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Date: Wed, 01 Jun 2011 20:31:51 -0700
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Lucas De Marchi <lucas.demarchi@...fusion.mobi>
Cc: linux-kernel@...r.kernel.org, kay.sievers@...y.org,
Nick Piggin <npiggin@...nel.dk>,
Al Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@....de>,
Stephen Rothwell <sfr@...b.auug.org.au>,
Andrew Morton <akpm@...ux-foundation.org>,
David Howells <dhowells@...hat.com>,
"Serge E. Hallyn" <serge.hallyn@...onical.com>,
Daniel Lezcano <daniel.lezcano@...e.fr>,
Jiri Slaby <jslaby@...e.cz>,
Greg Kroah-Hartman <gregkh@...e.de>,
James Morris <jmorris@...ei.org>
Subject: Re: [PATCH] sysctl: add support for poll()
Lucas De Marchi <lucas.demarchi@...fusion.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@...fusion.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@...fusion.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 ¤t->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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