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Subject[PATCH RFC] pidns: introduce syscall getvpid
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pid_t getvpid(pid_t pid, pid_t source, pid_t target);

This syscall converts pid from one pid-ns into pid in another pid-ns:
it takes @pid in namespace of @source task (zero for current) and
returns related pid in namespace of @target task (zero for current too).
If pid is unreachable from target pid-ns then it returns zero.

Such conversion is required for interaction between processes from
different pid-namespaces. For example when system service talks with
client from isolated container via socket about task in container:

getvpid(pid, client_pid, 0) -> pid in our pid namespace
getvpid(pid, 0, client_pid) -> pid in client pid namespace

Also service can get pid of init task and match it with container:

getvpid(1, client_pid, 0) -> pid of init task for client_pid

Seems like gdb and strace could use this too for converting pids of
newly forked tasks (IIRR they get pid from %rax) into pid from
correct namespace for further interaction.

As a bonus syscall getvpid can compare pid namespaces and
test isolation without mounted procfs:

getvpid(1, 0, pid) == 0 -> pid in our sub-pid-namespace
getvpid(1, 0, pid) == 1 -> pid in our pid-namespace
getvpid(1, pid1, pid2) == 0 -> pid1 isolated from pid2
getvpid(1, pid1, pid2) == 1 -> tasks are in one pid-namespace
getvpid(1, pid1, pid2) > 1 -> pid1 is in sub-pidns of pid2

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
include/linux/syscalls.h | 1 +
kernel/pid.c | 36 ++++++++++++++++++++++++++++++++
4 files changed, 39 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 7663c455b9f6..dadb55d42fc9 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -382,3 +382,4 @@
373 i386 shutdown sys_shutdown
374 i386 userfaultfd sys_userfaultfd
375 i386 membarrier sys_membarrier
+376 i386 getvpid sys_getvpid
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 278842fdf1f6..0338f2eb3b7c 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -331,6 +331,7 @@
322 64 execveat stub_execveat
323 common userfaultfd sys_userfaultfd
324 common membarrier sys_membarrier
+325 common getvpid sys_getvpid

#
# 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 a460e2ef2843..3405c30999e3 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -222,6 +222,7 @@ asmlinkage long sys_nanosleep(struct timespec __user *rqtp, struct timespec __us
asmlinkage long sys_alarm(unsigned int seconds);
asmlinkage long sys_getpid(void);
asmlinkage long sys_getppid(void);
+asmlinkage long sys_getvpid(pid_t pid, pid_t source, pid_t target);
asmlinkage long sys_getuid(void);
asmlinkage long sys_geteuid(void);
asmlinkage long sys_getgid(void);
diff --git a/kernel/pid.c b/kernel/pid.c
index ca368793808e..caa676ff7364 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -567,6 +567,42 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
return pid;
}

+/**
+ * sys_getvpid - convert pid from one pid-namespace into pid from another
+ *
+ * @pid - pid of requested task
+ * @source - pid of task in source pid-namespace, zero for current
+ * @target - pid of task in target pid-namespace, zero for current
+ *
+ * Returns pid from target pid-ns or zero if pid is unreachable.
+ * Returns -ESRCH if some of pids are not found.
+ */
+SYSCALL_DEFINE3(getvpid, pid_t, pid, pid_t, source, pid_t, target)
+{
+#ifdef CONFIG_PID_NS
+ struct pid_namespace *current_ns = task_active_pid_ns(current);
+ struct pid_namespace *source_ns = current_ns, *target_ns = current_ns;
+ struct pid *task_pid;
+ pid_t result = -ESRCH;
+
+ rcu_read_lock();
+ if (source)
+ source_ns = ns_of_pid(find_pid_ns(source, current_ns));
+ if (target)
+ target_ns = ns_of_pid(find_pid_ns(target, current_ns));
+ if (source_ns && target_ns) {
+ task_pid = find_pid_ns(pid, source_ns);
+ if (task_pid)
+ result = pid_nr_ns(task_pid, target_ns);
+ }
+ rcu_read_unlock();
+
+ return result;
+#else
+ return pid;
+#endif /* CONFIG_PID_NS */
+}
+
/*
* The pid hash table is scaled according to the amount of memory in the
* machine. From a minimum of 16 slots up to 4096 slots at one gigabyte or


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