[PATCH] Document signal-semantics changes of container-init

Sukadev Bhattiprolu sukadev at linux.vnet.ibm.com
Sat Mar 14 21:06:42 PDT 2009

Sukadev Bhattiprolu [sukadev at linux.vnet.ibm.com] wrote:
| Hi Michael,
| The kernel changes corresponding to his update were added to -mm
| on 2009-02-19. Please review and let me know if other man pages
| should be updated to document these semantics.

Grr, failed to mention about SIGKILL/SIGSTOP from parent ns.
Here is an updated patch.
From: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
Date: Sat, 14 Mar 2009 16:08:42 -0700
Subject: [PATCH] Document signal-semantics changes of container-init

Container-inits have some special signal semantics depending on
whether the sender is from an ancestor pid-namespace or from a
descendant process. These changes were implemented by the patchset:


Document the new semantics in the clone(2) and kill(2) man pages.

Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
 man2/clone.2 |    7 ++++++-
 man2/kill.2  |   29 +++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletions(-)

diff --git a/man2/clone.2 b/man2/clone.2
index 5d07f7c..975b321 100644
--- a/man2/clone.2
+++ b/man2/clone.2
@@ -396,7 +396,8 @@ will produce processes with PIDs that are unique within the namespace.
 The first process created in a new namespace
 (i.e., the process created using the
-flag) has the PID 1, and is the "init" process for the namespace.
+flag) has the PID 1, and is the "init" process for the namespace,
+also referred to as "container-init" process.
 Children that are orphaned within the namespace will be reparented
 to this process rather than
 .BR init (8).
@@ -405,6 +406,10 @@ Unlike the traditional
 process, the "init" process of a PID namespace can terminate,
 and if it does, all of the processes in the namespace are terminated.
+See NOTES section of
+.B kill(2)
+for special signal semantics of the "container-init" process.
 PID namespaces form a hierarchy.
 When a PID new namespace is created,
 the processes in that namespace are visible
diff --git a/man2/kill.2 b/man2/kill.2
index 5cfa9a4..a3bde50 100644
--- a/man2/kill.2
+++ b/man2/kill.2
@@ -128,6 +128,35 @@ has explicitly installed signal handlers.
 This is done to assure the
 system is not brought down accidentally.
+.I container-init
+process (i.e a process created by a call to
+.I clone() 
+flag) is also similarly immune to all "unhandled" signals sent by a
+descendant process.
+i.e the 
+.I container-init
+appears like an 
+.I init
+process to its descendant processes and they cannot accidentally terminate
+.I container-init
+and thereby, the entire pid-namespace.
+.I container-init
+also ignores all "unhandled" signals sent from an ancestor pid-namespace,
+except SIGKILL and SIGSTOP. i.e. if a
+.I container-init
+receives a SIGKILL or SIGSTOP from a process in an ancestor pid-namespace
+.I container-init
+behaves like a normal process allowing an administrator in ancestor container
+to stop/terminate the
+.I container-init.
 POSIX.1-2001 requires that \fIkill(\-1,sig)\fP send \fIsig\fP
 to all processes that the calling process may send signals to,
 except possibly for some implementation-defined system processes.

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