Sukadev Bhattiprolu wrote:
> From: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
> Date: Mon, 4 May 2009 01:17:45 -0700
> Subject: [RFC][PATCH 7/7][v2] Define clone_with_pids syscall
>
> clone_with_pids() is same as clone(), except that it takes a 'target_pid_set'
> paramter which lets caller choose a specific pid number for the child process
> in each of the child process's pid namespace. This system call would be needed
> to implement Checkpoint/Restart (i.e after a checkpoint, restart a process with
> its original pids).
>
> Call clone_with_pids as follows:
>
> pid_t pids[] = { 0, 77, 99 };
> struct target_pid_set pid_set;
>
> pid_set.num_pids = sizeof(pids) / sizeof(int);
> pid_set.target_pids = &pids;
>
> syscall(__NR_clone_with_pids, flags, stack, NULL, NULL, NULL, &pid_set);
>
> If a target-pid is 0, the kernel continues to assign a pid for the process in
> that namespace. In the above example, pids[0] is 0, meaning the kernel will
> assign next available pid to the process in init_pid_ns. But kernel will assign
> pid 77 in the child pid namespace 1 and pid 99 in pid namespace 2. If either
> 77 or 99 are taken, the system call fails with -EBUSY.
>
> If 'pid_set.num_pids' exceeds the current nesting level of pid namespaces,
> the system call fails with -EINVAL.
>
> Its mostly an exploratory patch seeking feedback on the interface.
>
> NOTE:
> 1. clone_with_pids(), at least for now, needs CAP_SYS_ADMIN to prevent
> misuse of the interface.
>
> 2. Compared to clone(), clone_with_pids() needs to pass in two more
> pieces of information:
>
> - number of pids in the set
> - user buffer containing the list of pids.
>
> But since clone() already takes 5 parameters, use a 'struct
> target_pid_set'.
>
> TODO:
> - Gently tested.
> - May need additional sanity checks in do_fork_with_pids().
> - Allow CLONE_NEWPID() with clone_with_pids() (ensure target-pid in
> the namespace is either 1 or 0).
>
> Changelog[v2]:
> - (Serge Hallyn) Mention CAP_SYS_ADMIN restriction in patch description.
> - (Oren Laadan) Add checks for 'num_pids < 0' (return -EINVAL) and
> 'num_pids == 0' (fall back to normal clone()).
> - Move arch-independent code (sanity checks and copy-in of target-pids)
> into kernel/fork.c and simplify sys_clone_with_pids()
>
> Changelog[v1]:
> - Fixed some compile errors (had fixed these errors earlier in my
> git tree but had not refreshed patches before emailing them)
>
> Signed-off-by: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
[...]
> index a16ef7b..f265a18 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1335,6 +1335,58 @@ struct task_struct * __cpuinit fork_idle(int cpu)
> }
>
> /*
> + * If user specified any 'target-pids' in @upid_setp, copy them from
> + * user and return a pointer to the list of target pids.
> + *
> + * If user did not specify any target pids, return NULL (caller should
> + * treat this like normal clone).
> + *
> + * On any errors, return the error code
> + */
> +static pid_t *copy_target_pids(void __user *upid_setp)
> +{
> + int rc;
> + int size;
> + int num_pids;
> + pid_t __user *utarget_pids;
> + pid_t *target_pids;
> + struct target_pid_set pid_set;
> +
> + if(!upid_setp)
> + return NULL;
> +
> + if (copy_from_user(&pid_set, upid_setp, sizeof(pid_set)))
> + return ERR_PTR(-EFAULT);
> +
> + num_pids = pid_set.num_pids;
> + utarget_pids = pid_set.target_pids;
> + size = num_pids * sizeof(pid_t);
> +
> + if (!num_pids)
> + return NULL;
> +
> + if (num_pids < 0 || num_pids > task_pid(current)->level + 1)
> + return ERR_PTR(-EINVAL);
What happens if (num_pids < task_pid(current->level) + 1) ?
Unless I missed something elsewhere, I suspect it will oops,
because in patch #4, you had:
for (i = ns->level; i >= 0; i--) {
- nr = alloc_pidmap(tmp, 0);
+ tpid = 0;
+ if (target_pids)
+ tpid = target_pids[i];
+
+ nr = alloc_pidmap(tmp, tpid);
if (nr < 0)
goto out_free;
In general, can a task figure out it's depth in the pid-ns hierarchy ?
I'm thinking of a case in which a checkpoint was taken of a (flat)
container which is in depth 2 of the global hierarchy, and then it
is restarted in depth 3, or in depth 1.
Perhaps the semantics of the syscall should be that target_pids will
indicate the desired pids _from the bottom up_. A value of "0" means
"don't care" and let the kernel assign. The remaining levels (upwards)
will be treated as zeros.
[...]
Oren.
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