kernel/nscommon.c

File Facts

System

Linux kernel

Corpus path

kernel/nscommon.c

Extension

.c

Size

9194 bytes

Lines

318

Domain

Core OS

Bucket

Scheduler, Processes, Timers, Sync, And Syscalls

Inferred role

Core OS: implementation source

Status

source implementation candidate

Why This File Exists

Core operating-system implementation surface: boot, tasks, memory, VFS, syscall-facing interfaces, synchronization, credentials, and isolation.

• Core operating-system implementation surface: boot, tasks, memory, VFS, syscall-facing interfaces, synchronization, credentials, and isolation.
• Uses kernel synchronization; read lock ordering, sleepability, and interrupt context assumptions before translating.
• Defines or uses C structs; map object ownership, embedded links, reference counts, and lock ownership.

Dependency Surface

• linux/ns_common.h
• linux/nstree.h
• linux/proc_ns.h
• linux/user_namespace.h
• linux/vfsdebug.h

Detected Declarations

• function ns_debug
• function __ns_common_init
• function __ns_common_free
• function ns_owner
• function user_ns1
• function user_ns1
• function may_see_all_namespaces

Annotated Snippet

if (!atomic_dec_and_test(&ns->__ns_ref_active)) {
			VFS_WARN_ON_ONCE(__ns_ref_active_read(ns) < 0);
			return;
		}
	}
}

/*
 * The active reference count works by having each namespace that gets
 * created take a single active reference on its owning user namespace.
 * That single reference is only released once the child namespace's
 * active count itself goes down. This makes it possible to efficiently
 * resurrect a namespace tree:
 *
 * A regular namespace tree might look as follow:
 * Legend:
 * + : adding active reference
 * - : dropping active reference
 * x : always active (initial namespace)
 *
 *
 *                 net_ns          pid_ns
 *                       \        /
 *                        +      +
 *                        user_ns1 (2)
 *                            |
 *                 ipc_ns     |     uts_ns
 *                       \    |    /
 *                        +   +   +
 *                        user_ns2 (3)
 *                            |
 *            cgroup_ns       |       mnt_ns
 *                     \      |      /
 *                      x     x     x
 *                      init_user_ns (1)
 *
 * If both net_ns and pid_ns put their last active reference on
 * themselves it will cascade to user_ns1 dropping its own active
 * reference and dropping one active reference on user_ns2:
 *
 *                 net_ns          pid_ns
 *                       \        /
 *                        -      -
 *                        user_ns1 (0)
 *                            |
 *                 ipc_ns     |     uts_ns
 *                       \    |    /
 *                        +   -   +
 *                        user_ns2 (2)
 *                            |
 *            cgroup_ns       |       mnt_ns
 *                     \      |      /
 *                      x     x     x
 *                      init_user_ns (1)
 *
 * Assume the whole tree is dead but all namespaces are still active:
 *
 *                 net_ns          pid_ns
 *                       \        /
 *                        -      -
 *                        user_ns1 (0)
 *                            |
 *                 ipc_ns     |     uts_ns
 *                       \    |    /
 *                        -   -   -
 *                        user_ns2 (0)
 *                            |
 *            cgroup_ns       |       mnt_ns
 *                     \      |      /
 *                      x     x     x
 *                      init_user_ns (1)
 *
 * Now assume the net_ns gets resurrected (.e.g., via the SIOCGSKNS ioctl()):
 *
 *                 net_ns          pid_ns
 *                       \        /
 *                        +      -
 *                        user_ns1 (0)
 *                            |
 *                 ipc_ns     |     uts_ns
 *                       \    |    /
 *                        -   +   -
 *                        user_ns2 (0)
 *                            |
 *            cgroup_ns       |       mnt_ns
 *                     \      |      /
 *                      x     x     x
 *                      init_user_ns (1)
 *
 * If net_ns had a zero reference count and we bumped it we also need to

Annotation

• Immediate include surface: `linux/ns_common.h`, `linux/nstree.h`, `linux/proc_ns.h`, `linux/user_namespace.h`, `linux/vfsdebug.h`.
• Detected declarations: `function ns_debug`, `function __ns_common_init`, `function __ns_common_free`, `function ns_owner`, `function user_ns1`, `function user_ns1`, `function may_see_all_namespaces`.
• Atlas domain: Core OS / Scheduler, Processes, Timers, Sync, And Syscalls.
• Implementation status: source implementation candidate.
• Synchronization appears in or near this file; preserve lock ordering, sleepability, and interrupt-context constraints.

Implementation Notes

• This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
• Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
• Driver-family pages are intentionally pattern-oriented unless they are part of the selected PCIe/NVMe representative device path.