Re: Commit 0be0ee71 ("fs: properly and reliably lock f_pos in fdget_pos()") breaking userspace

[Linus Torvalds]

On Mon, Nov 25, 2019 at 5:58 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> It might still be a bit noisy even with the above, but I think it will
> at least be better.

Yeah, I think I see what's going on.

I for some reason entirely missed the tty case. Oops. That was just
stupid of me, I should have thought of it.

There are other things that trigger the new informational line in that
hacky patch, and they _might_ matter, but I suspect it's the tty and
sound cases that causes the worst problems.

I suspect Kirill Smelkov even might have mentioned the tty case at one
point, and I just spaced out.

There are other things too that trigger that debug check, like the
sound file descriptors, and they might well matter too.

Anyway, I think the thing to do (for now) is to just say "character
devices are FMODE_STREAM files if they have no llseek operations".
That should take care of both tty's and the sound devices.

You can certainly have a character device that can do llseek, but it
sounds like a reasonable base rule.

Of course, this may fix the f_pos locking issue, but replace it with a
"oops, the character device driver tried to look at *pos anyway", and
that will give you a nice OOPS instead.

So this patch might just replace the failure mode with another failure
mode instead. At which point I think I'd have to revert that "get rid
of FMODE_ATOMIC_POS" trial balloon, but let's see if this patch solves
your problem and is sufficient..

I'd suggest using it _together_ with that "pr_info()" debug patch I
sent, to see what else might be going on..

                Linus
 fs/char_dev.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/char_dev.c b/fs/char_dev.c
index 00dfe17871ac..e5ffebdf80d5 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -367,6 +367,16 @@ void cdev_put(struct cdev *p)
 	}
 }
 
+static int might_be_stream(struct inode *inode, struct file *file)
+{
+	const struct file_operations *fops = file->f_op;
+
+	if (fops->llseek && fops->llseek != no_llseek)
+		return 0;
+
+	return stream_open(inode, file);
+}
+
 /*
  * Called every time a character special file is opened
  */
@@ -416,7 +426,7 @@ static int chrdev_open(struct inode *inode, struct file *filp)
 			goto out_cdev_put;
 	}
 
-	return 0;
+	return might_be_stream(inode, filp);
 
  out_cdev_put:
 	cdev_put(p);