try to get rid of races in hostfs open()

In case of mode mismatch, do *not* blindly close the descriptor
another openers might be using right now.  Open the underlying
file with currently sufficient mode, then
	* if current mode has grown so that it's sufficient for
us now, just close our new fd
	* if current mode has grown and our fd is *not* enough
to cover it, close and repeat.
	* otherwise, install our fd if the file hadn't been
opened at all or dup2() our fd over the current one (and close
our fd).
Critical section is protected by mutex; yes, system-wide.  All
we do under it is a bunch of comparison and maybe an overwriting
dup2() on host.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/hostfs/hostfs.h

@@ -74,6 +74,7 @@ extern void *open_dir(char *path, int *err_out);
 extern char *read_dir(void *stream, unsigned long long *pos,
 		      unsigned long long *ino_out, int *len_out);
 extern void close_file(void *stream);
+extern int replace_file(int oldfd, int fd);
 extern void close_dir(void *stream);
 extern int read_file(int fd, unsigned long long *offset, char *buf, int len);
 extern int write_file(int fd, unsigned long long *offset, const char *buf,

fs/hostfs/hostfs_kern.c

@@ -302,27 +302,22 @@ int hostfs_readdir(struct file *file, void *ent, filldir_t filldir)
 
 int hostfs_file_open(struct inode *ino, struct file *file)
 {
+	static DEFINE_MUTEX(open_mutex);
 	char *name;
 	fmode_t mode = 0;
+	int err;
 	int r = 0, w = 0, fd;
 
 	mode = file->f_mode & (FMODE_READ | FMODE_WRITE);
 	if ((mode & HOSTFS_I(ino)->mode) == mode)
 		return 0;
 
-	/*
-	 * The file may already have been opened, but with the wrong access,
-	 * so this resets things and reopens the file with the new access.
-	 */
-	if (HOSTFS_I(ino)->fd != -1) {
-		close_file(&HOSTFS_I(ino)->fd);
-		HOSTFS_I(ino)->fd = -1;
-	}
+	mode |= HOSTFS_I(ino)->mode;
 
-	HOSTFS_I(ino)->mode |= mode;
-	if (HOSTFS_I(ino)->mode & FMODE_READ)
+retry:
+	if (mode & FMODE_READ)
 		r = 1;
-	if (HOSTFS_I(ino)->mode & FMODE_WRITE)
+	if (mode & FMODE_WRITE)
 		w = 1;
 	if (w)
 		r = 1;
@@ -335,7 +330,31 @@ int hostfs_file_open(struct inode *ino, struct file *file)
 	__putname(name);
 	if (fd < 0)
 		return fd;
-	FILE_HOSTFS_I(file)->fd = fd;
+
+	mutex_lock(&open_mutex);
+	/* somebody else had handled it first? */
+	if ((mode & HOSTFS_I(ino)->mode) == mode) {
+		mutex_unlock(&open_mutex);
+		return 0;
+	}
+	if ((mode | HOSTFS_I(ino)->mode) != mode) {
+		mode |= HOSTFS_I(ino)->mode;
+		mutex_unlock(&open_mutex);
+		close_file(&fd);
+		goto retry;
+	}
+	if (HOSTFS_I(ino)->fd == -1) {
+		HOSTFS_I(ino)->fd = fd;
+	} else {
+		err = replace_file(fd, HOSTFS_I(ino)->fd);
+		close_file(&fd);
+		if (err < 0) {
+			mutex_unlock(&open_mutex);
+			return err;
+		}
+	}
+	HOSTFS_I(ino)->mode = mode;
+	mutex_unlock(&open_mutex);
 
 	return 0;
 }

fs/hostfs/hostfs_user.c

@@ -160,6 +160,11 @@ int fsync_file(int fd, int datasync)
 	return 0;
 }
 
+int replace_file(int oldfd, int fd)
+{
+	return dup2(oldfd, fd);
+}
+
 void close_file(void *stream)
 {
 	close(*((int *) stream));