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Date: Mon, 1 Dec 2014 08:21:24 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Konstantin Khlebnikov <koct9i@...il.com>
Cc: David Herrmann <dh.herrmann@...il.com>,
Linux API <linux-api@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Andrew Morton <akpm@...ux-foundation.org>,
systemd Mailing List <systemd-devel@...ts.freedesktop.org>
Subject: Re: [RFC PATCH] proc, pidns: Add highpid
On Sun, Nov 30, 2014 at 11:03 PM, Konstantin Khlebnikov
<koct9i@...il.com> wrote:
> Hmm. What about per-task/thread UUID? exported via separate file: /proc/PID/uuid
> It could be created at the first access, thus this wouldn't shlowdown clone().
> Also it could be droped at execve(), so it'll describe execution
> context more precisely than pid.
>
I'd be all for this if it weren't for two issues:
1. This thing needs to work as soon as init is started, and we can't
reliably generate random numbers that early.
2. Migration / CRIU is rather fundamentally at odds with making
anything universally unique, as opposed to just per-user-ns.
So, given that I don't think we can actually provide a universally
unique pid-like thing, I'd rather not even try.
That being said, I want to rework this a little bit. I think that
highpid should be restricted to the range 2^32 through 2^64 - 4096.
The former prevents anyone from confusing highpid with regular pid,
and the latter means that we don't need to worry about confusion
between errors and valid highpids (e.g. -1 will never be a highpid).
Implementing that will be only mildly annoying.
--Andy
> On Sat, Nov 29, 2014 at 2:05 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> Pid reuse is common, which means that it's difficult or impossible
>> to read information about a pid from /proc without races.
>>
>> This introduces a second number associated with each (task, pidns)
>> pair called highpid. Highpid is a 64-bit number, and, barring
>> extremely unlikely circumstances or outright error, a (highpid, pid)
>> will never be reused.
>>
>> With just this change, a program can open /proc/PID/status, read the
>> "Highpid" field, and confirm that it has the expected value. If the
>> pid has been reused, then highpid will be different.
>>
>> The initial implementation is straightforward: highpid is simply a
>> 64-bit counter. If a high-end system can fork every 3 ns (which
>> would be amazing, given that just allocating a pid requires at
>> atomic operation), it would take well over 1000 years for highpid to
>> wrap.
>>
>> For CRIU's benefit, the next highpid can be set by a privileged
>> user.
>>
>> NB: The sysctl stuff only works on 64-bit systems. If the approach
>> looks good, I'll fix that somehow.
>>
>> Signed-off-by: Andy Lutomirski <luto@...capital.net>
>> ---
>>
>> If this goes in, there's plenty of room to add new interfaces to
>> make this more useful. For example, we could add a fancier tgkill
>> that adds and validates hightgid and highpid, and we might want to
>> add a syscall to read one's own hightgid and highpid. These would
>> be quite useful for pidfiles.
>>
>> David, would this be useful for kdbus?
>>
>> CRIU people: will this be unduly difficult to support in CRIU?
>>
>> If you all think this is a good idea, I'll fix the sysctl stuff and
>> document it. It might also be worth adding "Hightgid" to status.
>>
>> fs/proc/array.c | 5 ++++-
>> include/linux/pid.h | 2 ++
>> include/linux/pid_namespace.h | 1 +
>> kernel/pid.c | 19 +++++++++++++++----
>> kernel/pid_namespace.c | 22 ++++++++++++++++++++++
>> 5 files changed, 44 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/proc/array.c b/fs/proc/array.c
>> index cd3653e4f35c..f1e0e69d19f9 100644
>> --- a/fs/proc/array.c
>> +++ b/fs/proc/array.c
>> @@ -159,6 +159,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
>> int g;
>> struct fdtable *fdt = NULL;
>> const struct cred *cred;
>> + const struct upid *upid;
>> pid_t ppid, tpid;
>>
>> rcu_read_lock();
>> @@ -170,12 +171,14 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
>> if (tracer)
>> tpid = task_pid_nr_ns(tracer, ns);
>> }
>> + upid = pid_upid_ns(pid, ns);
>> cred = get_task_cred(p);
>> seq_printf(m,
>> "State:\t%s\n"
>> "Tgid:\t%d\n"
>> "Ngid:\t%d\n"
>> "Pid:\t%d\n"
>> + "Highpid:\t%llu\n"
>> "PPid:\t%d\n"
>> "TracerPid:\t%d\n"
>> "Uid:\t%d\t%d\t%d\t%d\n"
>> @@ -183,7 +186,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
>> get_task_state(p),
>> task_tgid_nr_ns(p, ns),
>> task_numa_group_id(p),
>> - pid_nr_ns(pid, ns),
>> + upid ? upid->nr : 0, upid ? upid->highnr : 0,
>> ppid, tpid,
>> from_kuid_munged(user_ns, cred->uid),
>> from_kuid_munged(user_ns, cred->euid),
>> diff --git a/include/linux/pid.h b/include/linux/pid.h
>> index 23705a53abba..ece70b64d04c 100644
>> --- a/include/linux/pid.h
>> +++ b/include/linux/pid.h
>> @@ -51,6 +51,7 @@ struct upid {
>> /* Try to keep pid_chain in the same cacheline as nr for find_vpid */
>> int nr;
>> struct pid_namespace *ns;
>> + u64 highnr;
>> struct hlist_node pid_chain;
>> };
>>
>> @@ -170,6 +171,7 @@ static inline pid_t pid_nr(struct pid *pid)
>> }
>>
>> pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
>> +const struct upid *pid_upid_ns(struct pid *pid, struct pid_namespace *ns);
>> pid_t pid_vnr(struct pid *pid);
>>
>> #define do_each_pid_task(pid, type, task) \
>> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
>> index 1997ffc295a7..1f9f0455d7ef 100644
>> --- a/include/linux/pid_namespace.h
>> +++ b/include/linux/pid_namespace.h
>> @@ -26,6 +26,7 @@ struct pid_namespace {
>> struct rcu_head rcu;
>> int last_pid;
>> unsigned int nr_hashed;
>> + atomic64_t next_highpid;
>> struct task_struct *child_reaper;
>> struct kmem_cache *pid_cachep;
>> unsigned int level;
>> diff --git a/kernel/pid.c b/kernel/pid.c
>> index 9b9a26698144..863d11a9bfbf 100644
>> --- a/kernel/pid.c
>> +++ b/kernel/pid.c
>> @@ -312,6 +312,8 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>>
>> pid->numbers[i].nr = nr;
>> pid->numbers[i].ns = tmp;
>> + pid->numbers[i].highnr =
>> + atomic64_inc_return(&tmp->next_highpid) - 1;
>> tmp = tmp->parent;
>> }
>>
>> @@ -492,17 +494,26 @@ struct pid *find_get_pid(pid_t nr)
>> }
>> EXPORT_SYMBOL_GPL(find_get_pid);
>>
>> -pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
>> +const struct upid *pid_upid_ns(struct pid *pid, struct pid_namespace *ns)
>> {
>> struct upid *upid;
>> - pid_t nr = 0;
>>
>> if (pid && ns->level <= pid->level) {
>> upid = &pid->numbers[ns->level];
>> if (upid->ns == ns)
>> - nr = upid->nr;
>> + return upid;
>> }
>> - return nr;
>> + return NULL;
>> +}
>> +EXPORT_SYMBOL_GPL(pid_upid_ns);
>> +
>> +pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
>> +{
>> + const struct upid *upid = pid_upid_ns(pid, ns);
>> +
>> + if (!upid)
>> + return 0;
>> + return upid->nr;
>> }
>> EXPORT_SYMBOL_GPL(pid_nr_ns);
>>
>> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
>> index db95d8eb761b..e246802b4361 100644
>> --- a/kernel/pid_namespace.c
>> +++ b/kernel/pid_namespace.c
>> @@ -268,6 +268,22 @@ static int pid_ns_ctl_handler(struct ctl_table *table, int write,
>> return proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
>> }
>>
>> +static int pid_ns_next_highpid_handler(struct ctl_table *table, int write,
>> + void __user *buffer, size_t *lenp, loff_t *ppos)
>> +{
>> + struct pid_namespace *pid_ns = task_active_pid_ns(current);
>> + struct ctl_table tmp = *table;
>> +
>> + if (write && !ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
>> + return -EPERM;
>> +
>> + /* This needs to be fixed. */
>> + BUILD_BUG_ON(sizeof(u64) != sizeof(unsigned long));
>> +
>> + tmp.data = &pid_ns->next_highpid;
>> + return proc_dointvec(&tmp, write, buffer, lenp, ppos);
>> +}
>> +
>> extern int pid_max;
>> static int zero = 0;
>> static struct ctl_table pid_ns_ctl_table[] = {
>> @@ -279,6 +295,12 @@ static struct ctl_table pid_ns_ctl_table[] = {
>> .extra1 = &zero,
>> .extra2 = &pid_max,
>> },
>> + {
>> + .procname = "ns_next_highpid",
>> + .maxlen = sizeof(u64),
>> + .mode = 0666, /* permissions are checked in the handler */
>> + .proc_handler = pid_ns_next_highpid_handler,
>> + },
>> { }
>> };
>> static struct ctl_path kern_path[] = { { .procname = "kernel", }, { } };
>> --
>> 1.9.3
>>
>> --
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--
Andy Lutomirski
AMA Capital Management, LLC
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