00
security Merge branch 'work.mount0' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-07-19 10:42:02 -07:00
sound sound fixes for 5.3-rc1 2019-07-18 09:36:51 -07:00
tools tests: add pidfd poll tests 2019-08-06 19:52:41 +02:00
usr Kbuild updates for v5.3 (2nd) 2019-07-20 09:34:55 -07:00
virt KVM: Boost vCPUs that are delivering interrupts 2019-07-20 09:00:45 +02:00
.clang-format Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2019-04-17 11:26:25 -07:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes
.gitignore kbuild: create *.mod with full directory path and remove MODVERDIR 2019-07-18 02:19:31 +09:00
.mailmap MAINTAINERS: Update my email address 2019-06-18 14:37:27 +01:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS Remove references to dead website. 2019-07-19 12:22:04 -07:00
Kbuild Kbuild updates for v5.1 2019-03-10 17:48:21 -07:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-07-20 11:24:49 -07:00
Makefile Linus 5.3-rc1 2019-07-21 14:05:38 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.