[PATCH 3/5] tftp: emulate backwards seek by reopening the file

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

TFTP is a streaming protocol with no native seek support.
We emulate forward seeks are emulated by reading and discarding data.
Backwards seeks were previously rejected with -ENOSYS, breaking higher
level code that assumes seeking to be possible.

Given that we have a heuristic in-place for detecting excessive data
discards through seeking, let's implement backwards seeking to allow the
pattern of reading the start of a buffer to detect type at one location
and then reading everything at once at a later time.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 fs/tftp.c | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/fs/tftp.c b/fs/tftp.c
index e5a276ed5cae..8683c3841c68 100644
--- a/fs/tftp.c
+++ b/fs/tftp.c
@@ -822,6 +822,9 @@ static int tftp_close(struct inode *inode, struct file *f)
 {
 	struct file_priv *priv = f->private_data;
 
+	if (!priv)
+		return 0;
+
 	return tftp_do_close(priv);
 }
 
@@ -831,6 +834,9 @@ static int tftp_write(struct file *f, const void *inbuf, size_t insize)
 	size_t size, now;
 	int ret;
 
+	if (!priv)
+		return -EREMOTEIO;
+
 	pr_vdebug("%s: %zu\n", __func__, insize);
 
 	size = insize;
@@ -866,6 +872,9 @@ static int tftp_read(struct file *f, void *buf, size_t insize)
 	size_t outsize = 0, now;
 	int ret = 0;
 
+	if (!priv)
+		return -EREMOTEIO;
+
 	pr_vdebug("%s %zu\n", __func__, insize);
 
 	while (insize) {
@@ -902,14 +911,34 @@ static int tftp_read(struct file *f, void *buf, size_t insize)
 
 static int tftp_lseek(struct file *f, loff_t pos)
 {
+	struct file_priv *priv = f->private_data;
 	static loff_t seek_discard_total;
 	int ret = 0;
 	char *buf;
 	loff_t f_pos = f->f_pos;
 
+	if (!priv)
+		return -EREMOTEIO;
+
 	/* We cannot seek backwards without reloading or caching the file */
-	if (pos < f_pos)
-		return -ENOSYS;
+	if (pos < f_pos) {
+		/* We can reopen read streams, but not write streams */
+		if (priv->push)
+			return -ENOSYS;
+
+		tftp_do_close(priv);
+
+		priv = tftp_do_open(&f->fsdev->dev, f->f_flags,
+				    f->f_path.dentry, false);
+		if (IS_ERR(priv)) {
+			f->private_data = NULL;
+			return PTR_ERR(priv);
+		}
+
+		f->private_data = priv;
+		f->f_pos = 0;
+		f_pos = 0;
+	}
 
 	if (pos == f_pos)
 		return 0;
-- 
2.47.3