Re: [PATCH 1/6] seq_file: add seq_read_iter

[Al Viro]

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);