Use execveat syscall in fexecve (bug 22134)

By using execveat we no longer depend on /proc.  The execveat syscall was
introduced in 3.19, except for a few late comers.

ChangeLog

@@ -1,3 +1,21 @@
+2017-09-19  Andreas Schwab  <schwab@suse.de>
+
+	[BZ #22134]
+	* sysdeps/unix/sysv/linux/fexecve.c (fexecve) [__NR_execveat]: Try
+	execveat first.
+	[!__ASSUME_EXECVEAT]: Fall back to /proc if execveat is
+	unimplemented.
+	* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_EXECVEAT)
+	[__LINUX_KERNEL_VERSION >= 0x031300]: Define.
+	* sysdeps/unix/sysv/linux/alpha/kernel-features.h
+	(__ASSUME_EXECVEAT) [__LINUX_KERNEL_VERSION < 0x040200]: Undef.
+	* sysdeps/unix/sysv/linux/hppa/kernel-features.h
+	(__ASSUME_EXECVEAT) [__LINUX_KERNEL_VERSION < 0x040000]: Undef.
+	* sysdeps/unix/sysv/linux/microblaze/kernel-features.h
+	(__ASSUME_EXECVEAT) [__LINUX_KERNEL_VERSION < 0x040000]: Undef.
+	* posix/Makefile (tests): Add tst-fexecve.
+	* posix/tst-fexecve.c: New file.
+
 2017-09-19  Wilco Dijkstra  <wdijkstr@arm.com>
 
 	* benchtests/Makefile: Add logf benchmark.

posix/Makefile

@@ -93,7 +93,7 @@ tests		:= test-errno tstgetopt testfnm runtests runptests \
 		   tst-fnmatch3 bug-regex36 tst-getaddrinfo5 \
 		   tst-posix_spawn-fd tst-posix_spawn-setsid \
 		   tst-posix_fadvise tst-posix_fadvise64 \
-		   tst-sysconf-empty-chroot tst-glob_symlinks
+		   tst-sysconf-empty-chroot tst-glob_symlinks tst-fexecve
 tests-internal	:= bug-regex5 bug-regex20 bug-regex33 \
 		   tst-rfc3484 tst-rfc3484-2 tst-rfc3484-3
 xtests		:= bug-ga2

posix/tst-fexecve.c

@@ -0,0 +1,88 @@
+/* Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/wait.h>
+
+#include <support/check.h>
+
+/* Try executing "/bin/sh -c true", using FD opened on /bin/sh.  */
+static int
+try_fexecve (int fd)
+{
+  pid_t pid = fork ();
+
+  if (pid == 0)
+    {
+      static const char *const argv[] = {
+	"/bin/sh", "-c", "true", NULL
+      };
+      fexecve (fd, (char *const *) argv, environ);
+      _exit (errno);
+    }
+  if (pid < 0)
+    FAIL_RET ("fork failed: %m");
+
+  pid_t termpid;
+  int status;
+  termpid = TEMP_FAILURE_RETRY (waitpid (pid, &status, 0));
+  if (termpid == -1)
+    FAIL_RET ("waitpid failed: %m");
+  if (termpid != pid)
+    FAIL_RET ("waitpid returned %ld != %ld",
+	      (long int) termpid, (long int) pid);
+  if (!WIFEXITED (status))
+    FAIL_RET ("child hasn't exited normally");
+
+  /* If fexecve is unimplemented mark this test as UNSUPPORTED.  */
+  if (WEXITSTATUS (status) == ENOSYS)
+    FAIL_UNSUPPORTED ("fexecve is unimplemented");
+
+  if (WEXITSTATUS (status) != 0)
+    {
+      errno = WEXITSTATUS (status);
+      FAIL_RET ("fexecve failed: %m");
+    }
+  return 0;
+}
+
+static int
+do_test (void)
+{
+  int fd;
+  int ret;
+
+  fd = open ("/bin/sh", O_RDONLY);
+  if (fd < 0)
+    FAIL_UNSUPPORTED ("/bin/sh cannot be opened: %m");
+  ret = try_fexecve (fd);
+  close (fd);
+
+#ifdef O_PATH
+  fd = open ("/bin/sh", O_RDONLY | O_PATH);
+  if (fd < 0)
+    FAIL_UNSUPPORTED ("/bin/sh cannot be opened (O_PATH): %m");
+  ret |= try_fexecve (fd);
+  close (fd);
+#endif
+
+  return ret;
+}
+
+#include <support/test-driver.c>

sysdeps/unix/sysv/linux/alpha/kernel-features.h

@@ -35,4 +35,9 @@
 #define __ASSUME_RECV_SYSCALL	1
 #define __ASSUME_SEND_SYSCALL	1
 
+/* Support for the execveat syscall was added in 4.2.  */
+#if __LINUX_KERNEL_VERSION < 0x040200
+# undef __ASSUME_EXECVEAT
+#endif
+
 #endif /* _KERNEL_FEATURES_H */

sysdeps/unix/sysv/linux/fexecve.c

@@ -19,8 +19,13 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <unistd.h>
+#include <fcntl.h>
 #include <sys/stat.h>
 
+#include <sysdep.h>
+#include <sys/syscall.h>
+#include <kernel-features.h>
+
 
 /* Execute the file FD refers to, overlaying the running program image.
    ARGV and ENVP are passed to the new program, as for `execve'.  */
@@ -33,6 +38,15 @@ fexecve (int fd, char *const argv[], char *const envp[])
       return -1;
     }
 
+#ifdef __NR_execveat
+  INLINE_SYSCALL (execveat, 5, fd, "", argv, envp, AT_EMPTY_PATH);
+# ifndef __ASSUME_EXECVEAT
+  if (errno != ENOSYS)
+    return -1;
+# endif
+#endif
+
+#ifndef __ASSUME_EXECVEAT
   /* We use the /proc filesystem to get the information.  If it is not
      mounted we fail.  */
   char buf[sizeof "/proc/self/fd/" + sizeof (int) * 3];
@@ -50,6 +64,7 @@ fexecve (int fd, char *const argv[], char *const envp[])
     save = ENOSYS;
 
   __set_errno (save);
+#endif
 
   return -1;
 }

sysdeps/unix/sysv/linux/hppa/kernel-features.h

@@ -27,3 +27,8 @@
 
 #define __ASSUME_RECV_SYSCALL   1
 #define __ASSUME_SEND_SYSCALL	1
+
+/* Support for the execveat syscall was added in 4.0.  */
+#if __LINUX_KERNEL_VERSION < 0x040000
+# undef __ASSUME_EXECVEAT
+#endif

sysdeps/unix/sysv/linux/kernel-features.h

@@ -102,3 +102,8 @@
    implementation does not assume the __ASSUME_* and instead use a fallback
    implementation based on p{read,write}v and returning an error for
    non supported flags.  */
+
+/* Support for the execveat syscall was added in 3.19.  */
+#if __LINUX_KERNEL_VERSION >= 0x031300
+# define __ASSUME_EXECVEAT	1
+#endif

sysdeps/unix/sysv/linux/microblaze/kernel-features.h

@@ -47,3 +47,8 @@
 #if __LINUX_KERNEL_VERSION < 0x030300
 # undef __ASSUME_SENDMMSG_SYSCALL
 #endif
+
+/* Support for the execveat syscall was added in 4.0.  */
+#if __LINUX_KERNEL_VERSION < 0x040000
+# undef __ASSUME_EXECVEAT
+#endif