bcachefs: thread_with_file: allow creation of readonly files

Create a new run_thread_with_stdout function that opens a file in
O_RDONLY mode so that the kernel can write things to userspace but
userspace cannot write to the kernel.  This will be used to convey xfs
health event information to userspace.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/thread_with_file.c

@@ -231,6 +231,22 @@ static __poll_t thread_with_stdio_poll(struct file *file, struct poll_table_stru
 	return mask;
 }
 
+static __poll_t thread_with_stdout_poll(struct file *file, struct poll_table_struct *wait)
+{
+	struct thread_with_stdio *thr =
+		container_of(file->private_data, struct thread_with_stdio, thr);
+
+	poll_wait(file, &thr->stdio.output.wait, wait);
+
+	__poll_t mask = 0;
+
+	if (stdio_redirect_has_output(&thr->stdio))
+		mask |= EPOLLIN;
+	if (thr->thr.done)
+		mask |= EPOLLHUP|EPOLLERR;
+	return mask;
+}
+
 static const struct file_operations thread_with_stdio_fops = {
 	.llseek		= no_llseek,
 	.read		= thread_with_stdio_read,
@@ -239,6 +255,13 @@ static const struct file_operations thread_with_stdio_fops = {
 	.release	= thread_with_stdio_release,
 };
 
+static const struct file_operations thread_with_stdout_fops = {
+	.llseek		= no_llseek,
+	.read		= thread_with_stdio_read,
+	.poll		= thread_with_stdout_poll,
+	.release	= thread_with_stdio_release,
+};
+
 static int thread_with_stdio_fn(void *arg)
 {
 	struct thread_with_stdio *thr = arg;
@@ -261,6 +284,19 @@ int bch2_run_thread_with_stdio(struct thread_with_stdio *thr,
 	return bch2_run_thread_with_file(&thr->thr, &thread_with_stdio_fops, thread_with_stdio_fn);
 }
 
+int bch2_run_thread_with_stdout(struct thread_with_stdio *thr,
+				void (*exit)(struct thread_with_stdio *),
+				void (*fn)(struct thread_with_stdio *))
+{
+	stdio_buf_init(&thr->stdio.input);
+	stdio_buf_init(&thr->stdio.output);
+	thr->exit	= exit;
+	thr->fn		= fn;
+
+	return bch2_run_thread_with_file(&thr->thr, &thread_with_stdout_fops, thread_with_stdio_fn);
+}
+EXPORT_SYMBOL_GPL(bch2_run_thread_with_stdout);
+
 int bch2_stdio_redirect_read(struct stdio_redirect *stdio, char *ubuf, size_t len)
 {
 	struct stdio_buf *buf = &stdio->input;

fs/bcachefs/thread_with_file.h

@@ -59,6 +59,9 @@ struct thread_with_stdio {
 int bch2_run_thread_with_stdio(struct thread_with_stdio *,
 			       void (*exit)(struct thread_with_stdio *),
 			       void (*fn)(struct thread_with_stdio *));
+int bch2_run_thread_with_stdout(struct thread_with_stdio *,
+				void (*exit)(struct thread_with_stdio *),
+				void (*fn)(struct thread_with_stdio *));
 int bch2_stdio_redirect_read(struct stdio_redirect *, char *, size_t);
 int bch2_stdio_redirect_readline(struct stdio_redirect *, char *, size_t);