This seems like a very trivial bug that should have been squashed a long

time ago, but for some reason it was not. Basically, without this change
dlopen(3)'ing an empty .so file would just cause application to dump core
with SIGSEGV.

Make sure the file has enough data for at least the ELF header before
mmap'ing it.

Add a test case to check that dlopen an empty file return an error.

There were a separate discussion as to whether it should be SIGBUS
instead when you try to access region mapped from an empty file,
but it's definitely SIGSEGV now, so if anyone want to check that please
be my guest.
Reviewed by:	mjg, cem
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D5112

lib/libc/tests/gen/Makefile

@@ -11,6 +11,7 @@ ATF_TESTS_C+=		ftw_test
 ATF_TESTS_C+=		popen_test
 ATF_TESTS_C+=		posix_spawn_test
 ATF_TESTS_C+=		wordexp_test
+ATF_TESTS_C+=		dlopen_empty_test
 
 # TODO: t_closefrom, t_cpuset, t_fmtcheck, t_randomid, t_sleep
 # TODO: t_siginfo (fixes require further inspection)

lib/libc/tests/gen/dlopen_empty_test.c

@@ -0,0 +1,97 @@
+/*-
+ * Copyright (c) 2016 Maksym Sobolyev
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <sys/stat.h>
+#include <dlfcn.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include <atf-c.h>
+
+static const char *funname;
+static char *soname;
+
+static void
+sigsegv_handler(int sig __unused)
+{
+        unlink(soname);
+        free(soname);
+        atf_tc_fail("got SIGSEGV in the %s(3)", funname);
+}
+
+ATF_TC(dlopen_empty_test);
+ATF_TC_HEAD(dlopen_empty_test, tc)
+{
+        atf_tc_set_md_var(tc, "descr", "Tests the dlopen() of an empty file "
+                      "returns an error");
+}
+ATF_TC_BODY(dlopen_empty_test, tc)
+{
+        char tempname[] = "/tmp/temp.XXXXXX";
+        char *fname;
+        int fd;
+        void *dlh;
+        struct sigaction act, oact;
+
+        fname = mktemp(tempname);
+        ATF_REQUIRE_MSG(fname != NULL, "mktemp failed; errno=%d", errno);
+        asprintf(&soname, "%s.so", fname);
+        ATF_REQUIRE_MSG(soname != NULL, "asprintf failed; errno=%d", ENOMEM);
+        fd = open(soname, O_WRONLY | O_CREAT | O_TRUNC, DEFFILEMODE);
+        ATF_REQUIRE_MSG(fd != -1, "open(\"%s\") failed; errno=%d", soname, errno);
+        close(fd);
+
+        act.sa_handler = sigsegv_handler;
+        act.sa_flags = 0;
+        sigemptyset(&act.sa_mask);
+        ATF_CHECK_MSG(sigaction(SIGSEGV, &act, &oact) != -1,
+            "sigaction() failed");
+
+        funname = "dlopen";
+        dlh = dlopen(soname, RTLD_LAZY);
+        if (dlh != NULL) {
+                funname = "dlclose";
+                dlclose(dlh);
+        }
+        ATF_REQUIRE_MSG(dlh == NULL, "dlopen(\"%s\") did not fail", soname);
+        unlink(soname);
+        free(soname);
+}
+
+ATF_TP_ADD_TCS(tp)
+{
+
+        ATF_TP_ADD_TC(tp, dlopen_empty_test);
+
+        return (atf_no_error());
+}

libexec/rtld-elf/map_object.c

@@ -38,7 +38,7 @@
 #include "debug.h"
 #include "rtld.h"
 
-static Elf_Ehdr *get_elf_header(int, const char *);
+static Elf_Ehdr *get_elf_header(int, const char *, const struct stat *);
 static int convert_prot(int);	/* Elf flags -> mmap protection */
 static int convert_flags(int); /* Elf flags -> mmap flags */
 
@@ -91,7 +91,7 @@ map_object(int fd, const char *path, const struct stat *sb)
     char *note_map;
     size_t note_map_len;
 
-    hdr = get_elf_header(fd, path);
+    hdr = get_elf_header(fd, path, sb);
     if (hdr == NULL)
 	return (NULL);
 
@@ -324,10 +324,16 @@ map_object(int fd, const char *path, const struct stat *sb)
 }
 
 static Elf_Ehdr *
-get_elf_header(int fd, const char *path)
+get_elf_header(int fd, const char *path, const struct stat *sbp)
 {
 	Elf_Ehdr *hdr;
 
+	/* Make sure file has enough data for the ELF header */
+	if (sbp != NULL && sbp->st_size < sizeof(Elf_Ehdr)) {
+		_rtld_error("%s: invalid file format", path);
+		return (NULL);
+	}
+
 	hdr = mmap(NULL, PAGE_SIZE, PROT_READ, MAP_PRIVATE | MAP_PREFAULT_READ,
 	    fd, 0);
 	if (hdr == (Elf_Ehdr *)MAP_FAILED) {