Re: [PATCH] sysctl: add support for poll()

[ebiederm@xmission.com (Eric W. Biederman)]

Lucas De Marchi <lucas.demarchi@profusion.mobi> writes:

> CC'ing people as suggested by get_maintainer.pl
>
> On Wed, Jun 1, 2011 at 9:14 AM, Lucas De Marchi
> <lucas.demarchi@profusion.mobi> wrote:
>> 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.

Why do you want to do this?  What advantage does this bring to
userspace?

That feels like a pretty big special case.

Eric


>> 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
>>
>>