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Date:   Wed, 11 Nov 2020 22:21:16 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christoph Hellwig <hch@lst.de>,
        Greg KH <gregkh@linuxfoundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/6] seq_file: add seq_read_iter
Message-ID: <20201111222116.GA919131@ZenIV.linux.org.uk>
References: <20201104082738.1054792-1-hch@lst.de>
 <20201104082738.1054792-2-hch@lst.de>
 <20201110213253.GV3576660@ZenIV.linux.org.uk>
 <20201110213511.GW3576660@ZenIV.linux.org.uk>
 <20201110232028.GX3576660@ZenIV.linux.org.uk>
 <CAHk-=whTqr4Lp0NYR6k3yc2EbiF0RR17=TJPa4JBQATMR__XqA@mail.gmail.com>
 <20201111215220.GA3576660@ZenIV.linux.org.uk>
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Sender: Al Viro <viro@ftp.linux.org.uk>
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On Wed, Nov 11, 2020 at 09:52:20PM +0000, Al Viro wrote:

> That can be done, but I would rather go with
> 		n = copy_to_iter(m->buf + m->from, m->count, iter);
> 		m->count -= n;
> 		m->from += n;
>                 copied += n;
>                 if (!size)
>                         goto Done;
> 		if (m->count)
> 			goto Efault;
> if we do it that way.  Let me see if I can cook something
> reasonable along those lines...

Something like below (build-tested only):

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3b20e21604e7..07b33c1f34a9 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -168,7 +168,6 @@ EXPORT_SYMBOL(seq_read);
 ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 {
 	struct seq_file *m = iocb->ki_filp->private_data;
-	size_t size = iov_iter_count(iter);
 	size_t copied = 0;
 	size_t n;
 	void *p;
@@ -208,14 +207,11 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	}
 	/* if not empty - flush it first */
 	if (m->count) {
-		n = min(m->count, size);
-		if (copy_to_iter(m->buf + m->from, n, iter) != n)
-			goto Efault;
+		n = copy_to_iter(m->buf + m->from, m->count, iter);
 		m->count -= n;
 		m->from += n;
-		size -= n;
 		copied += n;
-		if (!size)
+		if (!iov_iter_count(iter) || m->count)
 			goto Done;
 	}
 	/* we need at least one record in buffer */
@@ -249,6 +245,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 	goto Done;
 Fill:
 	/* they want more? let's try to get some more */
+	/* m->count is positive and there's space left in iter */
 	while (1) {
 		size_t offs = m->count;
 		loff_t pos = m->index;
@@ -263,7 +260,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 			err = PTR_ERR(p);
 			break;
 		}
-		if (m->count >= size)
+		if (m->count >= iov_iter_count(iter))
 			break;
 		err = m->op->show(m, p);
 		if (seq_has_overflowed(m) || err) {
@@ -273,16 +270,14 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		}
 	}
 	m->op->stop(m, p);
-	n = min(m->count, size);
-	if (copy_to_iter(m->buf, n, iter) != n)
-		goto Efault;
+	n = copy_to_iter(m->buf, m->count, iter);
 	copied += n;
 	m->count -= n;
 	m->from = n;
 Done:
-	if (!copied)
-		copied = err;
-	else {
+	if (unlikely(!copied)) {
+		copied = m->count ? -EFAULT : err;
+	} else {
 		iocb->ki_pos += copied;
 		m->read_pos += copied;
 	}
@@ -291,9 +286,6 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 Enomem:
 	err = -ENOMEM;
 	goto Done;
-Efault:
-	err = -EFAULT;
-	goto Done;
 }
 EXPORT_SYMBOL(seq_read_iter);