[RFC][v6][PATCH 8/9]: Define clone_with_pids() syscall

[Sukadev Bhattiprolu]

Subject: [RFC][v6][PATCH 8/9]: Define clone_with_pids() syscall

Container restart requires that a task have the same pid it had when it was
checkpointed. When containers are nested the tasks within the containers
exist in multiple pid namespaces and hence have multiple pids to specify
during restart.

clone_with_pids(), intended for use during restart, is the same as clone(),
except that it takes a 'pid_set' paramter. This parameter lets caller choose
specific pid numbers for the child process, in the process's active and
ancestor pid namespaces. (Descendant pid namespaces in general don't matter
since processes don't have pids in them anyway, but see comments in
copy_target_pids() regarding CLONE_NEWPID).

Unlike clone(), clone_with_pids() needs CAP_SYS_ADMIN, at least for now, to
prevent unprivileged processes from misusing this interface.

Call clone_with_pids as follows:

	struct pid_set pid_set = { 3, {0, 97, 99} };
	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.

NOTE:
	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 pid_set'.


TODO:
	- May need additional sanity checks in do_fork_with_pids().

Changelog[v6]:
	- (Nathan Lynch, Arnd Bergmann, H. Peter Anvin, Linus Torvalds)
	  Change 'pid_set.pids' to a 'pid_t pids[]' so size of 'struct pid_set'
	  is constant across architectures.
	- (Nathan Lynch) Change pid_set.num_pids to unsigned and remove
	  'unum_pids < 0' check.

Changelog[v4]:
	- (Oren Laadan) rename 'struct target_pid_set' to 'struct pid_set'

Changelog[v3]:
	- (Oren Laadan) Allow CLONE_NEWPID flag (by allocating an extra pid
	  in the target_pids[] list and setting it 0. See copy_target_pids()).
	- (Oren Laadan) Specified target pids should apply only to youngest
	  pid-namespaces (see copy_target_pids())
	- (Matt Helsley) Update patch description.

Changelog[v2]:
	- Remove unnecessary printk and add a note to callers of
	  copy_target_pids() to free target_pids.
	- (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>
---
 arch/x86/include/asm/syscalls.h    |    2 
 arch/x86/include/asm/unistd_32.h   |    1 
 arch/x86/kernel/entry_32.S         |    1 
 arch/x86/kernel/process_32.c       |   21 +++++++
 arch/x86/kernel/syscall_table_32.S |    1 
 kernel/fork.c                      |  107 ++++++++++++++++++++++++++++++++++++-
 6 files changed, 132 insertions(+), 1 deletion(-)

Index: linux-2.6/arch/x86/include/asm/syscalls.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/syscalls.h	2009-09-09 19:06:20.000000000 -0700
+++ linux-2.6/arch/x86/include/asm/syscalls.h	2009-09-09 19:07:53.000000000 -0700
@@ -40,6 +40,8 @@ long sys_iopl(struct pt_regs *);
 
 /* kernel/process_32.c */
 int sys_clone(struct pt_regs *);
+int sys_clone_with_pids(struct pt_regs *);
+int sys_vfork(struct pt_regs *);
 int sys_execve(struct pt_regs *);
 
 /* kernel/signal.c */
Index: linux-2.6/arch/x86/include/asm/unistd_32.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/unistd_32.h	2009-09-09 19:06:20.000000000 -0700
+++ linux-2.6/arch/x86/include/asm/unistd_32.h	2009-09-09 19:07:53.000000000 -0700
@@ -342,6 +342,7 @@
 #define __NR_pwritev		334
 #define __NR_rt_tgsigqueueinfo	335
 #define __NR_perf_counter_open	336
+#define __NR_clone_with_pids	337
 
 #ifdef __KERNEL__
 
Index: linux-2.6/arch/x86/kernel/entry_32.S
===================================================================
--- linux-2.6.orig/arch/x86/kernel/entry_32.S	2009-09-09 19:06:20.000000000 -0700
+++ linux-2.6/arch/x86/kernel/entry_32.S	2009-09-09 19:07:53.000000000 -0700
@@ -718,6 +718,7 @@ ptregs_##name: \
 PTREGSCALL(iopl)
 PTREGSCALL(fork)
 PTREGSCALL(clone)
+PTREGSCALL(clone_with_pids)
 PTREGSCALL(vfork)
 PTREGSCALL(execve)
 PTREGSCALL(sigaltstack)
Index: linux-2.6/arch/x86/kernel/process_32.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/process_32.c	2009-09-09 19:06:20.000000000 -0700
+++ linux-2.6/arch/x86/kernel/process_32.c	2009-09-09 21:30:45.000000000 -0700
@@ -443,6 +443,27 @@ int sys_clone(struct pt_regs *regs)
 	return do_fork(clone_flags, newsp, regs, 0, parent_tidptr, child_tidptr);
 }
 
+int sys_clone_with_pids(struct pt_regs *regs)
+{
+	unsigned long clone_flags;
+	unsigned long newsp;
+	int __user *parent_tidptr;
+	int __user *child_tidptr;
+	struct pid_set __user *upid_setp;
+
+	clone_flags = regs->bx;
+	newsp = regs->cx;
+	parent_tidptr = (int __user *)regs->dx;
+	child_tidptr = (int __user *)regs->di;
+	upid_setp = (struct pid_set __user *)regs->bp;
+
+	if (!newsp)
+		newsp = regs->sp;
+
+	return do_fork_with_pids(clone_flags, newsp, regs, 0, parent_tidptr,
+			child_tidptr, upid_setp);
+}
+
 /*
  * sys_execve() executes a new program.
  */
Index: linux-2.6/arch/x86/kernel/syscall_table_32.S
===================================================================
--- linux-2.6.orig/arch/x86/kernel/syscall_table_32.S	2009-09-09 19:06:20.000000000 -0700
+++ linux-2.6/arch/x86/kernel/syscall_table_32.S	2009-09-09 19:07:53.000000000 -0700
@@ -336,3 +336,4 @@ ENTRY(sys_call_table)
 	.long sys_pwritev
 	.long sys_rt_tgsigqueueinfo	/* 335 */
 	.long sys_perf_counter_open
+	.long ptregs_clone_with_pids
Index: linux-2.6/kernel/fork.c
===================================================================
--- linux-2.6.orig/kernel/fork.c	2009-09-09 19:07:45.000000000 -0700
+++ linux-2.6/kernel/fork.c	2009-09-09 21:30:45.000000000 -0700
@@ -1327,6 +1327,96 @@ struct task_struct * __cpuinit fork_idle
 }
 
 /*
+ * If user specified any 'target-pids' in @upid_setp, copy them from
+ * user and return a pointer to a local copy of the list of pids. The
+ * caller must free the list, when they are done using it.
+ *
+ * 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(struct pid_set __user *upid_setp)
+{
+	int j;
+	int rc;
+	int size;
+	int unum_pids;		/* # of pids specified by user */
+	int knum_pids;		/* # of pids needed in kernel */
+	pid_t *target_pids;
+
+	if (!upid_setp)
+		return NULL;
+
+	knum_pids = task_pid(current)->level + 1;
+
+	/* First find the number of pids user is specifying */
+	rc = copy_from_user(&unum_pids, &upid_setp->num_pids, sizeof(int));
+	if (rc)
+		return ERR_PTR(-EFAULT);
+
+	if (!unum_pids)
+		return NULL;
+
+	if (unum_pids > knum_pids)
+		return ERR_PTR(-EINVAL);
+
+	/*
+	 * To keep alloc_pid() simple, allocate an extra pid_t in target_pids[]
+	 * and set it to 0. This last entry in target_pids[] corresponds to the
+	 * (yet-to-be-created) descendant pid-namespace if CLONE_NEWPID was
+	 * specified. If CLONE_NEWPID was not specified, this last entry will
+	 * simply be ignored.
+	 */
+	target_pids = kzalloc((knum_pids + 1) * sizeof(pid_t), GFP_KERNEL);
+	if (!target_pids)
+		return ERR_PTR(-ENOMEM);
+
+	/*
+	 * A process running in a level 2 pid namespace has three pid namespaces
+	 * and hence three pid numbers. If this process is checkpointed,
+	 * information about these three namespaces are saved. We refer to these
+	 * namespaces as 'known namespaces'.
+	 *
+	 * If this checkpointed process is however restarted in a level 3 pid
+	 * namespace, the restarted process has an extra ancestor pid namespace
+	 * (i.e 'unknown namespace') and 'knum_pids' exceeds 'unum_pids'.
+	 *
+	 * During restart, the process requests specific pids for its 'known
+	 * namespaces' and lets kernel assign pids to its 'unknown namespaces'.
+	 *
+	 * Since the requested-pids correspond to 'known namespaces' and since
+	 * 'known-namespaces' are younger than (i.e descendants of) 'unknown-
+	 * namespaces', copy requested pids to the back-end of target_pids[]
+	 * (i.e before the last entry for CLONE_NEWPID mentioned above).
+	 * Any entries in target_pids[] not corresponding to a requested pid
+	 * will be set to zero and kernel assigns a pid in those namespaces.
+	 *
+	 * NOTE: The order of pids in target_pids[] is oldest pid namespace to
+	 * 	 youngest (target_pids[0] corresponds to init_pid_ns). i.e.
+	 * 	 the order is:
+	 *
+	 * 		- pids for 'unknown-namespaces' (if any)
+	 * 		- pids for 'known-namespaces' (requested pids)
+	 * 		- 0 in the last entry (for CLONE_NEWPID).
+	 */
+	j = knum_pids - unum_pids;
+	size = unum_pids * sizeof(pid_t);
+
+	rc = copy_from_user(&target_pids[j], &upid_setp->pids[0], size);
+	if (rc) {
+		rc = -EFAULT;
+		goto out_free;
+	}
+
+	return target_pids;
+
+out_free:
+	kfree(target_pids);
+	return ERR_PTR(rc);
+}
+
+/*
  *  Ok, this is the main fork-routine.
  *
  * It copies the process, and if successful kick-starts
@@ -1343,7 +1433,7 @@ long do_fork_with_pids(unsigned long clo
 	struct task_struct *p;
 	int trace = 0;
 	long nr;
-	pid_t *target_pids = NULL;
+	pid_t *target_pids;
 
 	/*
 	 * Do some preliminary argument and permissions checking before we
@@ -1377,6 +1467,17 @@ long do_fork_with_pids(unsigned long clo
 		}
 	}
 
+	target_pids = copy_target_pids(pid_setp);
+
+	if (target_pids) {
+		if (IS_ERR(target_pids))
+			return PTR_ERR(target_pids);
+
+		nr = -EPERM;
+		if (!capable(CAP_SYS_ADMIN))
+			goto out_free;
+	}
+
 	/*
 	 * When called from kernel_thread, don't do user tracing stuff.
 	 */
@@ -1438,6 +1539,10 @@ long do_fork_with_pids(unsigned long clo
 	} else {
 		nr = PTR_ERR(p);
 	}
+
+out_free:
+	kfree(target_pids);
+
 	return nr;
 }
 
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