Commit dfef6dcd authored by Al Viro's avatar Al Viro

unfuck proc_sysctl ->d_compare()

a) struct inode is not going to be freed under ->d_compare();
however, the thing PROC_I(inode)->sysctl points to just might.
Fortunately, it's enough to make freeing that sucker delayed,
provided that we don't step on its ->unregistering, clear
the pointer to it in PROC_I(inode) before dropping the reference
and check if it's NULL in ->d_compare().

b) I'm not sure that we *can* walk into NULL inode here (we recheck
dentry->seq between verifying that it's still hashed / fetching
dentry->d_inode and passing it to ->d_compare() and there's no
negative hashed dentries in /proc/sys/*), but if we can walk into
that, we really should not have ->d_compare() return 0 on it!
Said that, I really suspect that this check can be simply killed.
Signed-off-by: default avatarAl Viro <>
parent 1858efd4
......@@ -27,6 +27,7 @@
static void proc_evict_inode(struct inode *inode)
struct proc_dir_entry *de;
struct ctl_table_header *head;
truncate_inode_pages(&inode->i_data, 0);
......@@ -38,8 +39,11 @@ static void proc_evict_inode(struct inode *inode)
de = PROC_I(inode)->pde;
if (de)
if (PROC_I(inode)->sysctl)
head = PROC_I(inode)->sysctl;
if (head) {
rcu_assign_pointer(PROC_I(inode)->sysctl, NULL);
struct vfsmount *proc_mnt;
......@@ -408,15 +408,18 @@ static int proc_sys_compare(const struct dentry *parent,
const struct dentry *dentry, const struct inode *inode,
unsigned int len, const char *str, const struct qstr *name)
struct ctl_table_header *head;
/* Although proc doesn't have negative dentries, rcu-walk means
* that inode here can be NULL */
/* AV: can it, indeed? */
if (!inode)
return 0;
return 1;
if (name->len != len)
return 1;
if (memcmp(name->name, str, len))
return 1;
return !sysctl_is_seen(PROC_I(inode)->sysctl);
head = rcu_dereference(PROC_I(inode)->sysctl);
return !head || !sysctl_is_seen(head);
static const struct dentry_operations proc_sys_dentry_operations = {
......@@ -25,6 +25,7 @@
#include <linux/kernel.h>
#include <linux/types.h>
#include <linux/compiler.h>
#include <linux/rcupdate.h>
struct completion;
......@@ -1037,10 +1038,15 @@ struct ctl_table_root {
struct ctl_table trees. */
struct ctl_table_header
struct ctl_table *ctl_table;
struct list_head ctl_entry;
int used;
int count;
union {
struct {
struct ctl_table *ctl_table;
struct list_head ctl_entry;
int used;
int count;
struct rcu_head rcu;
struct completion *unregistering;
struct ctl_table *ctl_table_arg;
struct ctl_table_root *root;
......@@ -194,9 +194,9 @@ static int sysrq_sysctl_handler(ctl_table *table, int write,
static struct ctl_table root_table[];
static struct ctl_table_root sysctl_table_root;
static struct ctl_table_header root_table_header = {
.count = 1,
{{.count = 1,
.ctl_table = root_table,
.ctl_entry = LIST_HEAD_INIT(sysctl_table_root.default_set.list),
.ctl_entry = LIST_HEAD_INIT(sysctl_table_root.default_set.list),}},
.root = &sysctl_table_root,
.set = &sysctl_table_root.default_set,
......@@ -1567,11 +1567,16 @@ void sysctl_head_get(struct ctl_table_header *head)
static void free_head(struct rcu_head *rcu)
kfree(container_of(rcu, struct ctl_table_header, rcu));
void sysctl_head_put(struct ctl_table_header *head)
if (!--head->count)
call_rcu(&head->rcu, free_head);
......@@ -1948,10 +1953,10 @@ void unregister_sysctl_table(struct ctl_table_header * header)
if (!--header->parent->count) {
call_rcu(&header->parent->rcu, free_head);
if (!--header->count)
call_rcu(&header->rcu, free_head);
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