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    Subject[PATCH v1 3/4] signal: support pidctl() with pidfd_send_signal()
    Date
    Let pidfd_send_signal() use pidfds retrieved via pidctl(). With this patch
    pidfd_send_signal() becomes independent of procfs. This fullfils the
    request made when we merged the pidfd_send_signal() patchset. The
    pidfd_send_signal() syscall is now always available allowing for it to be
    used by users without procfs mounted or even users without procfs support
    compiled into the kernel.

    Signed-off-by: Christian Brauner <christian@brauner.io>
    Reviewed-by: David Howells <dhowells@redhat.com>
    Acked-by: Serge Hallyn <serge@hallyn.com>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: "Eric W. Biederman" <ebiederm@xmission.com>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Jann Horn <jannh@google.com
    Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
    Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
    Cc: Jonathan Kowalski <bl0pbl33p@gmail.com>
    Cc: "Dmitry V. Levin" <ldv@altlinux.org>
    Cc: Andy Lutomirsky <luto@kernel.org>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>
    Cc: Aleksa Sarai <cyphar@cyphar.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    ---
    /* changelog */
    v1:
    - Jann Horn <jannh@google.com> in [1]:
    - make access_pidfd_pidns() more readable
    ---
    kernel/signal.c | 29 ++++++++++++-----------------
    kernel/sys_ni.c | 3 ---
    2 files changed, 12 insertions(+), 20 deletions(-)

    diff --git a/kernel/signal.c b/kernel/signal.c
    index b7953934aa99..7bdeda8333c8 100644
    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -3513,26 +3513,14 @@ SYSCALL_DEFINE2(kill, pid_t, pid, int, sig)
    return kill_something_info(sig, &info, pid);
    }

    -#ifdef CONFIG_PROC_FS
    /*
    * 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)
    +static inline 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;
    + return pidnscmp(task_active_pid_ns(current), ns_of_pid(pid)) >= 0;
    }

    static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
    @@ -3550,6 +3538,14 @@ static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo, siginfo_t *info)
    return copy_siginfo_from_user(kinfo, info);
    }

    +static struct pid *pidfd_to_pid(const struct file *file)
    +{
    + if (file->f_op == &pidfd_fops)
    + return file->private_data;
    +
    + return tgid_pidfd_to_pid(file);
    +}
    +
    /**
    * sys_pidfd_send_signal - send a signal to a process through a task file
    * descriptor
    @@ -3581,12 +3577,12 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
    if (flags)
    return -EINVAL;

    - f = fdget_raw(pidfd);
    + f = fdget(pidfd);
    if (!f.file)
    return -EBADF;

    /* Is this a pidfd? */
    - pid = tgid_pidfd_to_pid(f.file);
    + pid = pidfd_to_pid(f.file);
    if (IS_ERR(pid)) {
    ret = PTR_ERR(pid);
    goto err;
    @@ -3625,7 +3621,6 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
    fdput(f);
    return ret;
    }
    -#endif /* CONFIG_PROC_FS */

    static int
    do_send_specific(pid_t tgid, pid_t pid, int sig, struct kernel_siginfo *info)
    diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
    index d21f4befaea4..4d9ae5ea6caf 100644
    --- a/kernel/sys_ni.c
    +++ b/kernel/sys_ni.c
    @@ -167,9 +167,6 @@ COND_SYSCALL(syslog);

    /* kernel/sched/core.c */

    -/* kernel/signal.c */
    -COND_SYSCALL(pidfd_send_signal);
    -
    /* kernel/sys.c */
    COND_SYSCALL(setregid);
    COND_SYSCALL(setgid);
    --
    2.21.0
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