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

[Al Viro]

On Sat, Nov 14, 2020 at 08:50:00PM +0000, Al Viro wrote:

OK, I think I understand what's going on.  Could you check if
reverting the variant in -next and applying the following instead
fixes what you are seeing?

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3b20e21604e7..35667112bbd1 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;
@@ -208,16 +207,15 @@ 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)
-                       goto Done;
+               if (m->count)
+                       goto Efault;
        }
+       if (!iov_iter_count(iter))
+               goto Done;
        /* we need at least one record in buffer */
        m->from = 0;
        p = m->op->start(m, &m->index);
@@ -249,6 +247,7 @@ 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 +262,7 @@ 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) {
@@ -273,12 +272,12 @@ 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;
+       m->count -= n;
+       if (m->count)
+               goto Efault;
 Done:
        if (!copied)
                copied = err;