Date:   Mon, 18 Mar 2019 19:50:52 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Christian Brauner <christian@...uner.io>
Cc:     Daniel Colascione <dancol@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Tim Murray <timmurray@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-mm <linux-mm@...ck.org>,
        kernel-team <kernel-team@...roid.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

On Mon, Mar 18, 2019 at 01:29:51AM +0100, Christian Brauner wrote:
> On Sun, Mar 17, 2019 at 08:40:19AM -0700, Daniel Colascione wrote:
> > On Sun, Mar 17, 2019 at 4:42 AM Christian Brauner <christian@...uner.io> wrote:
> > >
> > > On Sat, Mar 16, 2019 at 09:53:06PM -0400, Joel Fernandes wrote:
> > > > On Sat, Mar 16, 2019 at 12:37:18PM -0700, Suren Baghdasaryan wrote:
> > > > > On Sat, Mar 16, 2019 at 11:57 AM Christian Brauner <christian@...uner.io> wrote:
> > > > > >
> > > > > > On Sat, Mar 16, 2019 at 11:00:10AM -0700, Daniel Colascione wrote:
> > > > > > > On Sat, Mar 16, 2019 at 10:31 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > > > > > >
> > > > > > > > On Fri, Mar 15, 2019 at 11:49 AM Joel Fernandes <joel@...lfernandes.org> wrote:
> > > > > > > > >
> > > > > > > > > On Fri, Mar 15, 2019 at 07:24:28PM +0100, Christian Brauner wrote:
> > > > > > > > > [..]
> > > > > > > > > > > why do we want to add a new syscall (pidfd_wait) though? Why not just use
> > > > > > > > > > > standard poll/epoll interface on the proc fd like Daniel was suggesting.
> > > > > > > > > > > AFAIK, once the proc file is opened, the struct pid is essentially pinned
> > > > > > > > > > > even though the proc number may be reused. Then the caller can just poll.
> > > > > > > > > > > We can add a waitqueue to struct pid, and wake up any waiters on process
> > > > > > > > > > > death (A quick look shows task_struct can be mapped to its struct pid) and
> > > > > > > > > > > also possibly optimize it using Steve's TIF flag idea. No new syscall is
> > > > > > > > > > > needed then, let me know if I missed something?
> > > > > > > > > >
> > > > > > > > > > Huh, I thought that Daniel was against the poll/epoll solution?
> > > > > > > > >
> > > > > > > > > Hmm, going through earlier threads, I believe so now. Here was Daniel's
> > > > > > > > > reasoning about avoiding a notification about process death through proc
> > > > > > > > > directory fd: http://lkml.iu.edu/hypermail/linux/kernel/1811.0/00232.html
> > > > > > > > >
> > > > > > > > > May be a dedicated syscall for this would be cleaner after all.
> > > > > > > >
> > > > > > > > Ah, I wish I've seen that discussion before...
> > > > > > > > syscall makes sense and it can be non-blocking and we can use
> > > > > > > > select/poll/epoll if we use eventfd.
> > > > > > >
> > > > > > > Thanks for taking a look.
> > > > > > >
> > > > > > > > I would strongly advocate for
> > > > > > > > non-blocking version or at least to have a non-blocking option.
> > > > > > >
> > > > > > > Waiting for FD readiness is *already* blocking or non-blocking
> > > > > > > according to the caller's desire --- users can pass options they want
> > > > > > > to poll(2) or whatever. There's no need for any kind of special
> > > > > > > configuration knob or non-blocking option. We already *have* a
> > > > > > > non-blocking option that works universally for everything.
> > > > > > >
> > > > > > > As I mentioned in the linked thread, waiting for process exit should
> > > > > > > work just like waiting for bytes to appear on a pipe. Process exit
> > > > > > > status is just another blob of bytes that a process might receive. A
> > > > > > > process exit handle ought to be just another information source. The
> > > > > > > reason the unix process API is so awful is that for whatever reason
> > > > > > > the original designers treated processes as some kind of special kind
> > > > > > > of resource instead of fitting them into the otherwise general-purpose
> > > > > > > unix data-handling API. Let's not repeat that mistake.
> > > > > > >
> > > > > > > > Something like this:
> > > > > > > >
> > > > > > > > evfd = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC);
> > > > > > > > // register eventfd to receive death notification
> > > > > > > > pidfd_wait(pid_to_kill, evfd);
> > > > > > > > // kill the process
> > > > > > > > pidfd_send_signal(pid_to_kill, ...)
> > > > > > > > // tend to other things
> > > > > > >
> > > > > > > Now you've lost me. pidfd_wait should return a *new* FD, not wire up
> > > > > > > an eventfd.
> > > > > > >
> > > > >
> > > > > Ok, I probably misunderstood your post linked by Joel. I though your
> > > > > original proposal was based on being able to poll a file under
> > > > > /proc/pid and then you changed your mind to have a separate syscall
> > > > > which I assumed would be a blocking one to wait for process exit.
> > > > > Maybe you can describe the new interface you are thinking about in
> > > > > terms of userspace usage like I did above? Several lines of code would
> > > > > explain more than paragraphs of text.
> > > >
> > > > Hey, Thanks Suren for the eventfd idea. I agree with Daniel on this. The idea
> > > > from Daniel here is to wait for process death and exit events by just
> > > > referring to a stable fd, independent of whatever is going on in /proc.
> > > >
> > > > What is needed is something like this (in highly pseudo-code form):
> > > >
> > > > pidfd = opendir("/proc/<pid>",..);
> > > > wait_fd = pidfd_wait(pidfd);
> > > > read or poll wait_fd (non-blocking or blocking whichever)
> > > >
> > > > wait_fd will block until the task has either died or reaped. In both these
> > > > cases, it can return a suitable string such as "dead" or "reaped" although an
> > > > integer with some predefined meaning is also Ok.
> > 
> > I want to return a siginfo_t: we already use this structure in other
> > contexts to report exit status.
> > 

Fine with me. I did a prototype (code is below) as a string but I can change
that to siginfo_t in the future.

> > > Having pidfd_wait() return another fd will make the syscall harder to
> > > swallow for a lot of people I reckon.
> > > What exactly prevents us from making the pidfd itself readable/pollable
> > > for the exit staus? They are "special" fds anyway. I would really like
> > > to avoid polluting the api with multiple different types of fds if possible.
> > 
> > If pidfds had been their own file type, I'd agree with you. But pidfds
> > are directories, which means that we're beholden to make them behave
> > like directories normally do. I'd rather introduce another FD than
> > heavily overload the semantics of a directory FD in one particular
> > context. In no other circumstances are directory FDs also weird
> > IO-data sources. Our providing a facility to get a new FD to which we
> > *can* give pipe-like behavior does no harm and *usage* cleaner and
> > easier to reason about.
> 
> I have two things I'm currently working on:
> - hijacking translate_pid()
> - pidfd_clone() essentially
> 
> My first goal is to talk to Eric about taking the translate_pid()
> syscall that has been sitting in his tree and expanding it.
> translate_pid() currently allows you to either get an fd for the pid
> namespace a pid resides in or the pid number of a given process in
> another pid namespace relative to a passed in pid namespace fd.

That's good to know. More comments below:

> I would
> like to make it possible for this syscall to also give us back pidfds.
> One question I'm currently struggling with is exactly what you said
> above: what type of file descriptor these are going to give back to us.
> It seems that a regular file instead of directory would make the most
> sense and would lead to a nicer API and I'm very much leaning towards
> that.

How about something like the following? We can plumb the new file as a pseudo
file that is invisible and linked to the fd. This is extremely rough (does
not do error handling, synchronizatoin etc) but just wanted to share the idea
of what the "frontend" could look like. It is also missing all the actual pid
status messages. It just takes care of the creating new fd from the pidfd
part and providing file read ops returning the "status" string.  It is also
written in signal.c and should likely go into proc fs files under fs.
Appreciate any suggestions (a test program did prove it works).

Also, I was able to translate a pidfd to a pid_namespace by referring to some
existing code but perhaps you may be able to suggest something better for
such translation..

---8<-----------------------

From: Joel Fernandes <joelaf@...gle.com>
Subject: [PATCH] Partial skeleton prototype of pidfd_wait frontend

Signed-off-by: Joel Fernandes <joelaf@...gle.com>
---
 arch/x86/entry/syscalls/syscall_32.tbl |  1 +
 arch/x86/entry/syscalls/syscall_64.tbl |  1 +
 include/linux/syscalls.h               |  1 +
 include/uapi/asm-generic/unistd.h      |  4 +-
 kernel/signal.c                        | 62 ++++++++++++++++++++++++++
 kernel/sys_ni.c                        |  3 ++
 6 files changed, 71 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 1f9607ed087c..2a63f1896b63 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -433,3 +433,4 @@
 425	i386	io_uring_setup		sys_io_uring_setup		__ia32_sys_io_uring_setup
 426	i386	io_uring_enter		sys_io_uring_enter		__ia32_sys_io_uring_enter
 427	i386	io_uring_register	sys_io_uring_register		__ia32_sys_io_uring_register
+428	i386	pidfd_wait		sys_pidfd_wait			__ia32_sys_pidfd_wait
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 92ee0b4378d4..cf2e08a8053b 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -349,6 +349,7 @@
 425	common	io_uring_setup		__x64_sys_io_uring_setup
 426	common	io_uring_enter		__x64_sys_io_uring_enter
 427	common	io_uring_register	__x64_sys_io_uring_register
+428	common	pidfd_wait		__x64_sys_pidfd_wait
 
 #
 # 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 e446806a561f..62160970ed3f 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -988,6 +988,7 @@ asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
 asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
 				       siginfo_t __user *info,
 				       unsigned int flags);
+asmlinkage long sys_pidfd_wait(int pidfd);
 
 /*
  * Architecture-specific system calls
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index dee7292e1df6..137aa8662230 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -832,9 +832,11 @@ __SYSCALL(__NR_io_uring_setup, sys_io_uring_setup)
 __SYSCALL(__NR_io_uring_enter, sys_io_uring_enter)
 #define __NR_io_uring_register 427
 __SYSCALL(__NR_io_uring_register, sys_io_uring_register)
+#define __NR_pidfd_wait 428
+__SYSCALL(__NR_pidfd_wait, sys_pidfd_wait)
 
 #undef __NR_syscalls
-#define __NR_syscalls 428
+#define __NR_syscalls 429
 
 /*
  * 32 bit systems traditionally used different
diff --git a/kernel/signal.c b/kernel/signal.c
index b7953934aa99..ebb550b87044 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3550,6 +3550,68 @@ static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
 	return copy_siginfo_from_user(kinfo, info);
 }
 
+static ssize_t pidfd_wait_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+	/*
+	 * This is just a test string, it will contain the actual
+	 * status of the pidfd in the future.
+	 */
+	char buf[] = "status";
+
+	return copy_to_iter(buf, strlen(buf)+1, to);
+}
+
+static const struct file_operations pidfd_wait_file_ops = {
+	.read_iter	= pidfd_wait_read_iter,
+};
+
+static struct inode *pidfd_wait_get_inode(struct super_block *sb)
+{
+	struct inode *inode = new_inode(sb);
+
+	inode->i_ino = get_next_ino();
+	inode_init_owner(inode, NULL, S_IFREG);
+
+	inode->i_op		= &simple_dir_inode_operations;
+	inode->i_fop		= &pidfd_wait_file_ops;
+
+	return inode;
+}
+
+SYSCALL_DEFINE1(pidfd_wait, int, pidfd)
+{
+	struct fd f;
+	struct inode *inode;
+	struct file *file;
+	int new_fd;
+	struct pid_namespace *pid_ns;
+	struct super_block *sb;
+	struct vfsmount *mnt;
+
+	f = fdget_raw(pidfd);
+	if (!f.file)
+		return -EBADF;
+
+	sb = file_inode(f.file)->i_sb;
+	pid_ns = sb->s_fs_info;
+
+	inode = pidfd_wait_get_inode(sb);
+
+	mnt = pid_ns->proc_mnt;
+
+	file = alloc_file_pseudo(inode, mnt, "pidfd_wait", O_RDONLY,
+			&pidfd_wait_file_ops);
+
+	file->f_mode |= FMODE_PREAD;
+
+	new_fd = get_unused_fd_flags(0);
+	fd_install(new_fd, file);
+
+	fdput(f);
+
+	return new_fd;
+}
+
 /**
  * sys_pidfd_send_signal - send a signal to a process through a task file
  *                          descriptor
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index d21f4befaea4..f52c4d864038 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -450,3 +450,6 @@ COND_SYSCALL(setuid16);
 
 /* restartable sequence */
 COND_SYSCALL(rseq);
+
+/* pidfd */
+COND_SYSCALL(pidfd_wait);
-- 
2.21.0.225.g810b269d1ac-goog

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