Fix buggy procfs_list_seq_next warning

The kernel seq_read() helper function expects ->next() to update
the passed position even there are no more entries.  Failure to
do so results in the following warning being logged.

    seq_file: buggy .next function procfs_list_seq_next [spl]
    did not update position index

Functionally there is no issue with the way procfs_list_seq_next()
is implemented and the warning is harmless.  However, we want to
silence this some what scary incorrect warning.  This commit
updates the Linux procfs code to advance the position even for
the last entry.

Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #10984 
Closes #10996 

1 files changed, 12 insertions, 0 deletions

diff --git a/module/os/linux/spl/spl-procfs-list.c b/module/os/linux/spl/spl-procfs-list.c
index 778da7bc4..cae13228c 100644
--- a/module/os/linux/spl/spl-procfs-list.c
+++ b/module/os/linux/spl/spl-procfs-list.c
@@ -89,7 +89,17 @@ procfs_list_next_node(procfs_list_cursor_t *cursor, loff_t *pos)
 		cursor->cached_node = next_node;
 		cursor->cached_pos = NODE_ID(procfs_list, cursor->cached_node);
 		*pos = cursor->cached_pos;
+	} else {
+		/*
+		 * seq_read() expects ->next() to update the position even
+		 * when there are no more entries. Advance the position to
+		 * prevent a warning from being logged.
+		 */
+		cursor->cached_node = NULL;
+		cursor->cached_pos++;
+		*pos = cursor->cached_pos;
 	}
+
 	return (next_node);
 }
 
@@ -105,6 +115,8 @@ procfs_list_seq_start(struct seq_file *f, loff_t *pos)
 		cursor->cached_node = SEQ_START_TOKEN;
 		cursor->cached_pos = 0;
 		return (SEQ_START_TOKEN);
+	} else if (cursor->cached_node == NULL) {
+		return (NULL);
 	}
 
 	/*