[PATCH v2] ns: introduce getnspid syscall

[Chen Hanxiao <chenhanxiao-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>]

We need a direct method of getting the pid inside containers.
If some issues occurred inside container guest, host user
could not know which process is in trouble just by guest pid:
the users of container guest only knew the pid inside containers.
This will bring obstacle for trouble shooting.

int getnspid(pid_t pid, int fd1, int fd2);

pid: the pid number need to be translated.

fd: a file descriptor referring to one of
    the namespace entries in a /proc/[pid]/ns/pid.
    fd1 for destination ns(ns1), where the pid came from.
    fd2 for reference ns(ns2), while fd2 = -2 means for current ns.

return value:
    >0 : translated pid in ns1(fd1) seen from ns2(fd2).
    <=0: on failure.

Signed-off-by: Chen Hanxiao <chenhanxiao-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
---
v2: drop pidtype
    check ns_ops before getting pid_namespace structure

 arch/x86/syscalls/syscall_32.tbl |  1 +
 arch/x86/syscalls/syscall_64.tbl |  1 +
 include/linux/syscalls.h         |  1 +
 kernel/nsproxy.c                 | 45 ++++++++++++++++++++++++++++++++++++++++
 4 files changed, 48 insertions(+)

diff --git a/arch/x86/syscalls/syscall_32.tbl b/arch/x86/syscalls/syscall_32.tbl
index d6b8679..9de0b32 100644
--- a/arch/x86/syscalls/syscall_32.tbl
+++ b/arch/x86/syscalls/syscall_32.tbl
@@ -360,3 +360,4 @@
 351	i386	sched_setattr		sys_sched_setattr
 352	i386	sched_getattr		sys_sched_getattr
 353	i386	renameat2		sys_renameat2
+354	i386	getnspid		sys_getnspid
diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl
index ec255a1..1630a8a 100644
--- a/arch/x86/syscalls/syscall_64.tbl
+++ b/arch/x86/syscalls/syscall_64.tbl
@@ -323,6 +323,7 @@
 314	common	sched_setattr		sys_sched_setattr
 315	common	sched_getattr		sys_sched_getattr
 316	common	renameat2		sys_renameat2
+317	common	getnspid		sys_getnspid
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index b0881a0..53dd0e8 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -866,4 +866,5 @@ asmlinkage long sys_process_vm_writev(pid_t pid,
 asmlinkage long sys_kcmp(pid_t pid1, pid_t pid2, int type,
 			 unsigned long idx1, unsigned long idx2);
 asmlinkage long sys_finit_module(int fd, const char __user *uargs, int flags);
+asmlinkage long sys_getpidns(pid_t pid, int fd1, int fd2);
 #endif
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 8e78110..9701ade 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -261,6 +261,51 @@ out:
 	return err;
 }
 
+SYSCALL_DEFINE3(getnspid, pid_t, pid, int, fd1, int, fd2)
+{
+	struct file *file1 = NULL, *file2 = NULL;
+	struct pid *pid_struct;
+	struct pid_namespace *ns1, *ns2;
+	struct proc_ns *ei;
+	int ret = -EINVAL;
+
+	file1 = proc_ns_fget(fd1);
+	if (IS_ERR(file1))
+		return PTR_ERR(file1);
+	ei = get_proc_ns(file_inode(file1));
+	if (ei->ns_ops != &pidns_operations)
+		goto out;
+	ns1 = (struct pid_namespace *)ei->ns;
+
+	/* fd == -2 for current pid ns */
+	if (fd2 == -2) {
+		ns2 = task_active_pid_ns(current);
+	} else {
+		file2 = proc_ns_fget(fd2);
+		if (IS_ERR(file2)) {
+			fput(file1);
+			return PTR_ERR(file2);
+		}
+		ei = get_proc_ns(file_inode(file2));
+		if (ei->ns_ops != &pidns_operations)
+			goto out;
+		ns2 = (struct pid_namespace *)ei->ns;
+	}
+
+	pid_struct = find_pid_ns(pid, ns1);
+	if (!pid_struct) {
+		ret = -ESRCH;
+		goto out;
+	}
+
+	ret = pid_nr_ns(pid_struct, ns2);
+out:
+	fput(file1);
+	if (file2)
+		fput(file2);
+	return ret;
+}
+
 int __init nsproxy_cache_init(void)
 {
 	nsproxy_cachep = KMEM_CACHE(nsproxy, SLAB_PANIC);
-- 
1.9.0