Message-ID: <20181217171004.zxqwp2nm7rq2mxex@brauner.io>
Date:   Mon, 17 Dec 2018 18:10:06 +0100
From:   Christian Brauner <christian@...uner.io>
To:     "Serge E. Hallyn" <serge@...lyn.com>, ebiederm@...ssion.com
Cc:     linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        luto@...nel.org, arnd@...db.de, keescook@...omium.org,
        jannh@...gle.com, akpm@...ux-foundation.org, oleg@...hat.com,
        cyphar@...har.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, dancol@...gle.com,
        timmurray@...gle.com, fweimer@...hat.com, tglx@...utronix.de,
        x86@...nel.org
Subject: Re: [PATCH v5 1/1] signal: add pidfd_send_signal() syscall

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:
> > 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" <ebiederm@...ssion.com>
> 
> 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

> 
> > Cc: Jann Horn <jannh@...gle.com>
> > Cc: Andy Lutomirsky <luto@...nel.org>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: Oleg Nesterov <oleg@...hat.com>
> > Cc: Al Viro <viro@...iv.linux.org.uk>
> > Cc: Florian Weimer <fweimer@...hat.com>
> > Signed-off-by: Christian Brauner <christian@...uner.io>
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > Acked-by: Arnd Bergmann <arnd@...db.de>
> > Acked-by: Serge Hallyn <serge@...lyn.com>
> > Acked-by: Aleksa Sarai <cyphar@...har.com>
> > ---
> > 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

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