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Message-ID: <20240210-abfinden-beimessen-2dbfea59b0da@brauner>
Date: Sat, 10 Feb 2024 11:23:12 +0100
From: Christian Brauner <brauner@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Andy Lutomirski <luto@...capital.net>,
"Eric W. Biederman" <ebiederm@...ssion.com>, Tycho Andersen <tycho@...ho.pizza>, linux-api@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] pidfd: change pidfd_send_signal() to respect
PIDFD_THREAD
> 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@...nel.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@...nel.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
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