libc: Add support to allow library calls to find appropriate shell executable for a process

Library calls like system() and popen() invoke the shell executable
pointed to by '_PATH_BSHELL' in order to run the command passed into the
function. The _PATH_BSHELL points to /system/bin/sh by default and thus
breaks any vendor process trying to use system() / popen(), as they are
denied access to system shell by selinux.

This CL make necessary changes, so the implmentations of system() and popen()
can use the appropriate shell (e.g. /vendor/bin/sh for processes running
out of /vendor partition). Also, changes the implementation of system()
and popen().

Bug: 64832610
Test: Manual, Using a test program running from /system/bin and
      /vendor/bin to ensure correct shell is being used.

Change-Id: Ie7168d69decb1ae98284446ae7db34dec930dc33
Merged-In: Ie7168d69decb1ae98284446ae7db34dec930dc33
Signed-off-by: Sandeep Patil <sspatil@google.com>
(cherry picked from commit aa3e32422c)

libc/Android.bp

@@ -1385,6 +1385,7 @@ cc_library_static {
     defaults: ["libc_defaults"],
     srcs: [
         "bionic/NetdClientDispatch.cpp",
+        "bionic/__bionic_get_shell_path.cpp",
         "bionic/__cmsg_nxthdr.cpp",
         "bionic/__errno.cpp",
         "bionic/__gnu_basename.cpp",

libc/bionic/__bionic_get_shell_path.cpp

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * 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 COPYRIGHT HOLDERS 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
+ * COPYRIGHT OWNER 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 <errno.h>
+#include <string.h>
+#include <sys/cdefs.h>
+#include <unistd.h>
+
+__LIBC_HIDDEN__ static const char* __libc_system_sh = "/system/bin/sh";
+__LIBC_HIDDEN__ static const char* __libc_vendor_sh = "/vendor/bin/sh";
+
+static const char* init_sh_path() {
+  /* look for /system or /vendor prefix */
+  char exe_path[7];
+  ssize_t len = readlink("/proc/self/exe", exe_path, sizeof(exe_path));
+  if (len != -1 && !strncmp(exe_path, __libc_vendor_sh, sizeof(exe_path))) {
+    return __libc_vendor_sh;
+  }
+
+  return __libc_system_sh;
+}
+
+__LIBC_HIDDEN__ extern "C" const char* __bionic_get_shell_path() {
+  static const char* sh_path = init_sh_path();
+  return sh_path;
+}

libc/include/paths.h

@@ -34,7 +34,9 @@
 
 #include <sys/cdefs.h>
 
+#ifndef _PATH_BSHELL
 #define	_PATH_BSHELL	"/system/bin/sh"
+#endif
 #define	_PATH_CONSOLE	"/dev/console"
 #define	_PATH_DEFPATH	"/sbin:/system/sbin:/system/bin:/system/xbin:/vendor/bin:/vendor/xbin"
 #define	_PATH_DEV	"/dev/"

libc/upstream-netbsd/android/include/netbsd-compat.h

@@ -44,4 +44,8 @@
 #include <stddef.h>
 int reallocarr(void*, size_t, size_t);
 
+/* Use appropriate shell depending on process's executable. */
+__LIBC_HIDDEN__ extern const char* __bionic_get_shell_path();
+#define _PATH_BSHELL __bionic_get_shell_path()
+
 #endif

libc/upstream-openbsd/android/include/openbsd-compat.h

@@ -71,6 +71,10 @@ extern const char* __progname;
  */
 #define _PATH_TMP "/data/local/tmp/"
 
+/* Use appropriate shell depending on process's executable. */
+__LIBC_HIDDEN__ extern const char* __bionic_get_shell_path();
+#define _PATH_BSHELL __bionic_get_shell_path()
+
 /* We have OpenBSD's getentropy_linux.c, but we don't mention getentropy in any header. */
 __LIBC_HIDDEN__ extern int getentropy(void*, size_t);