kill: Support mandating the presence of a userspace signal handler

In production we've had several incidents over the years where a process
has a signal handler registered for SIGHUP or one of the SIGUSR signals
which can be used to signal a request to reload configs, rotate log
files, and the like. While this may seem harmless enough, what we've
seen happen repeatedly is something like the following:

1. A process is using SIGHUP/SIGUSR[12] to request some
   application-handled state change -- reloading configs, rotating a log
   file, etc;
2. This kind of request is deprecated and removed, so the signal handler
   is removed. However, a site where the signal might be sent from is
   missed (often logrotate or a service manager);
3. Because the default disposition of these signals is terminal, sooner
   or later these applications are going to be sent SIGHUP or similar
   and end up unexpectedly killed.

I know for a fact that we're not the only organistion experiencing this:
in general, signal use is pretty tricky to reason about and safely
remove because of the fairly aggressive SIG_DFL behaviour for some
common signals, especially for SIGHUP which has a particularly ambiguous
meaning. Especially in a large, highly interconnected codebase,
reasoning about signal interactions between system configuration and
applications can be highly complex, and it's inevitable that on occasion
a callsite will be missed.

In some cases the right call to avoid this will be to migrate services
towards other forms of IPC for this purpose, but inevitably there will
be some services which must continue using signals, so we need a safe
way to support them.

This patch adds support for the -r/--require-handler flag, which checks
if a userspace handler is present for the signal being sent. If it is
not, the process will be skipped.

With this flag we can enforce that all SIGHUP reload cases and SIGUSR
equivalents use --require-handler. This effectively mitigates the case
we've seen time and time again where SIGHUP is used to rotate log files
or reload configs, but the sending site is mistakenly left present after
the removal of signal handler, resulting in unintended termination of
the process.

Signed-off-by: Chris Down <chris@chrisdown.name>

diff --git a/misc-utils/kill.1.adoc b/misc-utils/kill.1.adoc
index 4a6996a38015dc5945b14989f4ad76c8d4df5786..40ab0248d1e14cdce9c157ddc7c95ccb888ee3c2 100644 (file)
--- a/misc-utils/kill.1.adoc
+++ b/misc-utils/kill.1.adoc
@@ -62,6 +62,8 @@ Similar to *-l*, but it will print signal names and their corresponding numbers.
 Do not restrict the command-name-to-PID conversion to processes with the same UID as the present process.
 *-p*, *--pid*::
 Only print the process ID (PID) of the named processes, do not send any signals.
+*-r*, *--require-handler*::
+Do not send the signal if it is not caught in userspace by the signalled process.
 *--verbose*::
 Print PID(s) that will be signaled with *kill* along with the signal.
 *-q*, *--queue* _value_::

diff --git a/misc-utils/kill.c b/misc-utils/kill.c
index f557aacaabe0f3ef693b6f4e8cee1404f9e93b21..c469074b95f952cc1feaaca0f675f46a4e10b189 100644 (file)
--- a/misc-utils/kill.c
+++ b/misc-utils/kill.c
@@ -95,6 +95,7 @@ struct kill_control {
                check_all:1,
                do_kill:1,
                do_pid:1,
+               require_handler:1,
                use_sigval:1,
 #ifdef UL_HAVE_PIDFD
                timeout:1,
@@ -212,6 +213,7 @@ static void __attribute__((__noreturn__)) usage(void)
        fputs(_(" -p, --pid              print pids without signaling them\n"), out);
        fputs(_(" -l, --list[=<signal>]  list signal names, or convert a signal number to a name\n"), out);
        fputs(_(" -L, --table            list signal names and numbers\n"), out);
+       fputs(_(" -r, --require-handler  do not send signal if signal handler is not present\n"), out);
        fputs(_("     --verbose          print pids that will be signaled\n"), out);
 
        fputs(USAGE_SEPARATOR, out);
@@ -302,6 +304,10 @@ static char **parse_arguments(int argc, char **argv, struct kill_control *ctl)
                        print_all_signals(stdout, 1);
                        exit(EXIT_SUCCESS);
                }
+               if (!strcmp(arg, "-r") || !strcmp(arg, "--require-handler")) {
+                       ctl->require_handler = 1;
+                       continue;
+               }
                if (!strcmp(arg, "-p") || !strcmp(arg, "--pid")) {
                        ctl->do_pid = 1;
                        if (ctl->do_kill)
@@ -448,6 +454,32 @@ static int kill_verbose(const struct kill_control *ctl)
        return rc;
 }
 
+static int check_signal_handler(const struct kill_control *ctl)
+{
+       uintmax_t sigcgt = 0;
+       int rc = 0, has_hnd = 0;
+       struct path_cxt *pc;
+
+       if (!ctl->require_handler)
+               return 1;
+
+       pc = ul_new_procfs_path(ctl->pid, NULL);
+       if (!pc)
+               return -ENOMEM;
+
+       rc = procfs_process_get_stat_nth(pc, 34, &sigcgt);
+       if (rc)
+               return -EINVAL;
+
+       ul_unref_path(pc);
+
+       has_hnd = ((1UL << (ctl->numsig - 1)) & sigcgt) != 0;
+       if (ctl->verbose && !has_hnd)
+               printf(_("not signalling pid %d, it has no userspace handler for signal %d\n"), ctl->pid, ctl->numsig);
+
+       return has_hnd;
+}
+
 int main(int argc, char **argv)
 {
        struct kill_control ctl = { .numsig = SIGTERM };
@@ -470,6 +502,8 @@ int main(int argc, char **argv)
                errno = 0;
                ctl.pid = strtol(ctl.arg, &ep, 10);
                if (errno == 0 && ep && *ep == '\0' && ctl.arg < ep) {
+                       if (check_signal_handler(&ctl) <= 0)
+                               continue;
                        if (kill_verbose(&ctl) != 0)
                                nerrs++;
                        ct++;
@@ -491,6 +525,8 @@ int main(int argc, char **argv)
                                        continue;
                                if (procfs_dirent_get_pid(d, &ctl.pid) != 0)
                                        continue;
+                               if (check_signal_handler(&ctl) <= 0)
+                                       continue;
 
                                if (kill_verbose(&ctl) != 0)
                                        nerrs++;