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[PATCH] A terminating signal has to complete a bash process


From: Andrei Vagin
Subject: [PATCH] A terminating signal has to complete a bash process
Date: Mon, 30 Apr 2018 15:05:20 -0700

bash sets a handler for all terminating signals, which saves history,
executes traps, sets a default signal handler and re-sends the same
signal to itself. It expects that this signal will kill it.

Unfortunately it doesn't work in Linux, when a bash script is executed as
an init process in a pid namespaces, because all signals to the init
process, what are sent from the current pid namespace, are ignored.

man 7 pid_namespaces
  Only signals for which the "init" process has established a signal han‐
  dler can be sent to the "init" process by  other  members  of  the  PID
  namespace.   This restriction applies even to privileged processes, and
  prevents other members of the PID namespace from  accidentally  killing
  the "init" process.

Chet Ramey suggested to add a call to exit() after the kill(). This
patch adds this call for signals, which do not result in a core dump.
For other signals, a null pointer is dereferenced to get a core file.

Signed-off-by: Andrei Vagin <address@hidden>
---
 shell.h |  1 +
 sig.c   | 49 +++++++++++++++++++++++++++++++++++++++----------
 2 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/shell.h b/shell.h
index ce088791..3aa8485f 100644
--- a/shell.h
+++ b/shell.h
@@ -58,6 +58,7 @@ extern int EOF_Reached;
 #define EX_MISCERROR   2
 
 /* Special exit statuses used by the shell, internally and externally. */
+#define EX_KILLED      123
 #define EX_RETRYFAIL   124
 #define EX_WEXPCOMSUB  125
 #define EX_BINARY_FILE 126
diff --git a/sig.c b/sig.c
index e5bb7399..5c5e15b7 100644
--- a/sig.c
+++ b/sig.c
@@ -128,6 +128,7 @@ struct termsig {
      int signum;
      SigHandler *orig_handler;
      int orig_flags;
+     int core_dump;
 };
 
 #define NULL_HANDLER (SigHandler *)SIG_DFL
@@ -145,15 +146,15 @@ static struct termsig terminating_signals[] = {
 #endif
 
 #ifdef SIGILL
-{  SIGILL, NULL_HANDLER, 0 },
+{  SIGILL, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGTRAP
-{  SIGTRAP, NULL_HANDLER, 0 },
+{  SIGTRAP, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGIOT
-{  SIGIOT, NULL_HANDLER, 0 },
+{  SIGIOT, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGDANGER
@@ -165,19 +166,19 @@ static struct termsig terminating_signals[] = {
 #endif
 
 #ifdef SIGFPE
-{  SIGFPE, NULL_HANDLER, 0 },
+{  SIGFPE, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGBUS
-{  SIGBUS, NULL_HANDLER, 0 },
+{  SIGBUS, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGSEGV
-{  SIGSEGV, NULL_HANDLER, 0 },
+{  SIGSEGV, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGSYS
-{  SIGSYS, NULL_HANDLER, 0 },
+{  SIGSYS, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGPIPE
@@ -193,11 +194,11 @@ static struct termsig terminating_signals[] = {
 #endif
 
 #ifdef SIGXCPU
-{  SIGXCPU, NULL_HANDLER, 0 },
+{  SIGXCPU, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGXFSZ
-{  SIGXFSZ, NULL_HANDLER, 0 },
+{  SIGXFSZ, NULL_HANDLER, 0, 1 },
 #endif
 
 #ifdef SIGVTALRM
@@ -228,6 +229,7 @@ static struct termsig terminating_signals[] = {
 #define XSIG(x) (terminating_signals[x].signum)
 #define XHANDLER(x) (terminating_signals[x].orig_handler)
 #define XSAFLAGS(x) (terminating_signals[x].orig_flags)
+#define XCOREDUMP(x) (terminating_signals[x].core_dump)
 
 static int termsigs_initialized = 0;
 
@@ -375,7 +377,6 @@ reset_terminating_signals ()
 
   termsigs_initialized = 0;
 }
-#undef XSIG
 #undef XHANDLER
 
 /* Run some of the cleanups that should be performed when we run
@@ -557,6 +558,8 @@ termsig_handler (sig)
      int sig;
 {
   static int handling_termsig = 0;
+  int core_dump, i;
+  sigset_t mask;
 
   /* Simple semaphore to keep this function from being executed multiple
      times.  Since we no longer are running as a signal handler, we don't
@@ -598,9 +601,35 @@ termsig_handler (sig)
   executing_list = comsub_ignore_return = return_catch_flag = wait_intr_flag = 
0;
 
   run_exit_trap ();    /* XXX - run exit trap possibly in signal context? */
+
   set_signal_handler (sig, SIG_DFL);
+  sigprocmask (SIG_SETMASK, (sigset_t *)NULL, &mask);
+  sigdelset (&mask, sig);
+  sigprocmask (SIG_SETMASK, &mask, (sigset_t *)NULL);
+
   kill (getpid (), sig);
+
+  /* The previous signal had to kill this process, but it is still alive.
+     We know that this can happen in Linux, if bash is running as an init
+     process in a pid namespace, in this case the process can't kill
+     itself (man 7 pid_namespaces). */
+
+  core_dump = 0;
+  sigprocmask (SIG_SETMASK, (sigset_t *)NULL, &mask);
+  for (i = 0; i < TERMSIGS_LENGTH; i++) {
+    set_signal_handler (XSIG(i), SIG_DFL);
+    sigdelset (&mask, XSIG (i));
+    if (sig == XSIG(i))
+      core_dump = XCOREDUMP(i);
+  }
+  sigprocmask (SIG_SETMASK, &mask, (sigset_t *)NULL);
+
+  if (core_dump)
+    *((volatile unsigned long *) NULL) = 0xdead0000 + sig;
+
+  exit(EX_KILLED);
 }
+#undef XSIG
 
 /* What we really do when SIGINT occurs. */
 sighandler
-- 
2.14.3




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