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Date:	Mon, 1 Dec 2014 11:03:48 +0400
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	David Herrmann <dh.herrmann@...il.com>, 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

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.

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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