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Subject[PATCH] sysctl: add support for poll()
Date
Adding support for poll() in sysctl fs allows userspace to receive
notifications when an entry in sysctl changes. This way it's possible to
know when hostname/domainname is changed due to the respective syscall
has been called or its file under /proc/sys has been written to.

Signed-off-by: Lucas De Marchi <lucas.demarchi@profusion.mobi>
---
fs/proc/proc_sysctl.c | 40 ++++++++++++++++++++++++++++++++++++++++
include/linux/sysctl.h | 8 ++++++++
include/linux/utsname.h | 16 ++++++++++++++++
kernel/sys.c | 2 ++
kernel/utsname_sysctl.c | 36 ++++++++++++++++++++++++++++++++++++
5 files changed, 102 insertions(+), 0 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index f50133c..2e5d3ec 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -3,6 +3,7 @@
*/
#include <linux/init.h>
#include <linux/sysctl.h>
+#include <linux/poll.h>
#include <linux/proc_fs.h>
#include <linux/security.h>
#include <linux/namei.h>
@@ -176,6 +177,43 @@ static ssize_t proc_sys_write(struct file *filp, const char __user *buf,
return proc_sys_call_handler(filp, (void __user *)buf, count, ppos, 1);
}

+static int proc_sys_open(struct inode *inode, struct file *filp)
+{
+ struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+
+ if (table->poll) {
+ unsigned long event = atomic_read(&table->poll->event);
+
+ filp->private_data = (void *)event;
+ }
+
+ return 0;
+}
+
+static unsigned int proc_sys_poll(struct file *filp, poll_table *wait)
+{
+ struct inode *inode = filp->f_path.dentry->d_inode;
+ struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+ unsigned long event = (unsigned long)filp->private_data;
+ unsigned int ret = POLLIN | POLLRDNORM;
+
+ if (!table->proc_handler)
+ goto out;
+
+ if (!table->poll)
+ goto out;
+
+ poll_wait(filp, &table->poll->wait, wait);
+
+ if (event != atomic_read(&table->poll->event)) {
+ filp->private_data = (void *)(unsigned long)atomic_read(
+ &table->poll->event);
+ ret = POLLIN | POLLRDNORM | POLLERR | POLLPRI;
+ }
+
+out:
+ return ret;
+}

static int proc_sys_fill_cache(struct file *filp, void *dirent,
filldir_t filldir,
@@ -367,6 +405,8 @@ static int proc_sys_getattr(struct vfsmount *mnt, struct dentry *dentry, struct
}

static const struct file_operations proc_sys_file_operations = {
+ .open = proc_sys_open,
+ .poll = proc_sys_poll,
.read = proc_sys_read,
.write = proc_sys_write,
.llseek = default_llseek,
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 11684d9..96c89ba 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -25,6 +25,7 @@
#include <linux/kernel.h>
#include <linux/types.h>
#include <linux/compiler.h>
+#include <linux/wait.h>

struct completion;

@@ -1011,6 +1012,12 @@ extern int proc_do_large_bitmap(struct ctl_table *, int,
* cover common cases.
*/

+/* Support for userspace poll() to watch for changes */
+struct ctl_table_poll {
+ atomic_t event;
+ wait_queue_head_t wait;
+};
+
/* A sysctl table is an array of struct ctl_table: */
struct ctl_table
{
@@ -1021,6 +1028,7 @@ struct ctl_table
struct ctl_table *child;
struct ctl_table *parent; /* Automatically set */
proc_handler *proc_handler; /* Callback for text formatting */
+ struct ctl_table_poll *poll;
void *extra1;
void *extra2;
};
diff --git a/include/linux/utsname.h b/include/linux/utsname.h
index 4e5b021..c714ed7 100644
--- a/include/linux/utsname.h
+++ b/include/linux/utsname.h
@@ -37,6 +37,14 @@ struct new_utsname {
#include <linux/nsproxy.h>
#include <linux/err.h>

+enum uts_proc {
+ UTS_PROC_OSTYPE,
+ UTS_PROC_OSRELEASE,
+ UTS_PROC_VERSION,
+ UTS_PROC_HOSTNAME,
+ UTS_PROC_DOMAINNAME,
+};
+
struct user_namespace;
extern struct user_namespace init_user_ns;

@@ -80,6 +88,14 @@ static inline struct uts_namespace *copy_utsname(unsigned long flags,
}
#endif

+#ifdef CONFIG_PROC_SYSCTL
+extern void uts_proc_notify(enum uts_proc proc);
+#else
+static inline void uts_proc_notify(enum uts_proc proc)
+{
+}
+#endif
+
static inline struct new_utsname *utsname(void)
{
return &current->nsproxy->uts_ns->name;
diff --git a/kernel/sys.c b/kernel/sys.c
index e4128b2..ada9cd7 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1211,6 +1211,7 @@ SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
memset(u->nodename + len, 0, sizeof(u->nodename) - len);
errno = 0;
}
+ uts_proc_notify(UTS_PROC_HOSTNAME);
up_write(&uts_sem);
return errno;
}
@@ -1261,6 +1262,7 @@ SYSCALL_DEFINE2(setdomainname, char __user *, name, int, len)
memset(u->domainname + len, 0, sizeof(u->domainname) - len);
errno = 0;
}
+ uts_proc_notify(UTS_PROC_DOMAINNAME);
up_write(&uts_sem);
return errno;
}
diff --git a/kernel/utsname_sysctl.c b/kernel/utsname_sysctl.c
index a2cd77e..e96b766 100644
--- a/kernel/utsname_sysctl.c
+++ b/kernel/utsname_sysctl.c
@@ -13,6 +13,7 @@
#include <linux/uts.h>
#include <linux/utsname.h>
#include <linux/sysctl.h>
+#include <linux/wait.h>

static void *get_uts(ctl_table *table, int write)
{
@@ -51,12 +52,28 @@ static int proc_do_uts_string(ctl_table *table, int write,
uts_table.data = get_uts(table, write);
r = proc_dostring(&uts_table,write,buffer,lenp, ppos);
put_uts(table, write, uts_table.data);
+
+ if (write) {
+ atomic_inc(&table->poll->event);
+ wake_up_interruptible(&table->poll->wait);
+ }
+
return r;
}
#else
#define proc_do_uts_string NULL
#endif

+static struct ctl_table_poll hostname_poll = {
+ .event = ATOMIC_INIT(0),
+ .wait = __WAIT_QUEUE_HEAD_INITIALIZER(hostname_poll.wait),
+};
+
+static struct ctl_table_poll domainname_poll = {
+ .event = ATOMIC_INIT(0),
+ .wait = __WAIT_QUEUE_HEAD_INITIALIZER(domainname_poll.wait),
+};
+
static struct ctl_table uts_kern_table[] = {
{
.procname = "ostype",
@@ -85,6 +102,7 @@ static struct ctl_table uts_kern_table[] = {
.maxlen = sizeof(init_uts_ns.name.nodename),
.mode = 0644,
.proc_handler = proc_do_uts_string,
+ .poll = &hostname_poll,
},
{
.procname = "domainname",
@@ -92,6 +110,7 @@ static struct ctl_table uts_kern_table[] = {
.maxlen = sizeof(init_uts_ns.name.domainname),
.mode = 0644,
.proc_handler = proc_do_uts_string,
+ .poll = &domainname_poll,
},
{}
};
@@ -105,6 +124,23 @@ static struct ctl_table uts_root_table[] = {
{}
};

+#ifdef CONFIG_PROC_SYSCTL
+/*
+ * Notify userspace about a change in a certain entry of uts_kern_table,
+ * identified by the parameter proc.
+ */
+void uts_proc_notify(enum uts_proc proc)
+{
+ struct ctl_table *table = &uts_kern_table[proc];
+
+ if (!table->poll)
+ return;
+
+ atomic_inc(&table->poll->event);
+ wake_up_interruptible(&table->poll->wait);
+}
+#endif
+
static int __init utsname_sysctl_init(void)
{
register_sysctl_table(uts_root_table);
--
1.7.5.2


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