linux: Implement tmpfile with O_TMPFILE (BZ#21530)

This patch adds O_TMPFILE support to tmpfile on Linux.  This is
similar to the previous suggestion by Andreas Schwab [1] with the
difference the file descriptor creation is parameterized to
compartmentalize Linux only open flags (O_TMPFILE) on sysdeps.

Checked on x86_64-linux-gnu.

	Adhemerval Zanella  <adhemerval.zanella@linaro.org>
	Andreas Schwab  <schwab@suse.de>

	[BZ #21530]
	* include/stdio.h (__gen_tempfd): New function.
	* stdio-common/Makefile (routines): Add gentempfd.
	* stdio-common/gentempfd.c: New file.
	* sysdeps/unix/sysv/linux/gentempfd.c: Likewise.
	* stdio-common/tmpfile.c (tmpfile): First try to use a system specific
	unnamed file first.

[1] https://sourceware.org/ml/libc-alpha/2017-06/msg01293.html

3 files changed, 44 insertions, 10 deletions

diff --git a/stdio-common/Makefile b/stdio-common/Makefile
index 397e0c2..2c3c2e5 100644
--- a/stdio-common/Makefile
+++ b/stdio-common/Makefile
@@ -39,7 +39,7 @@ routines	:=							      \
 	flockfile ftrylockfile funlockfile				      \
 	isoc99_scanf isoc99_vscanf isoc99_fscanf isoc99_vfscanf isoc99_sscanf \
 	isoc99_vsscanf							      \
-	psiginfo
+	psiginfo gentempfd
 
 aux	:= errlist siglist printf-parsemb printf-parsewc fxprintf
 
diff --git a/stdio-common/gentempfd.c b/stdio-common/gentempfd.c
new file mode 100644
index 0000000..d40c57d
--- /dev/null
+++ b/stdio-common/gentempfd.c
@@ -0,0 +1,26 @@
+/* Generate a temporary file descriptor.  Generic/POSIX version.
+   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 <stdio.h>
+
+int
+__gen_tempfd (int flags)
+{
+  return -1;
+}
+libc_hidden_def (__gen_tempfd)
diff --git a/stdio-common/tmpfile.c b/stdio-common/tmpfile.c
index e6030be..c5cbb68 100644
--- a/stdio-common/tmpfile.c
+++ b/stdio-common/tmpfile.c
@@ -34,23 +34,31 @@
 FILE *
 tmpfile (void)
 {
-  char buf[FILENAME_MAX];
   int fd;
   FILE *f;
-
-  if (__path_search (buf, FILENAME_MAX, NULL, "tmpf", 0))
-    return NULL;
   int flags = 0;
 #ifdef FLAGS
   flags = FLAGS;
 #endif
-  fd = __gen_tempname (buf, 0, flags, __GT_FILE);
+
+  /* First try a system specific method.  */
+  fd = __gen_tempfd (flags);
+
   if (fd < 0)
-    return NULL;
+    {
+      char buf[FILENAME_MAX];
+
+      if (__path_search (buf, sizeof buf, NULL, "tmpf", 0))
+	return NULL;
+
+      fd = __gen_tempname (buf, 0, flags, __GT_FILE);
+      if (fd < 0)
+	return NULL;
 
-  /* Note that this relies on the Unix semantics that
-     a file is not really removed until it is closed.  */
-  (void) __unlink (buf);
+      /* Note that this relies on the Unix semantics that
+	 a file is not really removed until it is closed.  */
+      (void) __unlink (buf);
+    }
 
   if ((f = __fdopen (fd, "w+b")) == NULL)
     __close (fd);