libast: robustness fixes for temporary files and streams

From an OpenSUSE patch:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-pathtemp.dif

See src/lib/libast/man/path.3 for pathtemp()
and src/lib/libast/man/sfio.3 for sftmp()

src/lib/libast/path/pathtemp.c:
- Error check fix: add an access check wrapper function that checks
  if a path was given and if there is enough free space on the
  device, setting errno appropriately in case of trouble.

src/lib/libast/sfio/sftmp.c:
- On Linux, use the /dev/shm shared memory objects for the new
  temporary file descriptor -- that is, do not access HD or SSD but
  only the memory based tmpfs of the POSIX SHM.

src/lib/libast/path/pathtemp.c

@@ -73,6 +73,7 @@
 #include <ls.h>
 #include <tv.h>
 #include <tm.h>
+#include <error.h>
 
 #define ATTEMPT		10
 
@@ -81,7 +82,40 @@
 #define TMP1		"/tmp"
 #define TMP2		"/var/tmp"
 
-#define VALID(d)	(*(d)&&!eaccess(d,W_OK|X_OK))
+static inline int xaccess(const char *path, int mode)
+{
+	static size_t pgsz;
+	struct statvfs vfs;
+	int ret;
+
+	if (!pgsz)
+		pgsz = strtoul(astconf("PAGESIZE",NiL,NiL),NiL,0);
+
+	if (!path || !*path)
+	{
+		errno = EFAULT;
+		goto err;
+	}
+
+	do
+		ret = statvfs(path, &vfs);
+	while (ret < 0 && errno == EINTR);
+
+	if (ret < 0)
+		goto err;
+
+	if (vfs.f_frsize*vfs.f_bavail < pgsz)
+	{
+		errno = ENOSPC;
+		goto err;
+	}
+
+	return eaccess(path, mode);
+err:
+	return -1;
+}
+
+#define VALID(d)	(*(d)&&!xaccess(d,W_OK|X_OK))
 
 static struct
 {
@@ -182,7 +216,7 @@ pathtemp(char* buf, size_t len, const char* dir, const char* pfx, int* fdp)
 		tv.tv_nsec = 0;
 	else
 		tvgettime(&tv);
-	if (!(d = (char*)dir) || *d && eaccess(d, W_OK|X_OK))
+	if (!(d = (char*)dir) || (*d && xaccess(d, W_OK|X_OK)))
 	{
 		if (!tmp.vec)
 		{
@@ -227,7 +261,7 @@ pathtemp(char* buf, size_t len, const char* dir, const char* pfx, int* fdp)
 			tmp.dir = tmp.vec;
 			d = *tmp.dir++;
 		}
-		if (!d && (!*(d = astconf("TMP", NiL, NiL)) || eaccess(d, W_OK|X_OK)) && eaccess(d = TMP1, W_OK|X_OK) && eaccess(d = TMP2, W_OK|X_OK))
+		if (!d && (!*(d = astconf("TMP", NiL, NiL)) || xaccess(d, W_OK|X_OK)) && xaccess(d = TMP1, W_OK|X_OK) && xaccess(d = TMP2, W_OK|X_OK))
 			return 0;
 	}
 	if (!len)

src/lib/libast/sfio/sftmp.c

@@ -20,6 +20,14 @@
 *                                                                      *
 ***********************************************************************/
 #include	"sfhdr.h"
+#if _PACKAGE_ast
+# if defined(__linux__) && _lib_statfs
+#  include <sys/statfs.h>
+#  ifndef  TMPFS_MAGIC
+#   define TMPFS_MAGIC	0x01021994
+#  endif
+# endif
+#endif
 
 /*	Create a temporary stream for read/write.
 **	The stream is originally created as a memory-resident stream.
@@ -207,7 +215,24 @@ Sfio_t*	f;
 	int		fd;
 
 #if _PACKAGE_ast
+# if defined(__linux__) && _lib_statfs
+	/*
+	 * Use the area of POSIX shared memory objects for the new temporary file descriptor
+	 * that is do not access HD or SSD but only the memory based tmpfs of the POSIX SHM
+	 */
+	static int doshm;
+	static char *shm = "/dev/shm";
+	if (!doshm)
+	{
+		struct statfs fs;
+		if (statfs(shm, &fs) < 0 || fs.f_type != TMPFS_MAGIC || eaccess(shm, W_OK|X_OK))
+			shm = NiL;
+		doshm++;
+	}
+	if(!(file = pathtemp(NiL,PATH_MAX,shm,"sf",&fd)))
+# else
 	if(!(file = pathtemp(NiL,PATH_MAX,NiL,"sf",&fd)))
+# endif
 		return -1;
 	_rmtmp(f, file);
 	free(file);