Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect PIDFD_THREAD

[Christian Brauner <brauner@kernel.org>]

> and I forgot to mention, if you want to add PIDFD_SIGNAL_PRGP you can
> look at __kill_pgrp_info().

Yeah, I did that and there's a semantical twist in the old kill(2)
system call that made me think:

(1) kill(-1234) => kill process group with id 1234
(2) kill(0)     => kill process group of @current

which implementation wise is indicated by

__kill_pgrp_info(..., pid ? find_vpid(-pid) ? task_pgrp(current))

We're obviously not going to implement (2) as that doesn't really make a
sense for pidfd_send_signal().

But (1) is also wrong for pidfd_send_signal(). If we'd ever implement
(1) it should be via pidfd_open(1234, PIDFD_PROCESS_GROUP).

So if PIDFD_PROCESS_GROUP is set then we want to send a signal to the
process group that @pidfd is in. Unless there's reasons we can't do
this. I tried to draft this and what I have is a totally uncompiled
draft.

I was unsure how best to cleanly express how to take the process group
from the struct pid of that @pidfd. So I modeled it after how we do it
for PIDTYPE_TGID.

From 8d886b07cc1b17cc6dd3a9ebf19c51212282b6f5 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Fri, 9 Feb 2024 15:49:45 +0100
Subject: [PATCH] [RFC] pidfd: allow to override signal scope in
 pidfd_send_signal()

Right now we determine the scope of the signal based on the type of
pidfd. There are use-cases where it's useful to override the scope of
the signal. For example in [1]. Add flags to determine the scope of the
signal:

(1) PIDFD_SIGNAL_THREAD: send signal to specific thread reference by @pidfd
(2) PIDFD_SIGNAL_THREAD_GROUP: send signal to thread-group of @pidfd
(2) PIDFD_SIGNAL_PROCESS_GROUP: send signal to process-group of @pidfd

There's a semantical quirk in the old kill(2) system call that made me think:

(1) kill(-1234) => kill process group with id 1234
(2) kill(0)     => kill process group of @current

as indicated by

__kill_pgrp_info(..., pid ? find_vpid(-pid) ? task_pgrp(current))

We're obviously not going to implement (2) as that doesn't really make a
lot of sense for pidfd_send_signal().

But (1) is also wrong for pidfd_send_signal(). If we'd ever implement
(1) it should be via pidfd_open(..., PIDFD_PROCESS_GROUP).

Link: https://github.com/systemd/systemd/issues/31093 [1]
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/uapi/linux/pidfd.h |  5 +++++
 kernel/signal.c            | 46 ++++++++++++++++++++++++++++++++++----
 2 files changed, 47 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
index 2e6461459877..72ec000a97cd 100644
--- a/include/uapi/linux/pidfd.h
+++ b/include/uapi/linux/pidfd.h
@@ -10,4 +10,9 @@
 #define PIDFD_NONBLOCK	O_NONBLOCK
 #define PIDFD_THREAD	O_EXCL
 
+/* Flags for pidfd_send_signal(). */
+#define PIDFD_SIGNAL_THREAD		(1UL << 0)
+#define PIDFD_SIGNAL_THREAD_GROUP	(1UL << 1)
+#define PIDFD_SIGNAL_PROCESS_GROUP	(1UL << 2)
+
 #endif /* _UAPI_LINUX_PIDFD_H */
diff --git a/kernel/signal.c b/kernel/signal.c
index 8b8169623850..f0f9a5a822b4 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3873,6 +3873,23 @@ static struct pid *pidfd_to_pid(const struct file *file)
 	return tgid_pidfd_to_pid(file);
 }
 
+static int kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pid)
+{
+	struct task_struct *p;
+	int ret = -ESRCH;
+
+	read_lock(&tasklist_lock);
+	p = pid_task(pid, PIDTYPE_PID);
+	if (p)
+		ret = __kill_pgrp_info(sig, info, task_pgrp(p));
+	read_unlock(&tasklist_lock);
+	return ret;
+}
+
+#define PIDFD_SEND_SIGNAL_FLAGS                            \
+	(PIDFD_SIGNAL_THREAD | PIDFD_SIGNAL_THREAD_GROUP | \
+	 PIDFD_SIGNAL_PROCESS_GROUP)
+
 /**
  * sys_pidfd_send_signal - Signal a process through a pidfd
  * @pidfd:  file descriptor of the process
@@ -3897,7 +3914,11 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	enum pid_type type;
 
 	/* Enforce flags be set to 0 until we add an extension. */
-	if (flags)
+	if (flags & ~PIDFD_SEND_SIGNAL_FLAGS)
+		return -EINVAL;
+
+	/* Ensure that only a single signal scope determining flag is set. */
+	if (hweight32(flags & PIDFD_SEND_SIGNAL_FLAGS) > 1)
 		return -EINVAL;
 
 	f = fdget(pidfd);
@@ -3915,10 +3936,24 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	if (!access_pidfd_pidns(pid))
 		goto err;
 
-	if (f.file->f_flags & PIDFD_THREAD)
+	switch (flags) {
+	case 0:
+		/* Infer scope from the type of pidfd. */
+		if (f.file->f_flags & PIDFD_THREAD)
+			type = PIDTYPE_PID;
+		else
+			type = PIDTYPE_TGID;
+		break;
+	case PIDFD_SIGNAL_THREAD:
 		type = PIDTYPE_PID;
-	else
+		break;
+	case PIDFD_SIGNAL_THREAD_GROUP:
 		type = PIDTYPE_TGID;
+		break;
+	case PIDFD_SIGNAL_PROCESS_GROUP:
+		type = PIDTYPE_PGID;
+		break;
+	}
 
 	if (info) {
 		ret = copy_siginfo_from_user_any(&kinfo, info);
@@ -3938,7 +3973,10 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 		prepare_kill_siginfo(sig, &kinfo, type);
 	}
 
-	ret = kill_pid_info_type(sig, &kinfo, pid, type);
+	if (type == PIDFD_SIGNAL_PROCESS_GROUP)
+		ret = kill_pgrp_info(sig, &kinfo, pid);
+	else
+		ret = kill_pid_info_type(sig, &kinfo, pid, type);
 err:
 	fdput(f);
 	return ret;
-- 
2.43.0