With the addition of pidfd_open() it is possible for users to reference a
specific thread by doing:
int pidfd = pidfd_open(<tid>, 0);
This means we can extend pidfd_send_signal() to signal a specific thread.
As promised in the commit for pidfd_send_signal() [1] the extension is
based on a flag argument, i.e. the scope of the signal delivery is based on
the flag argument, not on the type of file descriptor.
To this end the flag PIDFD_SIGNAL_TID is added. With this change we now
cover most of the functionality of all the other signal sending functions
combined:
- pidfd_send_signal(<pidfd>, <sig>, NULL, 0);
which is equivalent to
kill(<positive-pid>, <signal>)
- pidfd_send_signal(<pidfd>, <sig>, <info>, 0);
which is equivalent to
rt_sigqueueinfo(<tgid>, <sig>, <uinfo>)
- pidfd_send_signal(<pidfd>, <sig>, NULL, PIDFD_SIGNAL_TID);
which is equivalent to
tgkill(<tgid>, <tid>, <signal)
rt_tgsigqueueinfo(<tgid>, <tid>, <sig>, <uinfo>)
- pidfd_send_signal(<pidfd>, <sig>, <info>, PIDFD_SIGNAL_TID);
which is equivalent to
rt_tgsigqueueinfo(<tgid>, <tid>, <sig>, <uinfo>)
/* References */
[1]: commit 3eb39f47934 ("signal: add pidfd_send_signal() syscall")
Signed-off-by: Christian Brauner <christ...@brauner.io>
Cc: Arnd Bergmann <a...@arndb.de>
Cc: "Eric W. Biederman" <ebied...@xmission.com>
Cc: Kees Cook <keesc...@chromium.org>
Cc: Alexey Dobriyan <adobri...@gmail.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Jann Horn <ja...@google.com
Cc: "Michael Kerrisk (man-pages)" <mtk.manpa...@gmail.com>
Cc: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
Cc: Jonathan Kowalski <bl0pbl...@gmail.com>
Cc: "Dmitry V. Levin" <l...@altlinux.org>
Cc: Andy Lutomirsky <l...@kernel.org>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Oleg Nesterov <o...@redhat.com>
Cc: Nagarathnam Muthusamy <nagarathnam.muthus...@oracle.com>
Cc: Aleksa Sarai <cyp...@cyphar.com>
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Florian Weimer <fwei...@redhat.com>
---
include/uapi/linux/wait.h | 3 +
kernel/signal.c | 116 ++++++++++++++++++++++++++------------
2 files changed, 82 insertions(+), 37 deletions(-)
diff --git a/include/uapi/linux/wait.h b/include/uapi/linux/wait.h
index d6c7c0701997..b72f0ef84fe5 100644
--- a/include/uapi/linux/wait.h
+++ b/include/uapi/linux/wait.h
@@ -21,4 +21,7 @@
/* Get a file descriptor for /proc/<pid> of the corresponding pidfd */
#define PIDFD_GET_PROCFD _IOR('p', 1, int)
+/* Flags to pass to pidfd_send_signal */
+#define PIDFD_SIGNAL_TID 1 /* Send signal to specific thread */
+
#endif /* _UAPI_LINUX_WAIT_H */
diff --git a/kernel/signal.c b/kernel/signal.c
index eb97d0cc6ef7..9f93da85b2b9 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3557,17 +3557,86 @@ static struct pid *pidfd_to_pid(const struct file *file)
return tgid_pidfd_to_pid(file);
}
+static int __do_send_specific(struct task_struct *p, int sig,
+ struct kernel_siginfo *info)
+{
+ int error = -ESRCH;
+
+ error = check_kill_permission(sig, info, p);
+ /*
+ * The null signal is a permissions and process existence probe.
+ * No signal is actually delivered.
+ */
+ if (!error && sig) {
+ error = do_send_sig_info(sig, info, p, PIDTYPE_PID);
+ /*
+ * If lock_task_sighand() failed we pretend the task
+ * dies after receiving the signal. The window is tiny,
+ * and the signal is private anyway.
+ */
+ if (unlikely(error == -ESRCH))
+ error = 0;
+ }
+
+ return error;
+}
+
+static int do_send_specific(pid_t tgid, pid_t pid, int sig,
+ struct kernel_siginfo *info)
+{
+ struct task_struct *p;
+ int error = -ESRCH;
+
+ rcu_read_lock();
+ p = find_task_by_vpid(pid);
+ if (p && (tgid <= 0 || task_tgid_vnr(p) == tgid))
+ error = __do_send_specific(p, sig, info);
+ rcu_read_unlock();
+
+ return error;
+}
+
+static int pidfd_send_signal_specific(struct pid *pid, int sig,
+ struct kernel_siginfo *info)
+{
+ struct task_struct *p;
+ int error = -ESRCH;
+
+ rcu_read_lock();
+ p = pid_task(pid, PIDTYPE_PID);
+ if (p)
+ error = __do_send_specific(p, sig, info);
+ rcu_read_unlock();
+
+ return error;
+}
+
/**
- * sys_pidfd_send_signal - send a signal to a process through a task file
- * descriptor
+ * sys_pidfd_send_signal - send a signal to a process through a pidfd
+
* @pidfd: the file descriptor of the process
* @sig: signal to be sent
* @info: the signal info
* @flags: future flags to be passed
*
- * The syscall currently only signals via PIDTYPE_PID which covers
- * kill(<positive-pid>, <signal>. It does not signal threads or process
- * groups.
+ * The syscall currently covers:
+ * - pidfd_send_signal(<pidfd>, <sig>, NULL, 0);
+ * which is equivalent to
+ * kill(<positive-pid>, <signal>)
+ *
+ * - pidfd_send_signal(<pidfd>, <sig>, <info>, 0);
+ * which is equivalent to
+ * rt_sigqueueinfo(<tgid>, <sig>, <uinfo>)
+ *
+ * - pidfd_send_signal(<pidfd>, <sig>, NULL, PIDFD_SIGNAL_TID);
+ * which is equivalent to
+ * tgkill(<tgid>, <tid>, <signal)
+ *
+ * rt_tgsigqueueinfo(<tgid>, <tid>, <sig>, <uinfo>)
+ * - pidfd_send_signal(<pidfd>, <sig>, <info>, PIDFD_SIGNAL_TID);
+ * which is equivalent to
+ * rt_tgsigqueueinfo(<tgid>, <tid>, <sig>, <uinfo>)
+ *
* In order to extend the syscall to threads and process groups the @flags
* argument should be used. In essence, the @flags argument will determine
* what is signaled and not the file descriptor itself. Put in other words,
@@ -3585,7 +3654,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
kernel_siginfo_t kinfo;
/* Enforce flags be set to 0 until we add an extension. */
- if (flags)
+ if (flags & ~PIDFD_SIGNAL_TID)
return -EINVAL;
f = fdget(pidfd);
@@ -3626,43 +3695,16 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
prepare_kill_siginfo(sig, &kinfo);
}
- ret = kill_pid_info(sig, &kinfo, pid);
+ if (flags & PIDFD_SIGNAL_TID)
+ ret = pidfd_send_signal_specific(pid, sig, &kinfo);
+ else
+ ret = kill_pid_info(sig, &kinfo, pid);
err:
fdput(f);
return ret;
}
-static int
-do_send_specific(pid_t tgid, pid_t pid, int sig, struct kernel_siginfo *info)
-{
- struct task_struct *p;
- int error = -ESRCH;
-
- rcu_read_lock();
- p = find_task_by_vpid(pid);
- if (p && (tgid <= 0 || task_tgid_vnr(p) == tgid)) {
- error = check_kill_permission(sig, info, p);
- /*
- * The null signal is a permissions and process existence
- * probe. No signal is actually delivered.
- */
- if (!error && sig) {
- error = do_send_sig_info(sig, info, p, PIDTYPE_PID);
- /*
- * If lock_task_sighand() failed we pretend the task
- * dies after receiving the signal. The window is tiny,
- * and the signal is private anyway.
- */
- if (unlikely(error == -ESRCH))
- error = 0;
- }
- }
- rcu_read_unlock();
-
- return error;
-}
-
static int do_tkill(pid_t tgid, pid_t pid, int sig)
{
struct kernel_siginfo info;
--
2.21.0
