[PATCH] [RFC] pidfd: allow to override signal scope in pidfd_send_signal()

[Christian Brauner]

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
(2) PIDFD_SIGNAL_THREAD_GROUP: send signal to thread-group

I've put off PIDFD_SIGNAL_PROCESS_GROUP for now since I need to stare at
the code a bit longer how this would work.

Link: https://github.com/systemd/systemd/issues/31093 [1]
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
---
 include/uapi/linux/pidfd.h |  4 ++++
 kernel/signal.c            | 35 ++++++++++++++++++++++++++++-------
 2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
index 2e6461459877..757ed5a668c6 100644
--- a/include/uapi/linux/pidfd.h
+++ b/include/uapi/linux/pidfd.h
@@ -10,4 +10,8 @@
 #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)
+
 #endif /* _UAPI_LINUX_PIDFD_H */
diff --git a/kernel/signal.c b/kernel/signal.c
index 9578ce17d85d..1d6586964099 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3872,6 +3872,9 @@ static struct pid *pidfd_to_pid(const struct file *file)
 	return tgid_pidfd_to_pid(file);
 }
 
+#define PIDFD_SEND_SIGNAL_FLAGS \
+	(PIDFD_SIGNAL_THREAD | PIDFD_SIGNAL_THREAD_GROUP)
+
 /**
  * sys_pidfd_send_signal - Signal a process through a pidfd
  * @pidfd:  file descriptor of the process
@@ -3889,14 +3892,19 @@ static struct pid *pidfd_to_pid(const struct file *file)
 SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 		siginfo_t __user *, info, unsigned int, flags)
 {
-	int ret;
+	int ret, si_code;
 	struct fd f;
 	struct pid *pid;
 	kernel_siginfo_t kinfo;
 	bool thread;
+	enum pid_type si_scope;
 
 	/* 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);
@@ -3914,7 +3922,22 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	if (!access_pidfd_pidns(pid))
 		goto err;
 
-	thread = f.file->f_flags & PIDFD_THREAD;
+	switch (flags) {
+	case 0:
+		/* Infer scope from the type of pidfd. */
+		thread = (f.file->f_flags & PIDFD_THREAD);
+		si_scope = thread ? PIDTYPE_PID : PIDTYPE_TGID;
+		si_code = thread ? SI_TKILL : SI_USER;
+		break;
+	case PIDFD_SIGNAL_THREAD:
+		si_scope = PIDTYPE_PID;
+		si_code = SI_TKILL;
+		break;
+	case PIDFD_SIGNAL_THREAD_GROUP:
+		si_scope = PIDTYPE_TGID;
+		si_code = SI_USER;
+		break;
+	}
 
 	if (info) {
 		ret = copy_siginfo_from_user_any(&kinfo, info);
@@ -3931,12 +3954,10 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
 			goto err;
 	} else {
-		prepare_kill_siginfo(sig, &kinfo,
-				     thread ? SI_TKILL : SI_USER);
+		prepare_kill_siginfo(sig, &kinfo, si_code);
 	}
 
-	ret = kill_pid_info_type(sig, &kinfo, pid,
-				 thread ? PIDTYPE_PID : PIDTYPE_TGID);
+	ret = kill_pid_info_type(sig, &kinfo, pid, si_scope);
 err:
 	fdput(f);
 	return ret;
-- 
2.43.0