[PATCH v5 0/1] signaling processes through pidfds

Hey everyone,

This is v5 of this patchset.
v5 does not introduce any functional changes since none were requested
or required in the thread. Instead, it focusses on updated documentation
making it very clear what the intentions are how to extend this syscall.

Eric, I dragged Serge into this and we went through the mails and tried
to very thoroughly address your concerns about whether the width of the
target should depend on flags or file descriptor types. As far as we
understand from the threads this was your ultimate worry and also the
reason why you withheld your agreement to the name of the syscall.
We have outlined how the syscall is intended to be extended and decided
that flags (e.g. PIDFD_TYPE_TID, PIDFD_TYPE_PGID) are the way to go. In
line with this we decided to accept "pidfd_" as prefix for the new
syscall.
All concerns we could identify and understand we tried to address. I
hope this will be sufficient for you to get behind the patch.
The relevant section in the commit message is titled:

/* sending signals to threads (tid) and process groups (pgid) */

Thanks!
Christian

Christian Brauner (1):
  signal: add pidfd_send_signal() syscall

 arch/x86/entry/syscalls/syscall_32.tbl |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl |   1 +
 fs/proc/base.c                         |  20 +++-
 include/linux/proc_fs.h                |  12 +++
 include/linux/syscalls.h               |   3 +
 include/uapi/asm-generic/unistd.h      |   4 +-
 kernel/signal.c                        | 141 +++++++++++++++++++++++--
 7 files changed, 173 insertions(+), 9 deletions(-)

-- 
2.19.1

The kill() syscall operates on process identifiers (pid). After a process
has exited its pid can be reused by another process. If a caller sends a
signal to a reused pid it will end up signaling the wrong process. This
issue has often surfaced and there has been a push to address this problem [1].

This patch uses file descriptors (fd) from proc/<pid> as stable handles on
struct pid. Even if a pid is recycled the handle will not change. The fd
can be used to send signals to the process it refers to.
Thus, the new syscall pidfd_send_signal() is introduced to solve this
problem. Instead of pids it operates on process fds (pidfd).

/* prototype and argument /*
long pidfd_send_signal(int pidfd, int sig, siginfo_t *info, unsigned int flags);

In addition to the pidfd and signal argument it takes an additional
siginfo_t and flags argument. If the siginfo_t argument is NULL then
pidfd_send_signal() is equivalent to kill(<positive-pid>, <signal>). If it
is not NULL pidfd_send_signal() is equivalent to rt_sigqueueinfo().
The flags argument is added to allow for future extensions of this syscall.
It currently needs to be passed as 0. Failing to do so will cause EINVAL.

/* pidfd_send_signal() replaces multiple pid-based syscalls */
The pidfd_send_signal() syscall currently takes on the job of
rt_sigqueueinfo(2) and parts of the functionality of kill(2), Namely, when a
positive pid is passed to kill(2). It will however be possible to also
replace tgkill(2) and rt_tgsigqueueinfo(2) if this syscall is extended.

/* sending signals to threads (tid) and process groups (pgid) */
Specifically, the pidfd_send_signal() syscall does currently not operate on
process groups or threads. This is left for future extensions.
In order to extend the syscall to allow sending signal to threads and
process groups appropriately named flags (e.g. PIDFD_TYPE_PGID, and
PIDFD_TYPE_TID) should be added. This implies that the flags argument will
determine what is signaled and not the file descriptor itself. Put in other
words, grouping in this api is a property of the flags argument not a
property of the file descriptor (cf. [13]).
When appropriate extensions through the flags argument are added then
pidfd_send_signal() can additionally replace the part of kill(2) which
operates on process groups as well as the tgkill(2) and
rt_tgsigqueueinfo(2) syscalls.
How such an extension could be implemented has been very roughly sketched
in [14], [15], and [16]. However, this should not be taken as a commitment
to a particular implementation. There might be better ways to do it.
Right now this is intentionally left out to keep this patchset as simple as
possible (cf. [4]). For example, if a pidfd for a tid from
/proc/<pid>/task/<tid> is passed EOPNOTSUPP will be returned to give
userspace a way to detect when I add support for signaling to threads (cf. [10]).

/* naming */
The syscall had various names throughout iterations of this patchset:
- procfd_signal()
- procfd_send_signal()
- taskfd_send_signal()
In the last round of reviews it was pointed out that given that if the
flags argument decides the scope of the signal instead of different types
of fds it might make sense to either settle for "procfd_" or "pidfd_" as
prefix. The community was willing to accept either (cf. [17] and [18]).
Given that one developer expressed strong preference for the "pidfd_"
prefix (cf. [13] and with other developers less opinionated about the name
we should settle for "pidfd_" to avoid further bikeshedding.

The  "_send_signal" suffix was chosen to reflect the fact that the syscall
takes on the job of multiple syscalls. It is therefore intentional that the
name is not reminiscent of neither kill(2) nor rt_sigqueueinfo(2). Not the
fomer because it might imply that pidfd_send_signal() is a replacement for
kill(2), and not the latter because it is a hassle to remember the correct
spelling - especially for non-native speakers - and because it is not
descriptive enough of what the syscall actually does. The name
"pidfd_send_signal" makes it very clear that its job is to send signals.

/* O_PATH file descriptors */
pidfds opened as O_PATH fds cannot be used to send signals to a process
(cf. [2]). Signaling processes through pidfds is the equivalent of writing
to a file. Thus, this is not an operation that operates "purely at the file
descriptor level" as required by the open(2) manpage.

/* zombies */
Zombies can be signaled just as any other process. No special error will be
reported since a zombie state is an unreliable state (cf. [3]). However,
this can be added as an extension through the @flags argument if the need
ever arises.

/* cross-namespace signals */
The patch currently enforces that the signaler and signalee either are in
the same pid namespace or that the signaler's pid namespace is an ancestor
of the signalee's pid namespace. This is done for the sake of simplicity
and because it is unclear to what values certain members of struct
siginfo_t would need to be set to (cf. [5], [6]).

/* compat syscalls */
It became clear that we would like to avoid adding compat syscalls
(cf. [7]).  The compat syscall handling is now done in kernel/signal.c
itself by adding __copy_siginfo_from_user_generic() which lets us avoid
compat syscalls (cf. [8]). It should be noted that the addition of
__copy_siginfo_from_user_any() is caused by a bug in the original
implementation of rt_sigqueueinfo(2) (cf. 12).
With upcoming rework for syscall handling things might improve
significantly (cf. [11]) and __copy_siginfo_from_user_any() will not gain
any additional callers.

/* testing */
This patch was tested on x64 and x86.

/* userspace usage */
An asciinema recording for the basic functionality can be found under [9].
With this patch a process can be killed via:

 #define _GNU_SOURCE
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <unistd.h>

 static inline int do_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
                                         unsigned int flags)
 {
 #ifdef __NR_pidfd_send_signal
         return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
 #else
         return -ENOSYS;
 #endif
 }

 int main(int argc, char *argv[])
 {
         int fd, ret, saved_errno, sig;

         if (argc < 3)
                 exit(EXIT_FAILURE);

         fd = open(argv[1], O_DIRECTORY | O_CLOEXEC);
         if (fd < 0) {
                 printf("%s - Failed to open \"%s\"\n", strerror(errno), argv[1]);
                 exit(EXIT_FAILURE);
         }

         sig = atoi(argv[2]);

         printf("Sending signal %d to process %s\n", sig, argv[1]);
         ret = do_pidfd_send_signal(fd, sig, NULL, 0);

         saved_errno = errno;
         close(fd);
         errno = saved_errno;

         if (ret < 0) {
                 printf("%s - Failed to send signal %d to process %s\n",
                        strerror(errno), sig, argv[1]);
                 exit(EXIT_FAILURE);
         }

         exit(EXIT_SUCCESS);
 }

[1]:  https://lore.kernel.org/lkml/20181029221037.87724-1-dancol@google.com/
[2]:  https://lore.kernel.org/lkml/874lbtjvtd.fsf@oldenburg2.str.redhat.com/
[3]:  https://lore.kernel.org/lkml/20181204132604.aspfupwjgjx6fhva@brauner.io/
[4]:  https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
[5]:  https://lore.kernel.org/lkml/20181121213946.GA10795@mail.hallyn.com/
[6]:  https://lore.kernel.org/lkml/20181120103111.etlqp7zop34v6nv4@brauner.io/
[7]:  https://lore.kernel.org/lkml/36323361-90BD-41AF-AB5B-EE0D7BA02C21@amacapital.net/
[8]:  https://lore.kernel.org/lkml/87tvjxp8pc.fsf@xmission.com/
[9]:  https://asciinema.org/a/IQjuCHew6bnq1cr78yuMv16cy
[10]: https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
[11]: https://lore.kernel.org/lkml/F53D6D38-3521-4C20-9034-5AF447DF62FF@amacapital.net/
[12]: https://lore.kernel.org/lkml/87zhtjn8ck.fsf@xmission.com/
[13]: https://lore.kernel.org/lkml/871s6u9z6u.fsf@xmission.com/
[14]: https://lore.kernel.org/lkml/20181206231742.xxi4ghn24z4h2qki@brauner.io/
[15]: https://lore.kernel.org/lkml/20181207003124.GA11160@mail.hallyn.com/
[16]: https://lore.kernel.org/lkml/20181207015423.4miorx43l3qhppfz@brauner.io/
[17]: https://lore.kernel.org/lkml/CAGXu5jL8PciZAXvOvCeCU3wKUEB_dU-O3q0tDw4uB_ojMvDEew@mail.gmail.com/
[18]: https://lore.kernel.org/lkml/20181206222746.GB9224@mail.hallyn.com/

Cc: "Eric W. Biederman" <redacted>
Cc: Jann Horn <jannh@google.com>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Florian Weimer <redacted>
Signed-off-by: Christian Brauner <christian@brauner.io>
Reviewed-by: Kees Cook <redacted>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Aleksa Sarai <redacted>
---
Changelog:
v5:
- s/may_signal_taskfd/access_taskfd_pidns/g
- make it clear that process grouping is a property of the @flags argument
  Eric has argued that he would like to know when we add thread and process
  group signal support whether grouping will be a property of the file
  descriptor or the flag argument and he would oppose this until a
  commitment has been made. It seems that the cleanest strategy is to make
  grouping a property of the @flags argument.
  He also argued that in this case the prefix of the syscall should be
  "pidfd_" (cf. https://lore.kernel.org/lkml/871s6u9z6u.fsf@xmission.com/).
- use "pidfd_" as prefix for the syscall since grouping will be a property
  of the @flags argument
- substantial rewrite of the commit message to reflect the discussion
v4:
- updated asciinema to use "taskfd_" prefix
- s/procfd_send_signal/taskfd_send_signal/g
- s/proc_is_tgid_procfd/tgid_taskfd_to_pid/b
- s/proc_is_tid_procfd/tid_taskfd_to_pid/b
- s/__copy_siginfo_from_user_generic/__copy_siginfo_from_user_any/g
- make it clear that __copy_siginfo_from_user_any() is a workaround caused
  by a bug in the original implementation of rt_sigqueueinfo()
- when spoofing signals turn them into regular kill signals if si_code is
  set to SI_USER
- make proc_is_t{g}id_procfd() return struct pid to allow proc_pid() to
  stay private to fs/proc/
v3:
- add __copy_siginfo_from_user_generic() to avoid adding compat syscalls
- s/procfd_signal/procfd_send_signal/g
- change type of flags argument from int to unsigned int
- add comment about what happens to zombies
- add proc_is_tid_procfd()
- return EOPNOTSUPP when /proc/<pid>/task/<tid> fd is passed so userspace
  has a way of knowing that tidfds are not supported currently.
v2:
- define __NR_procfd_signal in unistd.h
- wire up compat syscall
- s/proc_is_procfd/proc_is_tgid_procfd/g
- provide stubs when CONFIG_PROC_FS=n
- move proc_pid() to linux/proc_fs.h header
- use proc_pid() to grab struct pid from /proc/<pid> fd
v1:
- patch introduced
---
 arch/x86/entry/syscalls/syscall_32.tbl |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl |   1 +
 fs/proc/base.c                         |  20 +++-
 include/linux/proc_fs.h                |  12 +++
 include/linux/syscalls.h               |   3 +
 include/uapi/asm-generic/unistd.h      |   4 +-
 kernel/signal.c                        | 141 +++++++++++++++++++++++--
 7 files changed, 173 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 3cf7b533b3d1..6804c1e84b36 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl

@@ -398,3 +398,4 @@
 384	i386	arch_prctl		sys_arch_prctl			__ia32_compat_sys_arch_prctl
 385	i386	io_pgetevents		sys_io_pgetevents		__ia32_compat_sys_io_pgetevents
 386	i386	rseq			sys_rseq			__ia32_sys_rseq
+387	i386	pidfd_send_signal	sys_pidfd_send_signal		__ia32_sys_pidfd_send_signal

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f0b1709a5ffb..aa4b858fa0f1 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl

@@ -343,6 +343,7 @@
 332	common	statx			__x64_sys_statx
 333	common	io_pgetevents		__x64_sys_io_pgetevents
 334	common	rseq			__x64_sys_rseq
+335	common	pidfd_send_signal	__x64_sys_pidfd_send_signal
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ce3465479447..bf680b7b603a 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c

@@ -716,8 +716,6 @@ static int proc_pid_permission(struct inode *inode, int mask)
 	return generic_permission(inode, mask);
 }
 
-
-
 static const struct inode_operations proc_def_inode_operations = {
 	.setattr	= proc_setattr,
 };

@@ -3038,6 +3036,15 @@ static const struct file_operations proc_tgid_base_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+struct pid *tgid_pidfd_to_pid(const struct file *file)
+{
+	if (!d_is_dir(file->f_path.dentry) ||
+	    (file->f_op != &proc_tgid_base_operations))
+		return ERR_PTR(-EBADF);
+
+	return proc_pid(file_inode(file));
+}
+
 static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,

@@ -3422,6 +3429,15 @@ static const struct file_operations proc_tid_base_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+struct pid *tid_pidfd_to_pid(const struct file *file)
+{
+	if (!d_is_dir(file->f_path.dentry) ||
+	    (file->f_op != &proc_tid_base_operations))
+		return ERR_PTR(-EBADF);
+
+	return proc_pid(file_inode(file));
+}
+
 static const struct inode_operations proc_tid_base_inode_operations = {
 	.lookup		= proc_tid_base_lookup,
 	.getattr	= pid_getattr,

diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index d0e1f1522a78..eb150e5c0ab8 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h

@@ -73,6 +73,8 @@ struct proc_dir_entry *proc_create_net_single_write(const char *name, umode_t mo
 						    int (*show)(struct seq_file *, void *),
 						    proc_write_t write,
 						    void *data);
+extern struct pid *tgid_pidfd_to_pid(const struct file *file);
+extern struct pid *tid_pidfd_to_pid(const struct file *file);
 
 #else /* CONFIG_PROC_FS */
 

@@ -114,6 +116,16 @@ static inline int remove_proc_subtree(const char *name, struct proc_dir_entry *p
 #define proc_create_net(name, mode, parent, state_size, ops) ({NULL;})
 #define proc_create_net_single(name, mode, parent, show, data) ({NULL;})
 
+static inline struct pid *tgid_pidfd_to_pid(const struct file *file)
+{
+	return ERR_PTR(-EBADF);
+}
+
+static inline struct pid *tid_pidfd_to_pid(const struct file *file)
+{
+	return ERR_PTR(-EBADF);
+}
+
 #endif /* CONFIG_PROC_FS */
 
 struct net;

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 2ac3d13a915b..fd85b9045a9f 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -907,6 +907,9 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
 			  unsigned mask, struct statx __user *buffer);
 asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
 			 int flags, uint32_t sig);
+asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
+				       siginfo_t __user *info,
+				       unsigned int flags);
 
 /*
  * Architecture-specific system calls

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 538546edbfbd..0822abc5927a 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -738,9 +738,11 @@ __SYSCALL(__NR_statx,     sys_statx)
 __SC_COMP(__NR_io_pgetevents, sys_io_pgetevents, compat_sys_io_pgetevents)
 #define __NR_rseq 293
 __SYSCALL(__NR_rseq, sys_rseq)
+#define __NR_pidfd_send_signal 294
+__SYSCALL(__NR_pidfd_send_signal, sys_pidfd_send_signal)
 
 #undef __NR_syscalls
-#define __NR_syscalls 294
+#define __NR_syscalls 295
 
 /*
  * 32 bit systems traditionally used different

diff --git a/kernel/signal.c b/kernel/signal.c
index 9a32bc2088c9..3c83d3a5c7c5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c

@@ -19,7 +19,9 @@
 #include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
 #include <linux/sched/cputime.h>
+#include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/proc_fs.h>
 #include <linux/tty.h>
 #include <linux/binfmts.h>
 #include <linux/coredump.h>

@@ -3286,6 +3288,16 @@ COMPAT_SYSCALL_DEFINE4(rt_sigtimedwait, compat_sigset_t __user *, uthese,
 }
 #endif
 
+static inline void prepare_kill_siginfo(int sig, struct kernel_siginfo *info)
+{
+	clear_siginfo(info);
+	info->si_signo = sig;
+	info->si_errno = 0;
+	info->si_code = SI_USER;
+	info->si_pid = task_tgid_vnr(current);
+	info->si_uid = from_kuid_munged(current_user_ns(), current_uid());
+}
+
 /**
  *  sys_kill - send a signal to a process
  *  @pid: the PID of the process

@@ -3295,16 +3307,133 @@ SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
 {
 	struct kernel_siginfo info;
 
-	clear_siginfo(&info);
-	info.si_signo = sig;
-	info.si_errno = 0;
-	info.si_code = SI_USER;
-	info.si_pid = task_tgid_vnr(current);
-	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
+	prepare_kill_siginfo(sig, &info);
 
 	return kill_something_info(sig, &info, pid);
 }
 
+/*
+ * Verify that the signaler and signalee either are in the same pid namespace
+ * or that the signaler's pid namespace is an ancestor of the signalee's pid
+ * namespace.
+ */
+static bool access_pidfd_pidns(struct pid *pid)
+{
+	struct pid_namespace *active = task_active_pid_ns(current);
+	struct pid_namespace *p = ns_of_pid(pid);
+
+	for (;;) {
+		if (!p)
+			return false;
+		if (p == active)
+			break;
+		p = p->parent;
+	}
+
+	return true;
+}
+
+static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
+{
+#ifdef CONFIG_COMPAT
+	/*
+	 * Avoid hooking up compat syscalls and instead handle necessary
+	 * conversions here. Note, this is a stop-gap measure and should not be
+	 * considered a generic solution.
+	 */
+	if (in_compat_syscall())
+		return copy_siginfo_from_user32(
+			kinfo, (struct compat_siginfo __user *)info);
+#endif
+	return copy_siginfo_from_user(kinfo, info);
+}
+
+/**
+ * sys_pidfd_send_signal - send a signal to a process through a task file
+ *                          descriptor
+ * @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.
+ * 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,
+ * grouping is a property of the flags argument not a property of the file
+ * descriptor.
+ *
+ * Return: 0 on success, negative errno on failure
+ */
+SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
+		siginfo_t __user *, info, unsigned int, flags)
+{
+	int ret;
+	struct fd f;
+	struct pid *pid;
+	kernel_siginfo_t kinfo;
+
+	/* Enforce flags be set to 0 until we add an extension. */
+	if (flags)
+		return -EINVAL;
+
+	f = fdget_raw(pidfd);
+	if (!f.file)
+		return -EBADF;
+
+	pid = tid_pidfd_to_pid(f.file);
+	if (!IS_ERR(pid)) {
+		/*
+		 * Give userspace a way to detect /proc/<pid>/task/<tid>
+		 * support when we add it.
+		 */
+		ret = -EOPNOTSUPP;
+		goto err;
+	}
+
+	/* Is this a pidfd? */
+	pid = tgid_pidfd_to_pid(f.file);
+	if (IS_ERR(pid)) {
+		ret = PTR_ERR(pid);
+		goto err;
+	}
+
+	ret = -EINVAL;
+	if (!access_pidfd_pidns(pid))
+		goto err;
+
+	if (info) {
+		ret = copy_siginfo_from_user_any(&kinfo, info);
+		if (unlikely(ret))
+			goto err;
+
+		ret = -EINVAL;
+		if (unlikely(sig != kinfo.si_signo))
+			goto err;
+
+		if ((task_pid(current) != pid) &&
+		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)) {
+			/* Only allow sending arbitrary signals to yourself. */
+			ret = -EPERM;
+			if (kinfo.si_code != SI_USER)
+				goto err;
+
+			/* Turn this into a regular kill signal. */
+			prepare_kill_siginfo(sig, &kinfo);
+		}
+	} else {
+		prepare_kill_siginfo(sig, &kinfo);
+	}
+
+	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)
 {

-- 
2.19.1

Hi Christian,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on linus/master]
[also build test WARNING on v4.20-rc5]
[cannot apply to next-20181207]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Christian-Brauner/signaling-processes-through-pidfds/20181209-142857
config: ia64-allnoconfig (attached as .config)
compiler: ia64-linux-gcc (GCC) 8.1.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=8.1.0 make.cross ARCH=ia64 

All warnings (new ones prefixed by >>):

   <stdin>:1185:2: warning: #warning syscall perf_event_open not implemented [-Wcpp]
   <stdin>:1239:2: warning: #warning syscall seccomp not implemented [-Wcpp]
   <stdin>:1317:2: warning: #warning syscall pkey_mprotect not implemented [-Wcpp]
   <stdin>:1320:2: warning: #warning syscall pkey_alloc not implemented [-Wcpp]
   <stdin>:1323:2: warning: #warning syscall pkey_free not implemented [-Wcpp]
   <stdin>:1326:2: warning: #warning syscall statx not implemented [-Wcpp]
   <stdin>:1332:2: warning: #warning syscall io_pgetevents not implemented [-Wcpp]
   <stdin>:1335:2: warning: #warning syscall rseq not implemented [-Wcpp]

quoted

<stdin>:1338:2: warning: #warning syscall pidfd_send_signal not implemented [-Wcpp]

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

On Sat, Dec 08, 2018 at 06:40:59AM +0100, Christian Brauner wrote:

The kill() syscall operates on process identifiers (pid). After a process
has exited its pid can be reused by another process. If a caller sends a
signal to a reused pid it will end up signaling the wrong process. This
issue has often surfaced and there has been a push to address this problem [1].

This patch uses file descriptors (fd) from proc/<pid> as stable handles on
struct pid. Even if a pid is recycled the handle will not change. The fd
can be used to send signals to the process it refers to.
Thus, the new syscall pidfd_send_signal() is introduced to solve this
problem. Instead of pids it operates on process fds (pidfd).

/* prototype and argument /*
long pidfd_send_signal(int pidfd, int sig, siginfo_t *info, unsigned int flags);

In addition to the pidfd and signal argument it takes an additional
siginfo_t and flags argument. If the siginfo_t argument is NULL then
pidfd_send_signal() is equivalent to kill(<positive-pid>, <signal>). If it
is not NULL pidfd_send_signal() is equivalent to rt_sigqueueinfo().
The flags argument is added to allow for future extensions of this syscall.
It currently needs to be passed as 0. Failing to do so will cause EINVAL.

/* pidfd_send_signal() replaces multiple pid-based syscalls */
The pidfd_send_signal() syscall currently takes on the job of
rt_sigqueueinfo(2) and parts of the functionality of kill(2), Namely, when a
positive pid is passed to kill(2). It will however be possible to also
replace tgkill(2) and rt_tgsigqueueinfo(2) if this syscall is extended.

/* sending signals to threads (tid) and process groups (pgid) */
Specifically, the pidfd_send_signal() syscall does currently not operate on
process groups or threads. This is left for future extensions.
In order to extend the syscall to allow sending signal to threads and
process groups appropriately named flags (e.g. PIDFD_TYPE_PGID, and
PIDFD_TYPE_TID) should be added. This implies that the flags argument will
determine what is signaled and not the file descriptor itself. Put in other
words, grouping in this api is a property of the flags argument not a
property of the file descriptor (cf. [13]).
When appropriate extensions through the flags argument are added then
pidfd_send_signal() can additionally replace the part of kill(2) which
operates on process groups as well as the tgkill(2) and
rt_tgsigqueueinfo(2) syscalls.
How such an extension could be implemented has been very roughly sketched
in [14], [15], and [16]. However, this should not be taken as a commitment
to a particular implementation. There might be better ways to do it.
Right now this is intentionally left out to keep this patchset as simple as
possible (cf. [4]). For example, if a pidfd for a tid from
/proc/<pid>/task/<tid> is passed EOPNOTSUPP will be returned to give
userspace a way to detect when I add support for signaling to threads (cf. [10]).

/* naming */
The syscall had various names throughout iterations of this patchset:
- procfd_signal()
- procfd_send_signal()
- taskfd_send_signal()
In the last round of reviews it was pointed out that given that if the
flags argument decides the scope of the signal instead of different types
of fds it might make sense to either settle for "procfd_" or "pidfd_" as
prefix. The community was willing to accept either (cf. [17] and [18]).
Given that one developer expressed strong preference for the "pidfd_"
prefix (cf. [13] and with other developers less opinionated about the name
we should settle for "pidfd_" to avoid further bikeshedding.

The  "_send_signal" suffix was chosen to reflect the fact that the syscall
takes on the job of multiple syscalls. It is therefore intentional that the
name is not reminiscent of neither kill(2) nor rt_sigqueueinfo(2). Not the
fomer because it might imply that pidfd_send_signal() is a replacement for
kill(2), and not the latter because it is a hassle to remember the correct
spelling - especially for non-native speakers - and because it is not
descriptive enough of what the syscall actually does. The name
"pidfd_send_signal" makes it very clear that its job is to send signals.

/* O_PATH file descriptors */
pidfds opened as O_PATH fds cannot be used to send signals to a process
(cf. [2]). Signaling processes through pidfds is the equivalent of writing
to a file. Thus, this is not an operation that operates "purely at the file
descriptor level" as required by the open(2) manpage.

/* zombies */
Zombies can be signaled just as any other process. No special error will be
reported since a zombie state is an unreliable state (cf. [3]). However,
this can be added as an extension through the @flags argument if the need
ever arises.

/* cross-namespace signals */
The patch currently enforces that the signaler and signalee either are in
the same pid namespace or that the signaler's pid namespace is an ancestor
of the signalee's pid namespace. This is done for the sake of simplicity
and because it is unclear to what values certain members of struct
siginfo_t would need to be set to (cf. [5], [6]).

/* compat syscalls */
It became clear that we would like to avoid adding compat syscalls
(cf. [7]).  The compat syscall handling is now done in kernel/signal.c
itself by adding __copy_siginfo_from_user_generic() which lets us avoid
compat syscalls (cf. [8]). It should be noted that the addition of
__copy_siginfo_from_user_any() is caused by a bug in the original
implementation of rt_sigqueueinfo(2) (cf. 12).
With upcoming rework for syscall handling things might improve
significantly (cf. [11]) and __copy_siginfo_from_user_any() will not gain
any additional callers.

/* testing */
This patch was tested on x64 and x86.

/* userspace usage */
An asciinema recording for the basic functionality can be found under [9].
With this patch a process can be killed via:

 #define _GNU_SOURCE
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <unistd.h>

 static inline int do_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
                                         unsigned int flags)
 {
 #ifdef __NR_pidfd_send_signal
         return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
 #else
         return -ENOSYS;
 #endif
 }

 int main(int argc, char *argv[])
 {
         int fd, ret, saved_errno, sig;

         if (argc < 3)
                 exit(EXIT_FAILURE);

         fd = open(argv[1], O_DIRECTORY | O_CLOEXEC);
         if (fd < 0) {
                 printf("%s - Failed to open \"%s\"\n", strerror(errno), argv[1]);
                 exit(EXIT_FAILURE);
         }

         sig = atoi(argv[2]);

         printf("Sending signal %d to process %s\n", sig, argv[1]);
         ret = do_pidfd_send_signal(fd, sig, NULL, 0);

         saved_errno = errno;
         close(fd);
         errno = saved_errno;

         if (ret < 0) {
                 printf("%s - Failed to send signal %d to process %s\n",
                        strerror(errno), sig, argv[1]);
                 exit(EXIT_FAILURE);
         }

         exit(EXIT_SUCCESS);
 }

[1]:  https://lore.kernel.org/lkml/20181029221037.87724-1-dancol@google.com/
[2]:  https://lore.kernel.org/lkml/874lbtjvtd.fsf@oldenburg2.str.redhat.com/
[3]:  https://lore.kernel.org/lkml/20181204132604.aspfupwjgjx6fhva@brauner.io/
[4]:  https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
[5]:  https://lore.kernel.org/lkml/20181121213946.GA10795@mail.hallyn.com/
[6]:  https://lore.kernel.org/lkml/20181120103111.etlqp7zop34v6nv4@brauner.io/
[7]:  https://lore.kernel.org/lkml/36323361-90BD-41AF-AB5B-EE0D7BA02C21@amacapital.net/
[8]:  https://lore.kernel.org/lkml/87tvjxp8pc.fsf@xmission.com/
[9]:  https://asciinema.org/a/IQjuCHew6bnq1cr78yuMv16cy
[10]: https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
[11]: https://lore.kernel.org/lkml/F53D6D38-3521-4C20-9034-5AF447DF62FF@amacapital.net/
[12]: https://lore.kernel.org/lkml/87zhtjn8ck.fsf@xmission.com/
[13]: https://lore.kernel.org/lkml/871s6u9z6u.fsf@xmission.com/
[14]: https://lore.kernel.org/lkml/20181206231742.xxi4ghn24z4h2qki@brauner.io/
[15]: https://lore.kernel.org/lkml/20181207003124.GA11160@mail.hallyn.com/
[16]: https://lore.kernel.org/lkml/20181207015423.4miorx43l3qhppfz@brauner.io/
[17]: https://lore.kernel.org/lkml/CAGXu5jL8PciZAXvOvCeCU3wKUEB_dU-O3q0tDw4uB_ojMvDEew@mail.gmail.com/
[18]: https://lore.kernel.org/lkml/20181206222746.GB9224@mail.hallyn.com/

Cc: "Eric W. Biederman" <redacted>

Hi Eric,

have you had a chance to look at the latest version?

-serge

On Thu, Dec 13, 2018 at 04:23:00PM -0600, Serge Hallyn wrote:

On Sat, Dec 08, 2018 at 06:40:59AM +0100, Christian Brauner wrote:

quoted

The kill() syscall operates on process identifiers (pid). After a process
has exited its pid can be reused by another process. If a caller sends a
signal to a reused pid it will end up signaling the wrong process. This
issue has often surfaced and there has been a push to address this problem [1].

This patch uses file descriptors (fd) from proc/<pid> as stable handles on
struct pid. Even if a pid is recycled the handle will not change. The fd
can be used to send signals to the process it refers to.
Thus, the new syscall pidfd_send_signal() is introduced to solve this
problem. Instead of pids it operates on process fds (pidfd).

/* prototype and argument /*
long pidfd_send_signal(int pidfd, int sig, siginfo_t *info, unsigned int flags);

In addition to the pidfd and signal argument it takes an additional
siginfo_t and flags argument. If the siginfo_t argument is NULL then
pidfd_send_signal() is equivalent to kill(<positive-pid>, <signal>). If it
is not NULL pidfd_send_signal() is equivalent to rt_sigqueueinfo().
The flags argument is added to allow for future extensions of this syscall.
It currently needs to be passed as 0. Failing to do so will cause EINVAL.

/* pidfd_send_signal() replaces multiple pid-based syscalls */
The pidfd_send_signal() syscall currently takes on the job of
rt_sigqueueinfo(2) and parts of the functionality of kill(2), Namely, when a
positive pid is passed to kill(2). It will however be possible to also
replace tgkill(2) and rt_tgsigqueueinfo(2) if this syscall is extended.

/* sending signals to threads (tid) and process groups (pgid) */
Specifically, the pidfd_send_signal() syscall does currently not operate on
process groups or threads. This is left for future extensions.
In order to extend the syscall to allow sending signal to threads and
process groups appropriately named flags (e.g. PIDFD_TYPE_PGID, and
PIDFD_TYPE_TID) should be added. This implies that the flags argument will
determine what is signaled and not the file descriptor itself. Put in other
words, grouping in this api is a property of the flags argument not a
property of the file descriptor (cf. [13]).
When appropriate extensions through the flags argument are added then
pidfd_send_signal() can additionally replace the part of kill(2) which
operates on process groups as well as the tgkill(2) and
rt_tgsigqueueinfo(2) syscalls.
How such an extension could be implemented has been very roughly sketched
in [14], [15], and [16]. However, this should not be taken as a commitment
to a particular implementation. There might be better ways to do it.
Right now this is intentionally left out to keep this patchset as simple as
possible (cf. [4]). For example, if a pidfd for a tid from
/proc/<pid>/task/<tid> is passed EOPNOTSUPP will be returned to give
userspace a way to detect when I add support for signaling to threads (cf. [10]).

/* naming */
The syscall had various names throughout iterations of this patchset:
- procfd_signal()
- procfd_send_signal()
- taskfd_send_signal()
In the last round of reviews it was pointed out that given that if the
flags argument decides the scope of the signal instead of different types
of fds it might make sense to either settle for "procfd_" or "pidfd_" as
prefix. The community was willing to accept either (cf. [17] and [18]).
Given that one developer expressed strong preference for the "pidfd_"
prefix (cf. [13] and with other developers less opinionated about the name
we should settle for "pidfd_" to avoid further bikeshedding.

The  "_send_signal" suffix was chosen to reflect the fact that the syscall
takes on the job of multiple syscalls. It is therefore intentional that the
name is not reminiscent of neither kill(2) nor rt_sigqueueinfo(2). Not the
fomer because it might imply that pidfd_send_signal() is a replacement for
kill(2), and not the latter because it is a hassle to remember the correct
spelling - especially for non-native speakers - and because it is not
descriptive enough of what the syscall actually does. The name
"pidfd_send_signal" makes it very clear that its job is to send signals.

/* O_PATH file descriptors */
pidfds opened as O_PATH fds cannot be used to send signals to a process
(cf. [2]). Signaling processes through pidfds is the equivalent of writing
to a file. Thus, this is not an operation that operates "purely at the file
descriptor level" as required by the open(2) manpage.

/* zombies */
Zombies can be signaled just as any other process. No special error will be
reported since a zombie state is an unreliable state (cf. [3]). However,
this can be added as an extension through the @flags argument if the need
ever arises.

/* cross-namespace signals */
The patch currently enforces that the signaler and signalee either are in
the same pid namespace or that the signaler's pid namespace is an ancestor
of the signalee's pid namespace. This is done for the sake of simplicity
and because it is unclear to what values certain members of struct
siginfo_t would need to be set to (cf. [5], [6]).

/* compat syscalls */
It became clear that we would like to avoid adding compat syscalls
(cf. [7]).  The compat syscall handling is now done in kernel/signal.c
itself by adding __copy_siginfo_from_user_generic() which lets us avoid
compat syscalls (cf. [8]). It should be noted that the addition of
__copy_siginfo_from_user_any() is caused by a bug in the original
implementation of rt_sigqueueinfo(2) (cf. 12).
With upcoming rework for syscall handling things might improve
significantly (cf. [11]) and __copy_siginfo_from_user_any() will not gain
any additional callers.

/* testing */
This patch was tested on x64 and x86.

/* userspace usage */
An asciinema recording for the basic functionality can be found under [9].
With this patch a process can be killed via:

 #define _GNU_SOURCE
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <unistd.h>

 static inline int do_pidfd_send_signal(int pidfd, int sig, siginfo_t *info,
                                         unsigned int flags)
 {
 #ifdef __NR_pidfd_send_signal
         return syscall(__NR_pidfd_send_signal, pidfd, sig, info, flags);
 #else
         return -ENOSYS;
 #endif
 }

 int main(int argc, char *argv[])
 {
         int fd, ret, saved_errno, sig;

         if (argc < 3)
                 exit(EXIT_FAILURE);

         fd = open(argv[1], O_DIRECTORY | O_CLOEXEC);
         if (fd < 0) {
                 printf("%s - Failed to open \"%s\"\n", strerror(errno), argv[1]);
                 exit(EXIT_FAILURE);
         }

         sig = atoi(argv[2]);

         printf("Sending signal %d to process %s\n", sig, argv[1]);
         ret = do_pidfd_send_signal(fd, sig, NULL, 0);

         saved_errno = errno;
         close(fd);
         errno = saved_errno;

         if (ret < 0) {
                 printf("%s - Failed to send signal %d to process %s\n",
                        strerror(errno), sig, argv[1]);
                 exit(EXIT_FAILURE);
         }

         exit(EXIT_SUCCESS);
 }

[1]:  https://lore.kernel.org/lkml/20181029221037.87724-1-dancol@google.com/
[2]:  https://lore.kernel.org/lkml/874lbtjvtd.fsf@oldenburg2.str.redhat.com/
[3]:  https://lore.kernel.org/lkml/20181204132604.aspfupwjgjx6fhva@brauner.io/
[4]:  https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
[5]:  https://lore.kernel.org/lkml/20181121213946.GA10795@mail.hallyn.com/
[6]:  https://lore.kernel.org/lkml/20181120103111.etlqp7zop34v6nv4@brauner.io/
[7]:  https://lore.kernel.org/lkml/36323361-90BD-41AF-AB5B-EE0D7BA02C21@amacapital.net/
[8]:  https://lore.kernel.org/lkml/87tvjxp8pc.fsf@xmission.com/
[9]:  https://asciinema.org/a/IQjuCHew6bnq1cr78yuMv16cy
[10]: https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@brauner.io/
[11]: https://lore.kernel.org/lkml/F53D6D38-3521-4C20-9034-5AF447DF62FF@amacapital.net/
[12]: https://lore.kernel.org/lkml/87zhtjn8ck.fsf@xmission.com/
[13]: https://lore.kernel.org/lkml/871s6u9z6u.fsf@xmission.com/
[14]: https://lore.kernel.org/lkml/20181206231742.xxi4ghn24z4h2qki@brauner.io/
[15]: https://lore.kernel.org/lkml/20181207003124.GA11160@mail.hallyn.com/
[16]: https://lore.kernel.org/lkml/20181207015423.4miorx43l3qhppfz@brauner.io/
[17]: https://lore.kernel.org/lkml/CAGXu5jL8PciZAXvOvCeCU3wKUEB_dU-O3q0tDw4uB_ojMvDEew@mail.gmail.com/
[18]: https://lore.kernel.org/lkml/20181206222746.GB9224@mail.hallyn.com/

Cc: "Eric W. Biederman" <redacted>

Hi Eric,

have you had a chance to look at the latest version?

-serge

Hi Eric,

Gentle ping again: Have you had a chance to look at the latest version?
I'm not sure if you saw Serge's ping.
If there are no objections I would very much like to see this land in
-next for some testing. Especially, given that we have accumulated some
acks and reviews for the patch by now. :)

Christian

quoted

Cc: Jann Horn <jannh@google.com>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Florian Weimer <redacted>
Signed-off-by: Christian Brauner <christian@brauner.io>
Reviewed-by: Kees Cook <redacted>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Aleksa Sarai <redacted>
---
Changelog:
v5:
- s/may_signal_taskfd/access_taskfd_pidns/g
- make it clear that process grouping is a property of the @flags argument
  Eric has argued that he would like to know when we add thread and process
  group signal support whether grouping will be a property of the file
  descriptor or the flag argument and he would oppose this until a
  commitment has been made. It seems that the cleanest strategy is to make
  grouping a property of the @flags argument.
  He also argued that in this case the prefix of the syscall should be
  "pidfd_" (cf. https://lore.kernel.org/lkml/871s6u9z6u.fsf@xmission.com/).
- use "pidfd_" as prefix for the syscall since grouping will be a property
  of the @flags argument
- substantial rewrite of the commit message to reflect the discussion
v4:
- updated asciinema to use "taskfd_" prefix
- s/procfd_send_signal/taskfd_send_signal/g
- s/proc_is_tgid_procfd/tgid_taskfd_to_pid/b
- s/proc_is_tid_procfd/tid_taskfd_to_pid/b
- s/__copy_siginfo_from_user_generic/__copy_siginfo_from_user_any/g
- make it clear that __copy_siginfo_from_user_any() is a workaround caused
  by a bug in the original implementation of rt_sigqueueinfo()
- when spoofing signals turn them into regular kill signals if si_code is
  set to SI_USER
- make proc_is_t{g}id_procfd() return struct pid to allow proc_pid() to
  stay private to fs/proc/
v3:
- add __copy_siginfo_from_user_generic() to avoid adding compat syscalls
- s/procfd_signal/procfd_send_signal/g
- change type of flags argument from int to unsigned int
- add comment about what happens to zombies
- add proc_is_tid_procfd()
- return EOPNOTSUPP when /proc/<pid>/task/<tid> fd is passed so userspace
  has a way of knowing that tidfds are not supported currently.
v2:
- define __NR_procfd_signal in unistd.h
- wire up compat syscall
- s/proc_is_procfd/proc_is_tgid_procfd/g
- provide stubs when CONFIG_PROC_FS=n
- move proc_pid() to linux/proc_fs.h header
- use proc_pid() to grab struct pid from /proc/<pid> fd
v1:
- patch introduced
---
 arch/x86/entry/syscalls/syscall_32.tbl |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl |   1 +
 fs/proc/base.c                         |  20 +++-
 include/linux/proc_fs.h                |  12 +++
 include/linux/syscalls.h               |   3 +
 include/uapi/asm-generic/unistd.h      |   4 +-
 kernel/signal.c                        | 141 +++++++++++++++++++++++--
 7 files changed, 173 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 3cf7b533b3d1..6804c1e84b36 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl

@@ -398,3 +398,4 @@
 384	i386	arch_prctl		sys_arch_prctl			__ia32_compat_sys_arch_prctl
 385	i386	io_pgetevents		sys_io_pgetevents		__ia32_compat_sys_io_pgetevents
 386	i386	rseq			sys_rseq			__ia32_sys_rseq
+387	i386	pidfd_send_signal	sys_pidfd_send_signal		__ia32_sys_pidfd_send_signal

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index f0b1709a5ffb..aa4b858fa0f1 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl

@@ -343,6 +343,7 @@
 332	common	statx			__x64_sys_statx
 333	common	io_pgetevents		__x64_sys_io_pgetevents
 334	common	rseq			__x64_sys_rseq
+335	common	pidfd_send_signal	__x64_sys_pidfd_send_signal
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ce3465479447..bf680b7b603a 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c

@@ -716,8 +716,6 @@ static int proc_pid_permission(struct inode *inode, int mask)
 	return generic_permission(inode, mask);
 }
 
-
-
 static const struct inode_operations proc_def_inode_operations = {
 	.setattr	= proc_setattr,
 };

@@ -3038,6 +3036,15 @@ static const struct file_operations proc_tgid_base_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+struct pid *tgid_pidfd_to_pid(const struct file *file)
+{
+	if (!d_is_dir(file->f_path.dentry) ||
+	    (file->f_op != &proc_tgid_base_operations))
+		return ERR_PTR(-EBADF);
+
+	return proc_pid(file_inode(file));
+}
+
 static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,

@@ -3422,6 +3429,15 @@ static const struct file_operations proc_tid_base_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+struct pid *tid_pidfd_to_pid(const struct file *file)
+{
+	if (!d_is_dir(file->f_path.dentry) ||
+	    (file->f_op != &proc_tid_base_operations))
+		return ERR_PTR(-EBADF);
+
+	return proc_pid(file_inode(file));
+}
+
 static const struct inode_operations proc_tid_base_inode_operations = {
 	.lookup		= proc_tid_base_lookup,
 	.getattr	= pid_getattr,

diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index d0e1f1522a78..eb150e5c0ab8 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h

@@ -73,6 +73,8 @@ struct proc_dir_entry *proc_create_net_single_write(const char *name, umode_t mo
 						    int (*show)(struct seq_file *, void *),
 						    proc_write_t write,
 						    void *data);
+extern struct pid *tgid_pidfd_to_pid(const struct file *file);
+extern struct pid *tid_pidfd_to_pid(const struct file *file);
 
 #else /* CONFIG_PROC_FS */
 

@@ -114,6 +116,16 @@ static inline int remove_proc_subtree(const char *name, struct proc_dir_entry *p
 #define proc_create_net(name, mode, parent, state_size, ops) ({NULL;})
 #define proc_create_net_single(name, mode, parent, show, data) ({NULL;})
 
+static inline struct pid *tgid_pidfd_to_pid(const struct file *file)
+{
+	return ERR_PTR(-EBADF);
+}
+
+static inline struct pid *tid_pidfd_to_pid(const struct file *file)
+{
+	return ERR_PTR(-EBADF);
+}
+
 #endif /* CONFIG_PROC_FS */
 
 struct net;

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 2ac3d13a915b..fd85b9045a9f 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -907,6 +907,9 @@ asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
 			  unsigned mask, struct statx __user *buffer);
 asmlinkage long sys_rseq(struct rseq __user *rseq, uint32_t rseq_len,
 			 int flags, uint32_t sig);
+asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
+				       siginfo_t __user *info,
+				       unsigned int flags);
 
 /*
  * Architecture-specific system calls

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 538546edbfbd..0822abc5927a 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -738,9 +738,11 @@ __SYSCALL(__NR_statx,     sys_statx)
 __SC_COMP(__NR_io_pgetevents, sys_io_pgetevents, compat_sys_io_pgetevents)
 #define __NR_rseq 293
 __SYSCALL(__NR_rseq, sys_rseq)
+#define __NR_pidfd_send_signal 294
+__SYSCALL(__NR_pidfd_send_signal, sys_pidfd_send_signal)
 
 #undef __NR_syscalls
-#define __NR_syscalls 294
+#define __NR_syscalls 295
 
 /*
  * 32 bit systems traditionally used different

diff --git a/kernel/signal.c b/kernel/signal.c
index 9a32bc2088c9..3c83d3a5c7c5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c

@@ -19,7 +19,9 @@
 #include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
 #include <linux/sched/cputime.h>
+#include <linux/file.h>
 #include <linux/fs.h>
+#include <linux/proc_fs.h>
 #include <linux/tty.h>
 #include <linux/binfmts.h>
 #include <linux/coredump.h>

@@ -3286,6 +3288,16 @@ COMPAT_SYSCALL_DEFINE4(rt_sigtimedwait, compat_sigset_t __user *, uthese,
 }
 #endif
 
+static inline void prepare_kill_siginfo(int sig, struct kernel_siginfo *info)
+{
+	clear_siginfo(info);
+	info->si_signo = sig;
+	info->si_errno = 0;
+	info->si_code = SI_USER;
+	info->si_pid = task_tgid_vnr(current);
+	info->si_uid = from_kuid_munged(current_user_ns(), current_uid());
+}
+
 /**
  *  sys_kill - send a signal to a process
  *  @pid: the PID of the process

@@ -3295,16 +3307,133 @@ SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
 {
 	struct kernel_siginfo info;
 
-	clear_siginfo(&info);
-	info.si_signo = sig;
-	info.si_errno = 0;
-	info.si_code = SI_USER;
-	info.si_pid = task_tgid_vnr(current);
-	info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
+	prepare_kill_siginfo(sig, &info);
 
 	return kill_something_info(sig, &info, pid);
 }
 
+/*
+ * Verify that the signaler and signalee either are in the same pid namespace
+ * or that the signaler's pid namespace is an ancestor of the signalee's pid
+ * namespace.
+ */
+static bool access_pidfd_pidns(struct pid *pid)
+{
+	struct pid_namespace *active = task_active_pid_ns(current);
+	struct pid_namespace *p = ns_of_pid(pid);
+
+	for (;;) {
+		if (!p)
+			return false;
+		if (p == active)
+			break;
+		p = p->parent;
+	}
+
+	return true;
+}
+
+static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
+{
+#ifdef CONFIG_COMPAT
+	/*
+	 * Avoid hooking up compat syscalls and instead handle necessary
+	 * conversions here. Note, this is a stop-gap measure and should not be
+	 * considered a generic solution.
+	 */
+	if (in_compat_syscall())
+		return copy_siginfo_from_user32(
+			kinfo, (struct compat_siginfo __user *)info);
+#endif
+	return copy_siginfo_from_user(kinfo, info);
+}
+
+/**
+ * sys_pidfd_send_signal - send a signal to a process through a task file
+ *                          descriptor
+ * @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.
+ * 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,
+ * grouping is a property of the flags argument not a property of the file
+ * descriptor.
+ *
+ * Return: 0 on success, negative errno on failure
+ */
+SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
+		siginfo_t __user *, info, unsigned int, flags)
+{
+	int ret;
+	struct fd f;
+	struct pid *pid;
+	kernel_siginfo_t kinfo;
+
+	/* Enforce flags be set to 0 until we add an extension. */
+	if (flags)
+		return -EINVAL;
+
+	f = fdget_raw(pidfd);
+	if (!f.file)
+		return -EBADF;
+
+	pid = tid_pidfd_to_pid(f.file);
+	if (!IS_ERR(pid)) {
+		/*
+		 * Give userspace a way to detect /proc/<pid>/task/<tid>
+		 * support when we add it.
+		 */
+		ret = -EOPNOTSUPP;
+		goto err;
+	}
+
+	/* Is this a pidfd? */
+	pid = tgid_pidfd_to_pid(f.file);
+	if (IS_ERR(pid)) {
+		ret = PTR_ERR(pid);
+		goto err;
+	}
+
+	ret = -EINVAL;
+	if (!access_pidfd_pidns(pid))
+		goto err;
+
+	if (info) {
+		ret = copy_siginfo_from_user_any(&kinfo, info);
+		if (unlikely(ret))
+			goto err;
+
+		ret = -EINVAL;
+		if (unlikely(sig != kinfo.si_signo))
+			goto err;
+
+		if ((task_pid(current) != pid) &&
+		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)) {
+			/* Only allow sending arbitrary signals to yourself. */
+			ret = -EPERM;
+			if (kinfo.si_code != SI_USER)
+				goto err;
+
+			/* Turn this into a regular kill signal. */
+			prepare_kill_siginfo(sig, &kinfo);
+		}
+	} else {
+		prepare_kill_siginfo(sig, &kinfo);
+	}
+
+	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)
 {

-- 
2.19.1