The main motivation to add set_tid to clone3() is CRIU.
To restore a process with the same PID/TID CRIU currently uses
/proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
ns_last_pid and then (quickly) does a clone(). This works most of the
time, but it is racy. It is also slow as it requires multiple syscalls.
Extending clone3() to support set_tid makes it possible restore a
process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
race free (as long as the desired PID/TID is available).
This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
on clone3() with set_tid as they are currently in place for ns_last_pid.
Signed-off-by: Adrian Reber <[email protected]>
---
v2:
- Removed (size < sizeof(struct clone_args)) as discussed with
Christian and Dmitry
- Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
- Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
v3:
- Return EEXIST if PID is already in use (Christian)
- Drop CLONE_SET_TID (Christian and Oleg)
- Use idr_is_empty() instead of idr_get_cursor() (Oleg)
- Handle different `struct clone_args` sizes (Dmitry)
v4:
- Rework struct size check with defines (Christian)
- Reduce number of set_tid checks (Oleg)
- Less parentheses and more robust code (Oleg)
- Do ns_capable() on correct user_ns (Oleg, Christian)
---
include/linux/pid.h | 2 +-
include/linux/sched/task.h | 1 +
include/uapi/linux/sched.h | 1 +
kernel/fork.c | 25 +++++++++++++++++++++++--
kernel/pid.c | 34 +++++++++++++++++++++++++++-------
5 files changed, 53 insertions(+), 10 deletions(-)
diff --git a/include/linux/pid.h b/include/linux/pid.h
index 2a83e434db9d..052000db0ced 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
extern struct pid *find_get_pid(int nr);
extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
-extern struct pid *alloc_pid(struct pid_namespace *ns);
+extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
extern void free_pid(struct pid *pid);
extern void disable_pid_allocation(struct pid_namespace *ns);
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 0497091e40c1..4f2a80564332 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -26,6 +26,7 @@ struct kernel_clone_args {
unsigned long stack;
unsigned long stack_size;
unsigned long tls;
+ pid_t set_tid;
};
/*
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index b3105ac1381a..e1ce103a2c47 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -45,6 +45,7 @@ struct clone_args {
__aligned_u64 stack;
__aligned_u64 stack_size;
__aligned_u64 tls;
+ __aligned_u64 set_tid;
};
/*
diff --git a/kernel/fork.c b/kernel/fork.c
index 2852d0e76ea3..2a03f0e201e9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -117,6 +117,13 @@
*/
#define MAX_THREADS FUTEX_TID_MASK
+/*
+ * Different sizes of struct clone_args
+ */
+#define CLONE3_ARGS_SIZE_V0 64
+/* V1 includes set_tid */
+#define CLONE3_ARGS_SIZE_V1 72
+
/*
* Protected counters by write_lock_irq(&tasklist_lock)
*/
@@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct *copy_process(
stackleak_task_init(p);
if (pid != &init_struct_pid) {
- pid = alloc_pid(p->nsproxy->pid_ns_for_children);
+ if (args->set_tid && !ns_capable(
+ p->nsproxy->pid_ns_for_children->user_ns,
+ CAP_SYS_ADMIN)) {
+ retval = -EPERM;
+ goto bad_fork_cleanup_thread;
+ }
+ pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
if (IS_ERR(pid)) {
retval = PTR_ERR(pid);
goto bad_fork_cleanup_thread;
@@ -2535,9 +2548,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
if (unlikely(size > PAGE_SIZE))
return -E2BIG;
- if (unlikely(size < sizeof(struct clone_args)))
+ /* The struct needs to be at least the size of the original struct. */
+ if (unlikely(size < CLONE3_ARGS_SIZE_V0))
return -EINVAL;
+ if (size < sizeof(struct clone_args))
+ memset((void *)&args + size, 0,
+ sizeof(struct clone_args) - size);
+
if (unlikely(!access_ok(uargs, size)))
return -EFAULT;
@@ -2573,6 +2591,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
.tls = args.tls,
};
+ if (size >= CLONE3_ARGS_SIZE_V1)
+ kargs->set_tid = args.set_tid;
+
return 0;
}
diff --git a/kernel/pid.c b/kernel/pid.c
index 0a9f2e437217..9ce89c35c5be 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
call_rcu(&pid->rcu, delayed_put_pid);
}
-struct pid *alloc_pid(struct pid_namespace *ns)
+struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
{
struct pid *pid;
enum pid_type type;
@@ -186,12 +186,32 @@ struct pid *alloc_pid(struct pid_namespace *ns)
if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
pid_min = RESERVED_PIDS;
- /*
- * Store a null pointer so find_pid_ns does not find
- * a partially initialized PID (see below).
- */
- nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
- pid_max, GFP_ATOMIC);
+ if (set_tid) {
+ /*
+ * Also fail if a PID != 1 is requested
+ * and no PID 1 exists.
+ */
+ nr = -EINVAL;
+ if (set_tid < pid_max && set_tid > 0 &&
+ (set_tid == 1 || !idr_is_empty(&tmp->idr)))
+ nr = idr_alloc(&tmp->idr, NULL, set_tid,
+ set_tid + 1, GFP_ATOMIC);
+ /*
+ * If ENOSPC is returned it means that the PID is
+ * alreay in use. Return EEXIST in that case.
+ */
+ if (nr == -ENOSPC)
+ nr = -EEXIST;
+ /* Only use set_tid for one PID namespace. */
+ set_tid = 0;
+ } else {
+ /*
+ * Store a null pointer so find_pid_ns does not find
+ * a partially initialized PID (see below).
+ */
+ nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
+ pid_max, GFP_ATOMIC);
+ }
spin_unlock_irq(&pidmap_lock);
idr_preload_end();
--
2.21.0
On Thu, Aug 08, 2019 at 11:22:21PM +0200, Adrian Reber wrote:
> The main motivation to add set_tid to clone3() is CRIU.
>
> To restore a process with the same PID/TID CRIU currently uses
> /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> ns_last_pid and then (quickly) does a clone(). This works most of the
> time, but it is racy. It is also slow as it requires multiple syscalls.
>
> Extending clone3() to support set_tid makes it possible restore a
> process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> race free (as long as the desired PID/TID is available).
>
> This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> on clone3() with set_tid as they are currently in place for ns_last_pid.
>
> Signed-off-by: Adrian Reber <[email protected]>
> ---
> v2:
> - Removed (size < sizeof(struct clone_args)) as discussed with
> Christian and Dmitry
> - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
> - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
>
> v3:
> - Return EEXIST if PID is already in use (Christian)
> - Drop CLONE_SET_TID (Christian and Oleg)
> - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
> - Handle different `struct clone_args` sizes (Dmitry)
>
> v4:
> - Rework struct size check with defines (Christian)
> - Reduce number of set_tid checks (Oleg)
> - Less parentheses and more robust code (Oleg)
> - Do ns_capable() on correct user_ns (Oleg, Christian)
> ---
> include/linux/pid.h | 2 +-
> include/linux/sched/task.h | 1 +
> include/uapi/linux/sched.h | 1 +
> kernel/fork.c | 25 +++++++++++++++++++++++--
> kernel/pid.c | 34 +++++++++++++++++++++++++++-------
> 5 files changed, 53 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 2a83e434db9d..052000db0ced 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
> extern struct pid *find_get_pid(int nr);
> extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
>
> -extern struct pid *alloc_pid(struct pid_namespace *ns);
> +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
> extern void free_pid(struct pid *pid);
> extern void disable_pid_allocation(struct pid_namespace *ns);
>
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index 0497091e40c1..4f2a80564332 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -26,6 +26,7 @@ struct kernel_clone_args {
> unsigned long stack;
> unsigned long stack_size;
> unsigned long tls;
> + pid_t set_tid;
> };
>
> /*
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index b3105ac1381a..e1ce103a2c47 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -45,6 +45,7 @@ struct clone_args {
> __aligned_u64 stack;
> __aligned_u64 stack_size;
> __aligned_u64 tls;
> + __aligned_u64 set_tid;
> };
>
> /*
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2852d0e76ea3..2a03f0e201e9 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -117,6 +117,13 @@
> */
> #define MAX_THREADS FUTEX_TID_MASK
>
> +/*
> + * Different sizes of struct clone_args
> + */
> +#define CLONE3_ARGS_SIZE_V0 64
> +/* V1 includes set_tid */
> +#define CLONE3_ARGS_SIZE_V1 72
> +
> /*
> * Protected counters by write_lock_irq(&tasklist_lock)
> */
> @@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct *copy_process(
> stackleak_task_init(p);
>
> if (pid != &init_struct_pid) {
> - pid = alloc_pid(p->nsproxy->pid_ns_for_children);
> + if (args->set_tid && !ns_capable(
> + p->nsproxy->pid_ns_for_children->user_ns,
> + CAP_SYS_ADMIN)) {
> + retval = -EPERM;
> + goto bad_fork_cleanup_thread;
> + }
> + pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
> if (IS_ERR(pid)) {
> retval = PTR_ERR(pid);
> goto bad_fork_cleanup_thread;
> @@ -2535,9 +2548,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> if (unlikely(size > PAGE_SIZE))
> return -E2BIG;
>
> - if (unlikely(size < sizeof(struct clone_args)))
> + /* The struct needs to be at least the size of the original struct. */
I don't think you need that comment. I think the macro is pretty
self-explanatory. If you want it to be even clearer you could even make
it CLONE3_ARGS_SIZE_MIN but V0 is good enough. :)
> + if (unlikely(size < CLONE3_ARGS_SIZE_V0))
> return -EINVAL;
>
> + if (size < sizeof(struct clone_args))
> + memset((void *)&args + size, 0,
> + sizeof(struct clone_args) - size);
> +
> if (unlikely(!access_ok(uargs, size)))
> return -EFAULT;
>
> @@ -2573,6 +2591,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> .tls = args.tls,
> };
>
> + if (size >= CLONE3_ARGS_SIZE_V1)
> + kargs->set_tid = args.set_tid;
Hm, the if-condition is not needed though, right? At this point we will
have already copied from struct clone_args __user *uargs into struct
clone_args args. If we hit that codepath that means the kernel
definitely has a field for set_tid in its struct clone_args. :) So this
could probably just be:
.tls = args.tls,
.set_tid = args.set_tid,
}
?
> +
> return 0;
> }
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 0a9f2e437217..9ce89c35c5be 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
> call_rcu(&pid->rcu, delayed_put_pid);
> }
>
> -struct pid *alloc_pid(struct pid_namespace *ns)
> +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> {
> struct pid *pid;
> enum pid_type type;
> @@ -186,12 +186,32 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> pid_min = RESERVED_PIDS;
>
> - /*
> - * Store a null pointer so find_pid_ns does not find
> - * a partially initialized PID (see below).
> - */
> - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> - pid_max, GFP_ATOMIC);
> + if (set_tid) {
> + /*
> + * Also fail if a PID != 1 is requested
> + * and no PID 1 exists.
> + */
> + nr = -EINVAL;
> + if (set_tid < pid_max && set_tid > 0 &&
Hm, you're already in the if-branch hat verified if (set_tid) so the
set_tid > 0 conjunct seems redundant. :)
> + (set_tid == 1 || !idr_is_empty(&tmp->idr)))
> + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> + set_tid + 1, GFP_ATOMIC);
I'm confused, shouldn't this be
if (set_tid < pid_max || (set_tid == 1 && !idr_is_emtpy(&tmp->idf)))
> + /*
> + * If ENOSPC is returned it means that the PID is
> + * alreay in use. Return EEXIST in that case.
> + */
> + if (nr == -ENOSPC)
> + nr = -EEXIST;
> + /* Only use set_tid for one PID namespace. */
> + set_tid = 0;
> + } else {
> + /*
> + * Store a null pointer so find_pid_ns does not find
> + * a partially initialized PID (see below).
> + */
> + nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> + pid_max, GFP_ATOMIC);
> + }
> spin_unlock_irq(&pidmap_lock);
> idr_preload_end();
>
> --
> 2.21.0
>
On Sat, Aug 10, 2019 at 03:10:34AM +0200, Christian Brauner wrote:
> On Thu, Aug 08, 2019 at 11:22:21PM +0200, Adrian Reber wrote:
> > The main motivation to add set_tid to clone3() is CRIU.
> >
> > To restore a process with the same PID/TID CRIU currently uses
> > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > ns_last_pid and then (quickly) does a clone(). This works most of the
> > time, but it is racy. It is also slow as it requires multiple syscalls.
> >
> > Extending clone3() to support set_tid makes it possible restore a
> > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> > race free (as long as the desired PID/TID is available).
> >
> > This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> > on clone3() with set_tid as they are currently in place for ns_last_pid.
> >
> > Signed-off-by: Adrian Reber <[email protected]>
> > ---
> > v2:
> > - Removed (size < sizeof(struct clone_args)) as discussed with
> > Christian and Dmitry
> > - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
> > - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
> >
> > v3:
> > - Return EEXIST if PID is already in use (Christian)
> > - Drop CLONE_SET_TID (Christian and Oleg)
> > - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
> > - Handle different `struct clone_args` sizes (Dmitry)
> >
> > v4:
> > - Rework struct size check with defines (Christian)
> > - Reduce number of set_tid checks (Oleg)
> > - Less parentheses and more robust code (Oleg)
> > - Do ns_capable() on correct user_ns (Oleg, Christian)
> > ---
> > include/linux/pid.h | 2 +-
> > include/linux/sched/task.h | 1 +
> > include/uapi/linux/sched.h | 1 +
> > kernel/fork.c | 25 +++++++++++++++++++++++--
> > kernel/pid.c | 34 +++++++++++++++++++++++++++-------
> > 5 files changed, 53 insertions(+), 10 deletions(-)
> >
> > diff --git a/include/linux/pid.h b/include/linux/pid.h
> > index 2a83e434db9d..052000db0ced 100644
> > --- a/include/linux/pid.h
> > +++ b/include/linux/pid.h
> > @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
> > extern struct pid *find_get_pid(int nr);
> > extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
> >
> > -extern struct pid *alloc_pid(struct pid_namespace *ns);
> > +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
> > extern void free_pid(struct pid *pid);
> > extern void disable_pid_allocation(struct pid_namespace *ns);
> >
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index 0497091e40c1..4f2a80564332 100644
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -26,6 +26,7 @@ struct kernel_clone_args {
> > unsigned long stack;
> > unsigned long stack_size;
> > unsigned long tls;
> > + pid_t set_tid;
> > };
> >
> > /*
> > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> > index b3105ac1381a..e1ce103a2c47 100644
> > --- a/include/uapi/linux/sched.h
> > +++ b/include/uapi/linux/sched.h
> > @@ -45,6 +45,7 @@ struct clone_args {
> > __aligned_u64 stack;
> > __aligned_u64 stack_size;
> > __aligned_u64 tls;
> > + __aligned_u64 set_tid;
> > };
> >
> > /*
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 2852d0e76ea3..2a03f0e201e9 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -117,6 +117,13 @@
> > */
> > #define MAX_THREADS FUTEX_TID_MASK
> >
> > +/*
> > + * Different sizes of struct clone_args
> > + */
> > +#define CLONE3_ARGS_SIZE_V0 64
> > +/* V1 includes set_tid */
> > +#define CLONE3_ARGS_SIZE_V1 72
> > +
> > /*
> > * Protected counters by write_lock_irq(&tasklist_lock)
> > */
> > @@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct *copy_process(
> > stackleak_task_init(p);
> >
> > if (pid != &init_struct_pid) {
> > - pid = alloc_pid(p->nsproxy->pid_ns_for_children);
> > + if (args->set_tid && !ns_capable(
> > + p->nsproxy->pid_ns_for_children->user_ns,
> > + CAP_SYS_ADMIN)) {
> > + retval = -EPERM;
> > + goto bad_fork_cleanup_thread;
> > + }
> > + pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
> > if (IS_ERR(pid)) {
> > retval = PTR_ERR(pid);
> > goto bad_fork_cleanup_thread;
> > @@ -2535,9 +2548,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > if (unlikely(size > PAGE_SIZE))
> > return -E2BIG;
> >
> > - if (unlikely(size < sizeof(struct clone_args)))
> > + /* The struct needs to be at least the size of the original struct. */
>
> I don't think you need that comment. I think the macro is pretty
> self-explanatory. If you want it to be even clearer you could even make
> it CLONE3_ARGS_SIZE_MIN but V0 is good enough. :)
Will remove the comment.
> > + if (unlikely(size < CLONE3_ARGS_SIZE_V0))
> > return -EINVAL;
> >
> > + if (size < sizeof(struct clone_args))
> > + memset((void *)&args + size, 0,
> > + sizeof(struct clone_args) - size);
> > +
> > if (unlikely(!access_ok(uargs, size)))
> > return -EFAULT;
> >
> > @@ -2573,6 +2591,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > .tls = args.tls,
> > };
> >
> > + if (size >= CLONE3_ARGS_SIZE_V1)
> > + kargs->set_tid = args.set_tid;
>
> Hm, the if-condition is not needed though, right? At this point we will
> have already copied from struct clone_args __user *uargs into struct
> clone_args args. If we hit that codepath that means the kernel
> definitely has a field for set_tid in its struct clone_args. :) So this
> could probably just be:
>
> .tls = args.tls,
> .set_tid = args.set_tid,
> }
>
> ?
Right.
> > +
> > return 0;
> > }
> >
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 0a9f2e437217..9ce89c35c5be 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
> > call_rcu(&pid->rcu, delayed_put_pid);
> > }
> >
> > -struct pid *alloc_pid(struct pid_namespace *ns)
> > +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> > {
> > struct pid *pid;
> > enum pid_type type;
> > @@ -186,12 +186,32 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> > if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> > pid_min = RESERVED_PIDS;
> >
> > - /*
> > - * Store a null pointer so find_pid_ns does not find
> > - * a partially initialized PID (see below).
> > - */
> > - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > - pid_max, GFP_ATOMIC);
> > + if (set_tid) {
> > + /*
> > + * Also fail if a PID != 1 is requested
> > + * and no PID 1 exists.
> > + */
> > + nr = -EINVAL;
> > + if (set_tid < pid_max && set_tid > 0 &&
>
> Hm, you're already in the if-branch hat verified if (set_tid) so the
> set_tid > 0 conjunct seems redundant. :)
Yes, but I dropped all checks to see if set_tid is negative as suggested
by Oleg and moved it here.
> > + (set_tid == 1 || !idr_is_empty(&tmp->idr)))
> > + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> > + set_tid + 1, GFP_ATOMIC);
>
> I'm confused, shouldn't this be
>
> if (set_tid < pid_max || (set_tid == 1 && !idr_is_emtpy(&tmp->idf)))
Now I am also confused ;). This does not work. This will always return
true if set_tid is less than pid_max. So pid_max needs to be something
like 1 for the check after || to make sense, right? But you really got
me confused here right now. Right now I still think what I did is
correct.
> > + /*
> > + * If ENOSPC is returned it means that the PID is
> > + * alreay in use. Return EEXIST in that case.
> > + */
> > + if (nr == -ENOSPC)
> > + nr = -EEXIST;
> > + /* Only use set_tid for one PID namespace. */
> > + set_tid = 0;
> > + } else {
> > + /*
> > + * Store a null pointer so find_pid_ns does not find
> > + * a partially initialized PID (see below).
> > + */
> > + nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > + pid_max, GFP_ATOMIC);
> > + }
> > spin_unlock_irq(&pidmap_lock);
> > idr_preload_end();
> >
> > --
> > 2.21.0
> >
On Sat, Aug 10, 2019 at 07:59:18AM +0200, Adrian Reber wrote:
> On Sat, Aug 10, 2019 at 03:10:34AM +0200, Christian Brauner wrote:
> > On Thu, Aug 08, 2019 at 11:22:21PM +0200, Adrian Reber wrote:
> > > The main motivation to add set_tid to clone3() is CRIU.
> > >
> > > To restore a process with the same PID/TID CRIU currently uses
> > > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > > ns_last_pid and then (quickly) does a clone(). This works most of the
> > > time, but it is racy. It is also slow as it requires multiple syscalls.
> > >
> > > Extending clone3() to support set_tid makes it possible restore a
> > > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> > > race free (as long as the desired PID/TID is available).
> > >
> > > This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> > > on clone3() with set_tid as they are currently in place for ns_last_pid.
> > >
> > > Signed-off-by: Adrian Reber <[email protected]>
> > > ---
> > > v2:
> > > - Removed (size < sizeof(struct clone_args)) as discussed with
> > > Christian and Dmitry
> > > - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
> > > - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
> > >
> > > v3:
> > > - Return EEXIST if PID is already in use (Christian)
> > > - Drop CLONE_SET_TID (Christian and Oleg)
> > > - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
> > > - Handle different `struct clone_args` sizes (Dmitry)
> > >
> > > v4:
> > > - Rework struct size check with defines (Christian)
> > > - Reduce number of set_tid checks (Oleg)
> > > - Less parentheses and more robust code (Oleg)
> > > - Do ns_capable() on correct user_ns (Oleg, Christian)
> > > ---
> > > include/linux/pid.h | 2 +-
> > > include/linux/sched/task.h | 1 +
> > > include/uapi/linux/sched.h | 1 +
> > > kernel/fork.c | 25 +++++++++++++++++++++++--
> > > kernel/pid.c | 34 +++++++++++++++++++++++++++-------
> > > 5 files changed, 53 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/include/linux/pid.h b/include/linux/pid.h
> > > index 2a83e434db9d..052000db0ced 100644
> > > --- a/include/linux/pid.h
> > > +++ b/include/linux/pid.h
> > > @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
> > > extern struct pid *find_get_pid(int nr);
> > > extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
> > >
> > > -extern struct pid *alloc_pid(struct pid_namespace *ns);
> > > +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
> > > extern void free_pid(struct pid *pid);
> > > extern void disable_pid_allocation(struct pid_namespace *ns);
> > >
> > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > > index 0497091e40c1..4f2a80564332 100644
> > > --- a/include/linux/sched/task.h
> > > +++ b/include/linux/sched/task.h
> > > @@ -26,6 +26,7 @@ struct kernel_clone_args {
> > > unsigned long stack;
> > > unsigned long stack_size;
> > > unsigned long tls;
> > > + pid_t set_tid;
> > > };
> > >
> > > /*
> > > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> > > index b3105ac1381a..e1ce103a2c47 100644
> > > --- a/include/uapi/linux/sched.h
> > > +++ b/include/uapi/linux/sched.h
> > > @@ -45,6 +45,7 @@ struct clone_args {
> > > __aligned_u64 stack;
> > > __aligned_u64 stack_size;
> > > __aligned_u64 tls;
> > > + __aligned_u64 set_tid;
> > > };
> > >
> > > /*
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index 2852d0e76ea3..2a03f0e201e9 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -117,6 +117,13 @@
> > > */
> > > #define MAX_THREADS FUTEX_TID_MASK
> > >
> > > +/*
> > > + * Different sizes of struct clone_args
> > > + */
> > > +#define CLONE3_ARGS_SIZE_V0 64
> > > +/* V1 includes set_tid */
> > > +#define CLONE3_ARGS_SIZE_V1 72
> > > +
> > > /*
> > > * Protected counters by write_lock_irq(&tasklist_lock)
> > > */
> > > @@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct *copy_process(
> > > stackleak_task_init(p);
> > >
> > > if (pid != &init_struct_pid) {
> > > - pid = alloc_pid(p->nsproxy->pid_ns_for_children);
> > > + if (args->set_tid && !ns_capable(
> > > + p->nsproxy->pid_ns_for_children->user_ns,
> > > + CAP_SYS_ADMIN)) {
> > > + retval = -EPERM;
> > > + goto bad_fork_cleanup_thread;
> > > + }
> > > + pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
> > > if (IS_ERR(pid)) {
> > > retval = PTR_ERR(pid);
> > > goto bad_fork_cleanup_thread;
> > > @@ -2535,9 +2548,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > > if (unlikely(size > PAGE_SIZE))
> > > return -E2BIG;
> > >
> > > - if (unlikely(size < sizeof(struct clone_args)))
> > > + /* The struct needs to be at least the size of the original struct. */
> >
> > I don't think you need that comment. I think the macro is pretty
> > self-explanatory. If you want it to be even clearer you could even make
> > it CLONE3_ARGS_SIZE_MIN but V0 is good enough. :)
>
> Will remove the comment.
>
> > > + if (unlikely(size < CLONE3_ARGS_SIZE_V0))
> > > return -EINVAL;
> > >
> > > + if (size < sizeof(struct clone_args))
> > > + memset((void *)&args + size, 0,
> > > + sizeof(struct clone_args) - size);
> > > +
> > > if (unlikely(!access_ok(uargs, size)))
> > > return -EFAULT;
> > >
> > > @@ -2573,6 +2591,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > > .tls = args.tls,
> > > };
> > >
> > > + if (size >= CLONE3_ARGS_SIZE_V1)
> > > + kargs->set_tid = args.set_tid;
> >
> > Hm, the if-condition is not needed though, right? At this point we will
> > have already copied from struct clone_args __user *uargs into struct
> > clone_args args. If we hit that codepath that means the kernel
> > definitely has a field for set_tid in its struct clone_args. :) So this
> > could probably just be:
> >
> > .tls = args.tls,
> > .set_tid = args.set_tid,
> > }
> >
> > ?
>
> Right.
>
> > > +
> > > return 0;
> > > }
> > >
> > > diff --git a/kernel/pid.c b/kernel/pid.c
> > > index 0a9f2e437217..9ce89c35c5be 100644
> > > --- a/kernel/pid.c
> > > +++ b/kernel/pid.c
> > > @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
> > > call_rcu(&pid->rcu, delayed_put_pid);
> > > }
> > >
> > > -struct pid *alloc_pid(struct pid_namespace *ns)
> > > +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> > > {
> > > struct pid *pid;
> > > enum pid_type type;
> > > @@ -186,12 +186,32 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> > > if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> > > pid_min = RESERVED_PIDS;
> > >
> > > - /*
> > > - * Store a null pointer so find_pid_ns does not find
> > > - * a partially initialized PID (see below).
> > > - */
> > > - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > > - pid_max, GFP_ATOMIC);
> > > + if (set_tid) {
> > > + /*
> > > + * Also fail if a PID != 1 is requested
> > > + * and no PID 1 exists.
> > > + */
> > > + nr = -EINVAL;
> > > + if (set_tid < pid_max && set_tid > 0 &&
> >
> > Hm, you're already in the if-branch hat verified if (set_tid) so the
> > set_tid > 0 conjunct seems redundant. :)
>
> Yes, but I dropped all checks to see if set_tid is negative as suggested
> by Oleg and moved it here.
>
> > > + (set_tid == 1 || !idr_is_empty(&tmp->idr)))
> > > + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> > > + set_tid + 1, GFP_ATOMIC);
> >
> > I'm confused, shouldn't this be
> >
> > if (set_tid < pid_max || (set_tid == 1 && !idr_is_emtpy(&tmp->idf)))
>
> Now I am also confused ;). This does not work. This will always return
> true if set_tid is less than pid_max. So pid_max needs to be something
> like 1 for the check after || to make sense, right? But you really got
> me confused here right now. Right now I still think what I did is
> correct.
I missed the part where you reset set_tid to 0 below and mis-parsed the
rightmost conjunct in the if statement.
One thing I dislike is that you do
if (set_tid) {
if ([...] && set_tid > 0 && [...])
}
For consistency this should rather be:
if (set_tid) {
if ([...] && set_tid && [...])
}
Overall I'd rather write the second if as:
if (set_tid && set_tid < pid_max && (set_tid == 1 || !idr_is_empty(&tmp->idr)))
to place the most basic condition (i.e. set_tid is non-zero) leftmost.
The right-most conjunct (set_tid == 1 || !idr_is_empty(&tmp->idr)) is
non-trivial to parse. Someone not familiar with that code would need
time to figure out why and how exactly 1 is special-cased.
I think overall this is begging for alloc_pid() to be documented now if
you don't mind. :)
Christian
On Sun, Aug 11, 2019 at 08:51:48AM +0200, Christian Brauner wrote:
> On Sat, Aug 10, 2019 at 07:59:18AM +0200, Adrian Reber wrote:
> > On Sat, Aug 10, 2019 at 03:10:34AM +0200, Christian Brauner wrote:
> > > On Thu, Aug 08, 2019 at 11:22:21PM +0200, Adrian Reber wrote:
> > > > The main motivation to add set_tid to clone3() is CRIU.
> > > >
> > > > To restore a process with the same PID/TID CRIU currently uses
> > > > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > > > ns_last_pid and then (quickly) does a clone(). This works most of the
> > > > time, but it is racy. It is also slow as it requires multiple syscalls.
> > > >
> > > > Extending clone3() to support set_tid makes it possible restore a
> > > > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> > > > race free (as long as the desired PID/TID is available).
> > > >
> > > > This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> > > > on clone3() with set_tid as they are currently in place for ns_last_pid.
> > > >
> > > > Signed-off-by: Adrian Reber <[email protected]>
> > > > ---
> > > > v2:
> > > > - Removed (size < sizeof(struct clone_args)) as discussed with
> > > > Christian and Dmitry
> > > > - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
> > > > - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
> > > >
> > > > v3:
> > > > - Return EEXIST if PID is already in use (Christian)
> > > > - Drop CLONE_SET_TID (Christian and Oleg)
> > > > - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
> > > > - Handle different `struct clone_args` sizes (Dmitry)
> > > >
> > > > v4:
> > > > - Rework struct size check with defines (Christian)
> > > > - Reduce number of set_tid checks (Oleg)
> > > > - Less parentheses and more robust code (Oleg)
> > > > - Do ns_capable() on correct user_ns (Oleg, Christian)
> > > > ---
> > > > include/linux/pid.h | 2 +-
> > > > include/linux/sched/task.h | 1 +
> > > > include/uapi/linux/sched.h | 1 +
> > > > kernel/fork.c | 25 +++++++++++++++++++++++--
> > > > kernel/pid.c | 34 +++++++++++++++++++++++++++-------
> > > > 5 files changed, 53 insertions(+), 10 deletions(-)
> > > >
> > > > diff --git a/include/linux/pid.h b/include/linux/pid.h
> > > > index 2a83e434db9d..052000db0ced 100644
> > > > --- a/include/linux/pid.h
> > > > +++ b/include/linux/pid.h
> > > > @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
> > > > extern struct pid *find_get_pid(int nr);
> > > > extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
> > > >
> > > > -extern struct pid *alloc_pid(struct pid_namespace *ns);
> > > > +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
> > > > extern void free_pid(struct pid *pid);
> > > > extern void disable_pid_allocation(struct pid_namespace *ns);
> > > >
> > > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > > > index 0497091e40c1..4f2a80564332 100644
> > > > --- a/include/linux/sched/task.h
> > > > +++ b/include/linux/sched/task.h
> > > > @@ -26,6 +26,7 @@ struct kernel_clone_args {
> > > > unsigned long stack;
> > > > unsigned long stack_size;
> > > > unsigned long tls;
> > > > + pid_t set_tid;
> > > > };
> > > >
> > > > /*
> > > > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> > > > index b3105ac1381a..e1ce103a2c47 100644
> > > > --- a/include/uapi/linux/sched.h
> > > > +++ b/include/uapi/linux/sched.h
> > > > @@ -45,6 +45,7 @@ struct clone_args {
> > > > __aligned_u64 stack;
> > > > __aligned_u64 stack_size;
> > > > __aligned_u64 tls;
> > > > + __aligned_u64 set_tid;
> > > > };
> > > >
> > > > /*
> > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > index 2852d0e76ea3..2a03f0e201e9 100644
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -117,6 +117,13 @@
> > > > */
> > > > #define MAX_THREADS FUTEX_TID_MASK
> > > >
> > > > +/*
> > > > + * Different sizes of struct clone_args
> > > > + */
> > > > +#define CLONE3_ARGS_SIZE_V0 64
> > > > +/* V1 includes set_tid */
> > > > +#define CLONE3_ARGS_SIZE_V1 72
> > > > +
> > > > /*
> > > > * Protected counters by write_lock_irq(&tasklist_lock)
> > > > */
> > > > @@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct *copy_process(
> > > > stackleak_task_init(p);
> > > >
> > > > if (pid != &init_struct_pid) {
> > > > - pid = alloc_pid(p->nsproxy->pid_ns_for_children);
> > > > + if (args->set_tid && !ns_capable(
> > > > + p->nsproxy->pid_ns_for_children->user_ns,
> > > > + CAP_SYS_ADMIN)) {
> > > > + retval = -EPERM;
> > > > + goto bad_fork_cleanup_thread;
> > > > + }
> > > > + pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
> > > > if (IS_ERR(pid)) {
> > > > retval = PTR_ERR(pid);
> > > > goto bad_fork_cleanup_thread;
> > > > @@ -2535,9 +2548,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > > > if (unlikely(size > PAGE_SIZE))
> > > > return -E2BIG;
> > > >
> > > > - if (unlikely(size < sizeof(struct clone_args)))
> > > > + /* The struct needs to be at least the size of the original struct. */
> > >
> > > I don't think you need that comment. I think the macro is pretty
> > > self-explanatory. If you want it to be even clearer you could even make
> > > it CLONE3_ARGS_SIZE_MIN but V0 is good enough. :)
> >
> > Will remove the comment.
> >
> > > > + if (unlikely(size < CLONE3_ARGS_SIZE_V0))
> > > > return -EINVAL;
> > > >
> > > > + if (size < sizeof(struct clone_args))
> > > > + memset((void *)&args + size, 0,
> > > > + sizeof(struct clone_args) - size);
> > > > +
> > > > if (unlikely(!access_ok(uargs, size)))
> > > > return -EFAULT;
> > > >
> > > > @@ -2573,6 +2591,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > > > .tls = args.tls,
> > > > };
> > > >
> > > > + if (size >= CLONE3_ARGS_SIZE_V1)
> > > > + kargs->set_tid = args.set_tid;
> > >
> > > Hm, the if-condition is not needed though, right? At this point we will
> > > have already copied from struct clone_args __user *uargs into struct
> > > clone_args args. If we hit that codepath that means the kernel
> > > definitely has a field for set_tid in its struct clone_args. :) So this
> > > could probably just be:
> > >
> > > .tls = args.tls,
> > > .set_tid = args.set_tid,
> > > }
> > >
> > > ?
> >
> > Right.
> >
> > > > +
> > > > return 0;
> > > > }
> > > >
> > > > diff --git a/kernel/pid.c b/kernel/pid.c
> > > > index 0a9f2e437217..9ce89c35c5be 100644
> > > > --- a/kernel/pid.c
> > > > +++ b/kernel/pid.c
> > > > @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
> > > > call_rcu(&pid->rcu, delayed_put_pid);
> > > > }
> > > >
> > > > -struct pid *alloc_pid(struct pid_namespace *ns)
> > > > +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> > > > {
> > > > struct pid *pid;
> > > > enum pid_type type;
> > > > @@ -186,12 +186,32 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> > > > if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> > > > pid_min = RESERVED_PIDS;
> > > >
> > > > - /*
> > > > - * Store a null pointer so find_pid_ns does not find
> > > > - * a partially initialized PID (see below).
> > > > - */
> > > > - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > > > - pid_max, GFP_ATOMIC);
> > > > + if (set_tid) {
> > > > + /*
> > > > + * Also fail if a PID != 1 is requested
> > > > + * and no PID 1 exists.
> > > > + */
> > > > + nr = -EINVAL;
> > > > + if (set_tid < pid_max && set_tid > 0 &&
> > >
> > > Hm, you're already in the if-branch hat verified if (set_tid) so the
> > > set_tid > 0 conjunct seems redundant. :)
> >
> > Yes, but I dropped all checks to see if set_tid is negative as suggested
> > by Oleg and moved it here.
> >
> > > > + (set_tid == 1 || !idr_is_empty(&tmp->idr)))
> > > > + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> > > > + set_tid + 1, GFP_ATOMIC);
> > >
> > > I'm confused, shouldn't this be
> > >
> > > if (set_tid < pid_max || (set_tid == 1 && !idr_is_emtpy(&tmp->idf)))
> >
> > Now I am also confused ;). This does not work. This will always return
> > true if set_tid is less than pid_max. So pid_max needs to be something
> > like 1 for the check after || to make sense, right? But you really got
> > me confused here right now. Right now I still think what I did is
> > correct.
>
> I missed the part where you reset set_tid to 0 below and mis-parsed the
> rightmost conjunct in the if statement.
>
> One thing I dislike is that you do
>
> if (set_tid) {
> if ([...] && set_tid > 0 && [...])
> }
Thinking about this a bit more you probably did the explicit set_tid > 0 to
catch the case where it is negative? But also, I don't understand why we do any
work at all before having verified that set_tid is sensible, i.e. why do we
call kmem_cache_calloc(), do idr_preload(), and take a spinlock, before even
verifying that our parameters are sane? If there's no specific reason for this
I suggest to patch alloc_pid() like this:
-struct pid *alloc_pid(struct pid_namespace *ns)
+struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
{
struct pid *pid;
enum pid_type type;
@@ -166,6 +166,9 @@ struct pid *alloc_pid(struct pid_namespace *ns)
struct upid *upid;
int retval = -ENOMEM;
+ if (set_tid < 0 || set_tid >= pid_max)
+ return ERR_PTR(-EINVAL);
+
pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
if (!pid)
return ERR_PTR(retval);
@@ -186,12 +189,31 @@ struct pid *alloc_pid(struct pid_namespace *ns)
if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
pid_min = RESERVED_PIDS;
- /*
- * Store a null pointer so find_pid_ns does not find
- * a partially initialized PID (see below).
- */
- nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
- pid_max, GFP_ATOMIC);
+ if (set_tid) {
+ /*
+ * Also fail if a PID != 1 is requested
+ * and no PID 1 exists.
+ */
+ nr = -EINVAL;
+ if (set_tid == 1 || !idr_is_empty(&tmp->idr))
+ nr = idr_alloc(&tmp->idr, NULL, set_tid,
+ set_tid + 1, GFP_ATOMIC);
+ /*
+ * If ENOSPC is returned it means that the PID is
+ * alreay in use. Return EEXIST in that case.
+ */
+ if (nr == -ENOSPC)
+ nr = -EEXIST;
+ /* Only use set_tid for one PID namespace. */
+ set_tid = 0;
+ } else {
+ /*
+ * Store a null pointer so find_pid_ns does not find
+ * a partially initialized PID (see below).
+ */
+ nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
+ pid_max, GFP_ATOMIC);
+ }
spin_unlock_irq(&pidmap_lock);
idr_preload_end()
This makes things a lot more clearer in my opinion. First, verify that the
pre-conditions are met. Second, verify that the conditions are met which depend
on the state of the pid namespace, i.e. there's either already a pid 1 or pid 1
is requested.
We should also do this since alloc_pid() is exported in a header file and so we
can't and shouldn't rely on the fact that all callers will pass in something
sensible for set_tid.
>
> For consistency this should rather be:
>
> if (set_tid) {
> if ([...] && set_tid && [...])
> }
>
> Overall I'd rather write the second if as:
>
> if (set_tid && set_tid < pid_max && (set_tid == 1 || !idr_is_empty(&tmp->idr)))
>
> to place the most basic condition (i.e. set_tid is non-zero) leftmost.
>
> The right-most conjunct (set_tid == 1 || !idr_is_empty(&tmp->idr)) is
> non-trivial to parse. Someone not familiar with that code would need
> time to figure out why and how exactly 1 is special-cased.
> I think overall this is begging for alloc_pid() to be documented now if
> you don't mind. :)
>
> Christian
On Sun, Aug 11, 2019 at 09:06:59PM +0200, Christian Brauner wrote:
> On Sun, Aug 11, 2019 at 08:51:48AM +0200, Christian Brauner wrote:
> > On Sat, Aug 10, 2019 at 07:59:18AM +0200, Adrian Reber wrote:
> > > On Sat, Aug 10, 2019 at 03:10:34AM +0200, Christian Brauner wrote:
> > > > On Thu, Aug 08, 2019 at 11:22:21PM +0200, Adrian Reber wrote:
> > > > > The main motivation to add set_tid to clone3() is CRIU.
> > > > >
> > > > > To restore a process with the same PID/TID CRIU currently uses
> > > > > /proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
> > > > > ns_last_pid and then (quickly) does a clone(). This works most of the
> > > > > time, but it is racy. It is also slow as it requires multiple syscalls.
> > > > >
> > > > > Extending clone3() to support set_tid makes it possible restore a
> > > > > process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> > > > > race free (as long as the desired PID/TID is available).
> > > > >
> > > > > This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
> > > > > on clone3() with set_tid as they are currently in place for ns_last_pid.
> > > > >
> > > > > Signed-off-by: Adrian Reber <[email protected]>
> > > > > ---
> > > > > v2:
> > > > > - Removed (size < sizeof(struct clone_args)) as discussed with
> > > > > Christian and Dmitry
> > > > > - Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
> > > > > - Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)
> > > > >
> > > > > v3:
> > > > > - Return EEXIST if PID is already in use (Christian)
> > > > > - Drop CLONE_SET_TID (Christian and Oleg)
> > > > > - Use idr_is_empty() instead of idr_get_cursor() (Oleg)
> > > > > - Handle different `struct clone_args` sizes (Dmitry)
> > > > >
> > > > > v4:
> > > > > - Rework struct size check with defines (Christian)
> > > > > - Reduce number of set_tid checks (Oleg)
> > > > > - Less parentheses and more robust code (Oleg)
> > > > > - Do ns_capable() on correct user_ns (Oleg, Christian)
> > > > > ---
> > > > > include/linux/pid.h | 2 +-
> > > > > include/linux/sched/task.h | 1 +
> > > > > include/uapi/linux/sched.h | 1 +
> > > > > kernel/fork.c | 25 +++++++++++++++++++++++--
> > > > > kernel/pid.c | 34 +++++++++++++++++++++++++++-------
> > > > > 5 files changed, 53 insertions(+), 10 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/pid.h b/include/linux/pid.h
> > > > > index 2a83e434db9d..052000db0ced 100644
> > > > > --- a/include/linux/pid.h
> > > > > +++ b/include/linux/pid.h
> > > > > @@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
> > > > > extern struct pid *find_get_pid(int nr);
> > > > > extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
> > > > >
> > > > > -extern struct pid *alloc_pid(struct pid_namespace *ns);
> > > > > +extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
> > > > > extern void free_pid(struct pid *pid);
> > > > > extern void disable_pid_allocation(struct pid_namespace *ns);
> > > > >
> > > > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > > > > index 0497091e40c1..4f2a80564332 100644
> > > > > --- a/include/linux/sched/task.h
> > > > > +++ b/include/linux/sched/task.h
> > > > > @@ -26,6 +26,7 @@ struct kernel_clone_args {
> > > > > unsigned long stack;
> > > > > unsigned long stack_size;
> > > > > unsigned long tls;
> > > > > + pid_t set_tid;
> > > > > };
> > > > >
> > > > > /*
> > > > > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> > > > > index b3105ac1381a..e1ce103a2c47 100644
> > > > > --- a/include/uapi/linux/sched.h
> > > > > +++ b/include/uapi/linux/sched.h
> > > > > @@ -45,6 +45,7 @@ struct clone_args {
> > > > > __aligned_u64 stack;
> > > > > __aligned_u64 stack_size;
> > > > > __aligned_u64 tls;
> > > > > + __aligned_u64 set_tid;
> > > > > };
> > > > >
> > > > > /*
> > > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > > index 2852d0e76ea3..2a03f0e201e9 100644
> > > > > --- a/kernel/fork.c
> > > > > +++ b/kernel/fork.c
> > > > > @@ -117,6 +117,13 @@
> > > > > */
> > > > > #define MAX_THREADS FUTEX_TID_MASK
> > > > >
> > > > > +/*
> > > > > + * Different sizes of struct clone_args
> > > > > + */
> > > > > +#define CLONE3_ARGS_SIZE_V0 64
> > > > > +/* V1 includes set_tid */
> > > > > +#define CLONE3_ARGS_SIZE_V1 72
> > > > > +
> > > > > /*
> > > > > * Protected counters by write_lock_irq(&tasklist_lock)
> > > > > */
> > > > > @@ -2031,7 +2038,13 @@ static __latent_entropy struct task_struct *copy_process(
> > > > > stackleak_task_init(p);
> > > > >
> > > > > if (pid != &init_struct_pid) {
> > > > > - pid = alloc_pid(p->nsproxy->pid_ns_for_children);
> > > > > + if (args->set_tid && !ns_capable(
> > > > > + p->nsproxy->pid_ns_for_children->user_ns,
> > > > > + CAP_SYS_ADMIN)) {
> > > > > + retval = -EPERM;
> > > > > + goto bad_fork_cleanup_thread;
> > > > > + }
> > > > > + pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
> > > > > if (IS_ERR(pid)) {
> > > > > retval = PTR_ERR(pid);
> > > > > goto bad_fork_cleanup_thread;
> > > > > @@ -2535,9 +2548,14 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > > > > if (unlikely(size > PAGE_SIZE))
> > > > > return -E2BIG;
> > > > >
> > > > > - if (unlikely(size < sizeof(struct clone_args)))
> > > > > + /* The struct needs to be at least the size of the original struct. */
> > > >
> > > > I don't think you need that comment. I think the macro is pretty
> > > > self-explanatory. If you want it to be even clearer you could even make
> > > > it CLONE3_ARGS_SIZE_MIN but V0 is good enough. :)
> > >
> > > Will remove the comment.
> > >
> > > > > + if (unlikely(size < CLONE3_ARGS_SIZE_V0))
> > > > > return -EINVAL;
> > > > >
> > > > > + if (size < sizeof(struct clone_args))
> > > > > + memset((void *)&args + size, 0,
> > > > > + sizeof(struct clone_args) - size);
> > > > > +
> > > > > if (unlikely(!access_ok(uargs, size)))
> > > > > return -EFAULT;
> > > > >
> > > > > @@ -2573,6 +2591,9 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> > > > > .tls = args.tls,
> > > > > };
> > > > >
> > > > > + if (size >= CLONE3_ARGS_SIZE_V1)
> > > > > + kargs->set_tid = args.set_tid;
> > > >
> > > > Hm, the if-condition is not needed though, right? At this point we will
> > > > have already copied from struct clone_args __user *uargs into struct
> > > > clone_args args. If we hit that codepath that means the kernel
> > > > definitely has a field for set_tid in its struct clone_args. :) So this
> > > > could probably just be:
> > > >
> > > > .tls = args.tls,
> > > > .set_tid = args.set_tid,
> > > > }
> > > >
> > > > ?
> > >
> > > Right.
> > >
> > > > > +
> > > > > return 0;
> > > > > }
> > > > >
> > > > > diff --git a/kernel/pid.c b/kernel/pid.c
> > > > > index 0a9f2e437217..9ce89c35c5be 100644
> > > > > --- a/kernel/pid.c
> > > > > +++ b/kernel/pid.c
> > > > > @@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
> > > > > call_rcu(&pid->rcu, delayed_put_pid);
> > > > > }
> > > > >
> > > > > -struct pid *alloc_pid(struct pid_namespace *ns)
> > > > > +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> > > > > {
> > > > > struct pid *pid;
> > > > > enum pid_type type;
> > > > > @@ -186,12 +186,32 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> > > > > if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> > > > > pid_min = RESERVED_PIDS;
> > > > >
> > > > > - /*
> > > > > - * Store a null pointer so find_pid_ns does not find
> > > > > - * a partially initialized PID (see below).
> > > > > - */
> > > > > - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > > > > - pid_max, GFP_ATOMIC);
> > > > > + if (set_tid) {
> > > > > + /*
> > > > > + * Also fail if a PID != 1 is requested
> > > > > + * and no PID 1 exists.
> > > > > + */
> > > > > + nr = -EINVAL;
> > > > > + if (set_tid < pid_max && set_tid > 0 &&
> > > >
> > > > Hm, you're already in the if-branch hat verified if (set_tid) so the
> > > > set_tid > 0 conjunct seems redundant. :)
> > >
> > > Yes, but I dropped all checks to see if set_tid is negative as suggested
> > > by Oleg and moved it here.
> > >
> > > > > + (set_tid == 1 || !idr_is_empty(&tmp->idr)))
> > > > > + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> > > > > + set_tid + 1, GFP_ATOMIC);
> > > >
> > > > I'm confused, shouldn't this be
> > > >
> > > > if (set_tid < pid_max || (set_tid == 1 && !idr_is_emtpy(&tmp->idf)))
> > >
> > > Now I am also confused ;). This does not work. This will always return
> > > true if set_tid is less than pid_max. So pid_max needs to be something
> > > like 1 for the check after || to make sense, right? But you really got
> > > me confused here right now. Right now I still think what I did is
> > > correct.
> >
> > I missed the part where you reset set_tid to 0 below and mis-parsed the
> > rightmost conjunct in the if statement.
> >
> > One thing I dislike is that you do
> >
> > if (set_tid) {
> > if ([...] && set_tid > 0 && [...])
> > }
>
> Thinking about this a bit more you probably did the explicit set_tid > 0 to
> catch the case where it is negative? But also, I don't understand why we do any
> work at all before having verified that set_tid is sensible, i.e. why do we
> call kmem_cache_calloc(), do idr_preload(), and take a spinlock, before even
> verifying that our parameters are sane? If there's no specific reason for this
> I suggest to patch alloc_pid() like this:
>
> -struct pid *alloc_pid(struct pid_namespace *ns)
> +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> {
> struct pid *pid;
> enum pid_type type;
> @@ -166,6 +166,9 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> struct upid *upid;
> int retval = -ENOMEM;
>
> + if (set_tid < 0 || set_tid >= pid_max)
> + return ERR_PTR(-EINVAL);
> +
> pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
> if (!pid)
> return ERR_PTR(retval);
> @@ -186,12 +189,31 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> pid_min = RESERVED_PIDS;
>
> - /*
> - * Store a null pointer so find_pid_ns does not find
> - * a partially initialized PID (see below).
> - */
> - nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> - pid_max, GFP_ATOMIC);
> + if (set_tid) {
> + /*
> + * Also fail if a PID != 1 is requested
> + * and no PID 1 exists.
> + */
> + nr = -EINVAL;
> + if (set_tid == 1 || !idr_is_empty(&tmp->idr))
> + nr = idr_alloc(&tmp->idr, NULL, set_tid,
> + set_tid + 1, GFP_ATOMIC);
> + /*
> + * If ENOSPC is returned it means that the PID is
> + * alreay in use. Return EEXIST in that case.
> + */
> + if (nr == -ENOSPC)
> + nr = -EEXIST;
> + /* Only use set_tid for one PID namespace. */
> + set_tid = 0;
> + } else {
> + /*
> + * Store a null pointer so find_pid_ns does not find
> + * a partially initialized PID (see below).
> + */
> + nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> + pid_max, GFP_ATOMIC);
> + }
> spin_unlock_irq(&pidmap_lock);
> idr_preload_end()
>
> This makes things a lot more clearer in my opinion. First, verify that the
> pre-conditions are met. Second, verify that the conditions are met which depend
> on the state of the pid namespace, i.e. there's either already a pid 1 or pid 1
> is requested.
> We should also do this since alloc_pid() is exported in a header file and so we
> can't and shouldn't rely on the fact that all callers will pass in something
> sensible for set_tid.
I like that. I will update the patch and send it out to match this
suggestion.
Adrian