Re: [PATCH v7 1/2] signal: add pidfd_send_signal() syscall

[Christian Brauner]

On Wed, Jan 02, 2019 at 05:16:53PM +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]). Clarification for this has been
> requested by Eric (cf. [19]).
> 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.
> 
> /* 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);
>  }
> 
> /* Q&A
>  * Given that it seems the same questions get asked again by people who are
>  * late to the party it makes sense to add a Q&A section to the commit
>  * message so it's hopefully easier to avoid duplicate threads.
>  *
>  * For the sake of progress please consider these arguments settled unless
>  * there is a new point that desperately needs to be addressed. Please make
>  * sure to check the links to the threads in this commit message whether
>  * this has not already been covered.
>  */
> Q-01: (Florian Weimer [20], Andrew Morton [21])
>       What happens when the target process has exited?
> A-01: Sending the signal will fail with ESRCH (cf. [22]).
> 
> Q-02:  (Andrew Morton [21])
>        Is the task_struct pinned by the fd?
> A-02:  No. A reference to struct pid is kept. struct pid - as far as I
>        understand - was created exactly for the reason to not require to
>        pin struct task_struct (cf. [22]).
> 
> Q-03: (Andrew Morton [21])
>       Does the entire procfs directory remain visible? Just one entry
>       within it?
> A-03: The same thing that happens right now when you hold a file descriptor
>       to /proc/<pid> open (cf. [22]).
> 
> Q-04: (Andrew Morton [21])
>       Does the pid remain reserved?
> A-04: No. This patchset guarantees a stable handle not that pids are not
>       recycled (cf. [22]).
> 
> Q-05: (Andrew Morton [21])
>       Do attempts to signal that fd return errors?
> A-05: See {Q,A}-01.
> 
> Q-06: (Andrew Morton [22])
>       Is there a cleaner way of obtaining the fd? Another syscall perhaps.
> A-06: Userspace can already trivially retrieve file descriptors from procfs
>       so this is something that we will need to support anyway. Hence,
>       there's no immediate need to add another syscalls just to make
>       pidfd_send_signal() not dependent on the presence of procfs. However,
>       adding a syscalls to get such file descriptors is planned for a
>       future patchset (cf. [22]).
> 
> Q-07: (Andrew Morton [21] and others)
>       This fd-for-a-process sounds like a handy thing and people may well
>       think up other uses for it in the future, probably unrelated to
>       signals. Are the code and the interface designed to permit such
>       future applications?
> A-07: Yes (cf. [22]).
> 
> Q-08: (Andrew Morton [21] and others)
>       Now I think about it, why a new syscall? This thing is looking
>       rather like an ioctl?
> A-08: This has been extensively discussed. It was agreed that a syscall is
>       preferred for a variety or reasons. Here are just a few taken from
>       prior threads. Syscalls are safer than ioctl()s especially when
>       signaling to fds. Processes are a core kernel concept so a syscall
>       seems more appropriate. The layout of the syscall with its four
>       arguments would require the addition of a custom struct for the
>       ioctl() thereby causing at least the same amount or even more
>       complexity for userspace than a simple syscall. The new syscall will
>       replace multiple other pid-based syscalls (see description above).
>       The file-descriptors-for-processes concept introduced with this
>       syscall will be extended with other syscalls in the future. See also
>       [22], [23] and various other threads already linked in here.
> 
> Q-09: (Florian Weimer [24])
>       What happens if you use the new interface with an O_PATH descriptor?
> A-09:
>       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. See also [4].
> 
> /* References */
> [1]:  https://lore.kernel.org/lkml/20181029221037.87724-1-dancol@xxxxxxxxxx/
> [2]:  https://lore.kernel.org/lkml/874lbtjvtd.fsf@xxxxxxxxxxxxxxxxxxxxxxxxx/
> [3]:  https://lore.kernel.org/lkml/20181204132604.aspfupwjgjx6fhva@xxxxxxxxxx/
> [4]:  https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@xxxxxxxxxx/
> [5]:  https://lore.kernel.org/lkml/20181121213946.GA10795@xxxxxxxxxxxxxxx/
> [6]:  https://lore.kernel.org/lkml/20181120103111.etlqp7zop34v6nv4@xxxxxxxxxx/
> [7]:  https://lore.kernel.org/lkml/36323361-90BD-41AF-AB5B-EE0D7BA02C21@xxxxxxxxxxxxxx/
> [8]:  https://lore.kernel.org/lkml/87tvjxp8pc.fsf@xxxxxxxxxxxx/
> [9]:  https://asciinema.org/a/IQjuCHew6bnq1cr78yuMv16cy
> [10]: https://lore.kernel.org/lkml/20181203180224.fkvw4kajtbvru2ku@xxxxxxxxxx/
> [11]: https://lore.kernel.org/lkml/F53D6D38-3521-4C20-9034-5AF447DF62FF@xxxxxxxxxxxxxx/
> [12]: https://lore.kernel.org/lkml/87zhtjn8ck.fsf@xxxxxxxxxxxx/
> [13]: https://lore.kernel.org/lkml/871s6u9z6u.fsf@xxxxxxxxxxxx/
> [14]: https://lore.kernel.org/lkml/20181206231742.xxi4ghn24z4h2qki@xxxxxxxxxx/
> [15]: https://lore.kernel.org/lkml/20181207003124.GA11160@xxxxxxxxxxxxxxx/
> [16]: https://lore.kernel.org/lkml/20181207015423.4miorx43l3qhppfz@xxxxxxxxxx/
> [17]: https://lore.kernel.org/lkml/CAGXu5jL8PciZAXvOvCeCU3wKUEB_dU-O3q0tDw4uB_ojMvDEew@xxxxxxxxxxxxxx/
> [18]: https://lore.kernel.org/lkml/20181206222746.GB9224@xxxxxxxxxxxxxxx/
> [19]: https://lore.kernel.org/lkml/20181208054059.19813-1-christian@xxxxxxxxxx/
> [20]: https://lore.kernel.org/lkml/8736rebl9s.fsf@xxxxxxxxxxxxxxxxxxxxxxxx/
> [21]: https://lore.kernel.org/lkml/20181228152012.dbf0508c2508138efc5f2bbe@xxxxxxxxxxxxxxxxxxxx/
> [22]: https://lore.kernel.org/lkml/20181228233725.722tdfgijxcssg76@xxxxxxxxxx/
> [23]: https://lwn.net/Articles/773459/
> [24]: https://lore.kernel.org/lkml/8736rebl9s.fsf@xxxxxxxxxxxxxxxxxxxxxxxx/
> 
> Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Andy Lutomirsky <luto@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Florian Weimer <fweimer@xxxxxxxxxx>
> Signed-off-by: Christian Brauner <christian@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
> Acked-by: Aleksa Sarai <cyphar@xxxxxxxxxx>

We now have a separate repo on kernel.org for future work related to
pidfds [1]. This should be the target tree from which we can send prs
for new syscalls etc. The target branch is named "pidfd".
Patches for a new merge window will be placed in the "for-next" branch.
The "for-next" branch is already tracked by Stephen in linux-next.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/

Thanks!
Christian