Bug#21723: Should be able to dump core after setuid() under Linux

In many cases, binaries can no longer dump core after calling setuid().

Where the PR_SET_DUMPABLE macro is set, use the prctl() system call 
to tell the kernel that it's allowed to dump the core of the server.


configure.in:
  Test system for "sys/prctl.h", to get access to prctl().
sql/mysqld.cc:
  Add a process-control operation that tells the Linux kernel that it 
  is allowed to dump core after setuid().

configure.in

@@ -776,6 +776,7 @@ AC_CHECK_HEADERS(fcntl.h float.h floatingpoint.h ieeefp.h limits.h \
  sys/timeb.h sys/types.h sys/un.h sys/vadvise.h sys/wait.h term.h \
  unistd.h utime.h sys/utime.h termio.h termios.h sched.h crypt.h alloca.h \
  sys/ioctl.h malloc.h sys/malloc.h sys/ipc.h sys/shm.h linux/config.h \
+ sys/prctl.h \
  sys/resource.h sys/param.h)
 
 #--------------------------------------------------------------------

sql/mysqld.cc

@@ -32,6 +32,9 @@
 #ifdef HAVE_NDBCLUSTER_DB
 #include "ha_ndbcluster.h"
 #endif
+#ifdef HAVE_SYS_PRCTL_H
+#include <sys/prctl.h>
+#endif
 
 #ifdef HAVE_INNOBASE_DB
 #define OPT_INNODB_DEFAULT 1
@@ -1365,6 +1368,15 @@ static struct passwd *check_user(const char *user)
 err:
   sql_print_error("Fatal error: Can't change to run as user '%s' ;  Please check that the user exists!\n",user);
   unireg_abort(1);
+
+#ifdef PR_SET_DUMPABLE
+  if (test_flags & TEST_CORE_ON_SIGNAL)
+  {
+    /* inform kernel that process is dumpable */
+    (void) prctl(PR_SET_DUMPABLE, 1);
+  }
+#endif
+
 #endif
   return NULL;
 }