From e447e5bad3ce6508c9d52f3dcbcc0c9c84aba823 Mon Sep 17 00:00:00 2001
From: Michal Sekletar <msekleta@redhat.com>
Date: Thu, 11 Jul 2019 14:53:24 +0200
Subject: [PATCH] signal.7: Clarify that siginfo_t isn't changed on coalescing

Confirmed by experiment by mtk:

$ cat siginfo_nonqueuing.c

                        } while (0)

static void
grimReaper(int sig, siginfo_t *si, void *ucontext)
{
    printf("caught signal %d\n", sig);

    printf("    si_pid=%ld, si_uid=%ld, si_status=%d\n",
            (long) si->si_pid, (long) si->si_uid, si->si_status);
}

static void
child(int sleepTime, uid_t uid, int status)
{
    switch (fork()) {
    case -1:
        errExit("fork");
    case 0:
        sleep(sleepTime);
        if (geteuid() == 0)
            setuid(uid);

        printf("Child %ld with UID %ld exiting with status %d\n",
                (long) getpid(),(long) getuid(), status);
        exit(status);
    default:
        return;
    }
}

int
main(int argc, char *argv[])
{
    struct sigaction sa;
    sigset_t blocking;

    sa.sa_sigaction = grimReaper;
    sa.sa_flags = SA_SIGINFO;
    sigemptyset(&sa.sa_mask);

    if (sigaction(SIGCHLD, &sa, NULL) == -1)
        errExit("sigaction");

    sigemptyset(&blocking);
    sigaddset(&blocking, SIGCHLD);
    if (sigprocmask(SIG_BLOCK, &blocking, NULL) == -1)
        errExit("sigprocmask");

    child(2, 20000, 20);
    child(3, 30000, 30);
    child(1, 10000, 10);

    sleep(5);

    if (sigprocmask(SIG_UNBLOCK, &blocking, NULL) == -1)
        errExit("sigprocmask");

    exit(EXIT_SUCCESS);
}
$ ./siginfo_nonqueuing
Child 4042 with UID 1000 exiting with status 10
Child 4040 with UID 1000 exiting with status 20
Child 4041 with UID 1000 exiting with status 30
caught signal 17
    si_pid=4042, si_uid=1000, si_status=10

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Lennart Poettering <lennart@poettering.net>

Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Michal Sekletar <msekleta@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
---
 man7/signal.7 | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/man7/signal.7 b/man7/signal.7
index 3074f573e..0250e83d3 100644
--- a/man7/signal.7
+++ b/man7/signal.7
@@ -436,6 +436,11 @@ Real-time signals are distinguished by the following:
 Multiple instances of real-time signals can be queued.
 By contrast, if multiple instances of a standard signal are delivered
 while that signal is currently blocked, then only one instance is queued.
+Note that the
+.I siginfo_t
+structure associated with the signal already in queue is not overwritten
+on arrival of subsequent instances of the same signal hence the process would
+receive metadata associated with the first instance of the signal.
 .IP 2. 4
 If the signal is sent using
 .BR sigqueue (3),