linux/kernel/user.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * The "user cache".
 *
 * (C) Copyright 1991-2000 Linus Torvalds
 *
 * We have a per-user structure to keep track of how many
 * processes, files etc the user has claimed, in order to be
 * able to have per-user limits for system resources. 
 */

#include <linux/init.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/bitops.h>
#include <linux/key.h>
#include <linux/sched/user.h>
#include <linux/interrupt.h>
#include <linux/export.h>
#include <linux/user_namespace.h>
#include <linux/binfmts.h>
#include <linux/proc_ns.h>

#if IS_ENABLED(CONFIG_BINFMT_MISC)
struct binfmt_misc init_binfmt_misc = …;
EXPORT_SYMBOL_GPL(…);
#endif

/*
 * userns count is 1 for root user, 1 for init_uts_ns,
 * and 1 for... ?
 */
struct user_namespace init_user_ns = …;
EXPORT_SYMBOL_GPL(…);

/*
 * UID task count cache, to get fast user lookup in "alloc_uid"
 * when changing user ID's (ie setuid() and friends).
 */

#define UIDHASH_BITS …
#define UIDHASH_SZ …
#define UIDHASH_MASK …
#define __uidhashfn(uid) …
#define uidhashentry(uid) …

static struct kmem_cache *uid_cachep;
static struct hlist_head uidhash_table[UIDHASH_SZ];

/*
 * The uidhash_lock is mostly taken from process context, but it is
 * occasionally also taken from softirq/tasklet context, when
 * task-structs get RCU-freed. Hence all locking must be softirq-safe.
 * But free_uid() is also called with local interrupts disabled, and running
 * local_bh_enable() with local interrupts disabled is an error - we'll run
 * softirq callbacks, and they can unconditionally enable interrupts, and
 * the caller of free_uid() didn't expect that..
 */
static DEFINE_SPINLOCK(uidhash_lock);

/* root_user.__count is 1, for init task cred */
struct user_struct root_user = …;

/*
 * These routines must be called with the uidhash spinlock held!
 */
static void uid_hash_insert(struct user_struct *up, struct hlist_head *hashent)
{ … }

static void uid_hash_remove(struct user_struct *up)
{ … }

static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
{ … }

static int user_epoll_alloc(struct user_struct *up)
{ … }

static void user_epoll_free(struct user_struct *up)
{ … }

/* IRQs are disabled and uidhash_lock is held upon function entry.
 * IRQ state (as stored in flags) is restored and uidhash_lock released
 * upon function exit.
 */
static void free_user(struct user_struct *up, unsigned long flags)
	__releases(&uidhash_lock)
{ … }

/*
 * Locate the user_struct for the passed UID.  If found, take a ref on it.  The
 * caller must undo that ref with free_uid().
 *
 * If the user_struct could not be found, return NULL.
 */
struct user_struct *find_user(kuid_t uid)
{ … }

void free_uid(struct user_struct *up)
{ … }
EXPORT_SYMBOL_GPL(…);

struct user_struct *alloc_uid(kuid_t uid)
{ … }

static int __init uid_cache_init(void)
{ … }
subsys_initcall(uid_cache_init);