pidfd: add PIDFD_SELF* sentinels to refer to own thread/process

It is useful to be able to utilise the pidfd mechanism to reference the
current thread or process (from a userland point of view - thread group
leader from the kernel's point of view).

Therefore introduce PIDFD_SELF_THREAD to refer to the current thread, and
PIDFD_SELF_THREAD_GROUP to refer to the current thread group leader.

For convenience and to avoid confusion from userland's perspective we alias
these:

* PIDFD_SELF is an alias for PIDFD_SELF_THREAD - This is nearly always what
  the user will want to use, as they would find it surprising if for
  instance fd's were unshared()'d and they wanted to invoke pidfd_getfd()
  and that failed.

* PIDFD_SELF_PROCESS is an alias for PIDFD_SELF_THREAD_GROUP - Most users
  have no concept of thread groups or what a thread group leader is, and
  from userland's perspective and nomenclature this is what userland
  considers to be a process.

We adjust pidfd_get_task() and the pidfd_send_signal() system call with
specific handling for this, implementing this functionality for
process_madvise(), process_mrelease() (albeit, using it here wouldn't
really make sense) and pidfd_send_signal().

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/r/24315a16a3d01a548dd45c7515f7d51c767e954e.1738268370.git.lorenzo.stoakes@oracle.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 63 insertions, 42 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 875e97f6205a..081f19a24506 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -4009,6 +4009,47 @@ static struct pid *pidfd_to_pid(const struct file *file)
 	(PIDFD_SIGNAL_THREAD | PIDFD_SIGNAL_THREAD_GROUP | \
 	 PIDFD_SIGNAL_PROCESS_GROUP)
 
+static int do_pidfd_send_signal(struct pid *pid, int sig, enum pid_type type,
+				siginfo_t __user *info, unsigned int flags)
+{
+	kernel_siginfo_t kinfo;
+
+	switch (flags) {
+	case PIDFD_SIGNAL_THREAD:
+		type = PIDTYPE_PID;
+		break;
+	case PIDFD_SIGNAL_THREAD_GROUP:
+		type = PIDTYPE_TGID;
+		break;
+	case PIDFD_SIGNAL_PROCESS_GROUP:
+		type = PIDTYPE_PGID;
+		break;
+	}
+
+	if (info) {
+		int ret;
+
+		ret = copy_siginfo_from_user_any(&kinfo, info);
+		if (unlikely(ret))
+			return ret;
+
+		if (unlikely(sig != kinfo.si_signo))
+			return -EINVAL;
+
+		/* Only allow sending arbitrary signals to yourself. */
+		if ((task_pid(current) != pid || type > PIDTYPE_TGID) &&
+		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
+			return -EPERM;
+	} else {
+		prepare_kill_siginfo(sig, &kinfo, type);
+	}
+
+	if (type == PIDTYPE_PGID)
+		return kill_pgrp_info(sig, &kinfo, pid);
+
+	return kill_pid_info_type(sig, &kinfo, pid, type);
+}
+
 /**
  * sys_pidfd_send_signal - Signal a process through a pidfd
  * @pidfd:  file descriptor of the process
@@ -4026,9 +4067,7 @@ 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;
 	struct pid *pid;
-	kernel_siginfo_t kinfo;
 	enum pid_type type;
 
 	/* Enforce flags be set to 0 until we add an extension. */
@@ -4039,57 +4078,39 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 	if (hweight32(flags & PIDFD_SEND_SIGNAL_FLAGS) > 1)
 		return -EINVAL;
 
-	CLASS(fd, f)(pidfd);
-	if (fd_empty(f))
-		return -EBADF;
+	switch (pidfd) {
+	case PIDFD_SELF_THREAD:
+		pid = get_task_pid(current, PIDTYPE_PID);
+		type = PIDTYPE_PID;
+		break;
+	case PIDFD_SELF_THREAD_GROUP:
+		pid = get_task_pid(current, PIDTYPE_TGID);
+		type = PIDTYPE_TGID;
+		break;
+	default: {
+		CLASS(fd, f)(pidfd);
+		if (fd_empty(f))
+			return -EBADF;
 
-	/* Is this a pidfd? */
-	pid = pidfd_to_pid(fd_file(f));
-	if (IS_ERR(pid))
-		return PTR_ERR(pid);
+		/* Is this a pidfd? */
+		pid = pidfd_to_pid(fd_file(f));
+		if (IS_ERR(pid))
+			return PTR_ERR(pid);
 
-	if (!access_pidfd_pidns(pid))
-		return -EINVAL;
+		if (!access_pidfd_pidns(pid))
+			return -EINVAL;
 
-	switch (flags) {
-	case 0:
 		/* Infer scope from the type of pidfd. */
 		if (fd_file(f)->f_flags & PIDFD_THREAD)
 			type = PIDTYPE_PID;
 		else
 			type = PIDTYPE_TGID;
-		break;
-	case PIDFD_SIGNAL_THREAD:
-		type = PIDTYPE_PID;
-		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);
-		if (unlikely(ret))
-			return ret;
-
-		if (unlikely(sig != kinfo.si_signo))
-			return -EINVAL;
-
-		/* Only allow sending arbitrary signals to yourself. */
-		if ((task_pid(current) != pid || type > PIDTYPE_TGID) &&
-		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
-			return -EPERM;
-	} else {
-		prepare_kill_siginfo(sig, &kinfo, type);
+		return do_pidfd_send_signal(pid, sig, type, info, flags);
+	}
 	}
 
-	if (type == PIDTYPE_PGID)
-		return kill_pgrp_info(sig, &kinfo, pid);
-	else
-		return kill_pid_info_type(sig, &kinfo, pid, type);
+	return do_pidfd_send_signal(pid, sig, type, info, flags);
 }
 
 static int