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From: Christian Brauner <christian.brauner@ubuntu.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: linux-kernel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux API <linux-api@vger.kernel.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Linux Security Module <linux-security-module@vger.kernel.org>,
	Akinobu Mita <akinobu.mita@gmail.com>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Daniel Micay <danielmicay@gmail.com>,
	Djalal Harouni <tixxdz@gmail.com>,
	"Dmitry V . Levin" <ldv@altlinux.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	"J . Bruce Fields" <bfields@fieldses.org>,
	Jeff Layton <jlayton@poochiereds.net>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Alexey Gladkov <gladkov.alexey@gmail.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jeff Dike <jdike@addtoit.com>,
	Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>
Subject: Re: [PATCH 4/3] pid: Improve the comment about waiting in zap_pid_ns_processes
Date: Sat, 29 Feb 2020 03:59:48 +0100
Message-ID: <20200229025948.mfla2guxzvo7mdwg@wittgenstein> (raw)
In-Reply-To: <878skmpcib.fsf_-_@x220.int.ebiederm.org>

On Fri, Feb 28, 2020 at 04:34:20PM -0600, Eric W. Biederman wrote:
> 
> Oleg wrote a very informative comment, but with the removal of
> proc_cleanup_work it is no longer accurate.
> 
> Rewrite the comment so that it only talks about the details
> that are still relevant, and hopefully is a little clearer.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>  kernel/pid_namespace.c | 31 +++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index 318fcc6ba301..01f8ba32cc0c 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -224,20 +224,27 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>  	} while (rc != -ECHILD);
>  
>  	/*
> -	 * kernel_wait4() above can't reap the EXIT_DEAD children but we do not
> -	 * really care, we could reparent them to the global init. We could
> -	 * exit and reap ->child_reaper even if it is not the last thread in
> -	 * this pid_ns, free_pid(pid_allocated == 0) calls proc_cleanup_work(),
> -	 * pid_ns can not go away until proc_kill_sb() drops the reference.
> +	 * kernel_wait4() misses EXIT_DEAD children, and EXIT_ZOMBIE
> +	 * process whose parents processes are outside of the pid
> +	 * namespace.  Such processes are created with setns()+fork().
>  	 *
> -	 * But this ns can also have other tasks injected by setns()+fork().
> -	 * Again, ignoring the user visible semantics we do not really need
> -	 * to wait until they are all reaped, but they can be reparented to
> -	 * us and thus we need to ensure that pid->child_reaper stays valid
> -	 * until they all go away. See free_pid()->wake_up_process().
> +	 * If those EXIT_ZOMBIE processes are not reaped by their
> +	 * parents before their parents exit, they will be reparented
> +	 * to pid_ns->child_reaper.  Thus pidns->child_reaper needs to
> +	 * stay valid until they all go away.
>  	 *
> -	 * We rely on ignored SIGCHLD, an injected zombie must be autoreaped
> -	 * if reparented.
> +	 * The code relies on the the pid_ns->child_reaper ignoring

s/the the/the/

Hm, can we maybe reformulate this to:

"The code relies on having made pid_ns->child_reaper ignore SIGCHLD above
causing EXIT_ZOMBIE processes to be autoreaped if reparented."

Which imho makes it clearer that it was us ensuring that SIGCHLD is
ignored. Someone not too familiar with the exit codepaths might be
looking at zap_pid_ns_processes() not knowing that it is only called
when namespace init is exiting.

Otherwise

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

  reply index

Thread overview: 85+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-10 15:05 [PATCH v8 00/11] proc: modernize proc to support multiple private instances Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 01/11] proc: Rename struct proc_fs_info to proc_fs_opts Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 02/11] proc: add proc_fs_info struct to store proc information Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 03/11] proc: move /proc/{self|thread-self} dentries to proc_fs_info Alexey Gladkov
2020-02-10 18:23   ` Andy Lutomirski
2020-02-12 15:00     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 04/11] proc: move hide_pid, pid_gid from pid_namespace " Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 05/11] proc: add helpers to set and get proc hidepid and gid mount options Alexey Gladkov
2020-02-10 18:30   ` Andy Lutomirski
2020-02-12 14:57     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 06/11] proc: support mounting procfs instances inside same pid namespace Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances Alexey Gladkov
2020-02-10 17:46   ` Linus Torvalds
2020-02-10 19:23     ` Al Viro
2020-02-11  1:36   ` Eric W. Biederman
2020-02-11  4:01     ` Eric W. Biederman
2020-02-12 14:49     ` Alexey Gladkov
2020-02-12 14:59       ` Eric W. Biederman
2020-02-12 17:08         ` Alexey Gladkov
2020-02-12 18:45         ` Linus Torvalds
2020-02-12 19:16           ` Eric W. Biederman
2020-02-12 19:49             ` Linus Torvalds
2020-02-12 20:03               ` Al Viro
2020-02-12 20:35                 ` Linus Torvalds
2020-02-12 20:38                   ` Al Viro
2020-02-12 20:41                     ` Al Viro
2020-02-12 21:02                       ` Linus Torvalds
2020-02-12 21:46                         ` Eric W. Biederman
2020-02-13  0:48                           ` Linus Torvalds
2020-02-13  4:37                             ` Eric W. Biederman
2020-02-13  5:55                               ` Al Viro
2020-02-13 21:30                                 ` Linus Torvalds
2020-02-13 22:23                                   ` Al Viro
2020-02-13 22:47                                     ` Linus Torvalds
2020-02-14 14:15                                       ` Eric W. Biederman
2020-02-14  3:48                                 ` Eric W. Biederman
2020-02-20 20:46                               ` [PATCH 0/7] proc: Dentry flushing without proc_mnt Eric W. Biederman
2020-02-20 20:47                                 ` [PATCH 1/7] proc: Rename in proc_inode rename sysctl_inodes sibling_inodes Eric W. Biederman
2020-02-20 20:48                                 ` [PATCH 2/7] proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache Eric W. Biederman
2020-02-20 20:49                                 ` [PATCH 3/7] proc: Mov rcu_read_(lock|unlock) in proc_prune_siblings_dcache Eric W. Biederman
2020-02-20 22:33                                   ` Linus Torvalds
2020-02-20 20:49                                 ` [PATCH 4/7] proc: Use d_invalidate " Eric W. Biederman
2020-02-20 22:43                                   ` Linus Torvalds
2020-02-20 22:54                                   ` Al Viro
2020-02-20 23:00                                     ` Linus Torvalds
2020-02-20 23:03                                     ` Al Viro
2020-02-20 23:39                                       ` Eric W. Biederman
2020-02-20 20:51                                 ` [PATCH 5/7] proc: Clear the pieces of proc_inode that proc_evict_inode cares about Eric W. Biederman
2020-02-20 20:52                                 ` [PATCH 6/7] proc: Use a list of inodes to flush from proc Eric W. Biederman
2020-02-20 20:52                                 ` [PATCH 7/7] proc: Ensure we see the exit of each process tid exactly once Eric W. Biederman
2020-02-21 16:50                                   ` Oleg Nesterov
2020-02-22 15:46                                     ` Eric W. Biederman
2020-02-20 23:02                                 ` [PATCH 0/7] proc: Dentry flushing without proc_mnt Linus Torvalds
2020-02-20 23:07                                   ` Al Viro
2020-02-20 23:37                                     ` Eric W. Biederman
2020-02-24 16:25                                 ` [PATCH v2 0/6] " Eric W. Biederman
2020-02-24 16:26                                   ` [PATCH v2 1/6] proc: Rename in proc_inode rename sysctl_inodes sibling_inodes Eric W. Biederman
2020-02-24 16:27                                   ` [PATCH v2 2/6] proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache Eric W. Biederman
2020-02-24 16:27                                   ` [PATCH v2 3/6] proc: In proc_prune_siblings_dcache cache an aquired super block Eric W. Biederman
2020-02-24 16:28                                   ` [PATCH v2 4/6] proc: Use d_invalidate in proc_prune_siblings_dcache Eric W. Biederman
2020-02-24 16:28                                   ` [PATCH v2 5/6] proc: Clear the pieces of proc_inode that proc_evict_inode cares about Eric W. Biederman
2020-02-24 16:29                                   ` [PATCH v2 6/6] proc: Use a list of inodes to flush from proc Eric W. Biederman
2020-02-28 20:17                                   ` [PATCH 0/3] proc: Actually honor the mount options Eric W. Biederman
2020-02-28 20:18                                     ` [PATCH 1/3] uml: Don't consult current to find the proc_mnt in mconsole_proc Eric W. Biederman
2020-02-28 20:18                                     ` [PATCH 2/3] uml: Create a private mount of proc for mconsole Eric W. Biederman
2020-02-28 20:30                                       ` Christian Brauner
2020-02-28 21:28                                         ` Eric W. Biederman
2020-02-28 21:59                                           ` Christian Brauner
2020-02-28 20:19                                     ` [PATCH 3/3] proc: Remove the now unnecessary internal mount of proc Eric W. Biederman
2020-02-28 20:39                                       ` Christian Brauner
2020-02-28 21:40                                         ` Eric W. Biederman
2020-02-28 22:34                                     ` [PATCH 4/3] pid: Improve the comment about waiting in zap_pid_ns_processes Eric W. Biederman
2020-02-29  2:59                                       ` Christian Brauner [this message]
2020-02-14  3:49                     ` [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances Eric W. Biederman
2020-02-12 19:47           ` Al Viro
2020-02-11 22:45   ` Al Viro
2020-02-12 14:26     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 08/11] proc: instantiate only pids that we can ptrace on 'hidepid=4' mount option Alexey Gladkov
2020-02-10 16:29   ` Jordan Glover
2020-02-12 14:34     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 09/11] proc: add option to mount only a pids subset Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 10/11] docs: proc: add documentation for "hidepid=4" and "subset=pidfs" options and new mount behavior Alexey Gladkov
2020-02-10 18:29   ` Andy Lutomirski
2020-02-12 16:03     ` Alexey Gladkov
2020-02-10 15:05 ` [PATCH v8 11/11] proc: Move hidepid values to uapi as they are user interface to mount Alexey Gladkov

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