On Sat, 14 Oct 2017 11:17:47 +0300 Konstantin Khlebnikov [off-list ref] wrote:

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pid_t translate_pid(pid_t pid, int source, int target);

This syscall converts pid from source pid-ns into pid in target pid-ns.
If pid is unreachable from target pid-ns it returns zero.

Pid-namespaces are referred file descriptors opened to proc files
/proc/[pid]/ns/pid or /proc/[pid]/ns/pid_for_children. Negative argument
refers to current pid namespace, same as file /proc/self/ns/pid.

Kernel expose virtual pids in /proc/[pid]/status:NSpid, but backward
translation requires scanning all tasks. Also pids could be translated
by sending them through unix socket between namespaces, this method is
slow and insecure because other side is exposed inside pid namespace.

Andrew asked why we might need this.

Such conversion is required for interaction between processes across pid-namespaces.
For example to identify process in container by pid file looking from outside.

Two years ago I've solved this in project of mine with monstrous code which
forks couple times just to convert pid, lucky for me performance wasn't important.

That's a single user who needed this a single time, and found a
userspace-based solution anyway.  This is not exactly compelling!

Is there a stronger case to be made?  How does this change benefit our
users?  Sell it to us!