[PATCH bpf-next] bpf: fix pos computation for bpf_iter seq_ops->start()

[Yonghong Song <yhs@fb.com>]

Currently, the pos pointer in bpf iterator map/task/task_file
seq_ops->start() is always incremented.
This is incorrect. It should be increased only if
*pos is 0 (for SEQ_START_TOKEN) since these start()
function actually returns the first real object.
If *pos is not 0, it merely found the object
based on the state in seq->private, and not really
advancing the *pos. This patch fixed this issue
by only incrementing *pos if it is 0.

Note that the old *pos calculation, although not
correct, does not affect correctness of bpf_iter
as bpf_iter seq_file->read() does not support llseek.

This patch also renamed "mid" in bpf_map iterator
seq_file private data to "map_id" for better clarity.

Fixes: 6086d29def80 ("bpf: Add bpf_map iterator")
Fixes: eaaacd23910f ("bpf: Add task and task/file iterator targets")
Signed-off-by: Yonghong Song <yhs@fb.com>
---
 kernel/bpf/map_iter.c  | 16 ++++++----------
 kernel/bpf/task_iter.c |  6 ++++--
 2 files changed, 10 insertions(+), 12 deletions(-)

I targeted this to bpf-next as I think this patch
does not need to backport to bpf tree as it does not
impact correctness.

Alexei, I also made the change of "mid"->"map_id"
and simplified the logic in map_iter seq_file->next()
the same as your patch in
  https://lore.kernel.org/bpf/20200717044031.56412-2-alexei.starovoitov@gmail.com/T
the goal is to reduce conflicts as we touch
the same file and close lines.

diff --git a/kernel/bpf/map_iter.c b/kernel/bpf/map_iter.c
index 8a7af11b411f..5926c76d854e 100644
--- a/kernel/bpf/map_iter.c
+++ b/kernel/bpf/map_iter.c
@@ -7,7 +7,7 @@
 #include <linux/btf_ids.h>
 
 struct bpf_iter_seq_map_info {
-	u32 mid;
+	u32 map_id;
 };
 
 static void *bpf_map_seq_start(struct seq_file *seq, loff_t *pos)
@@ -15,27 +15,23 @@ static void *bpf_map_seq_start(struct seq_file *seq, loff_t *pos)
 	struct bpf_iter_seq_map_info *info = seq->private;
 	struct bpf_map *map;
 
-	map = bpf_map_get_curr_or_next(&info->mid);
+	map = bpf_map_get_curr_or_next(&info->map_id);
 	if (!map)
 		return NULL;
 
-	++*pos;
+	if (*pos == 0)
+		++*pos;
 	return map;
 }
 
 static void *bpf_map_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
 	struct bpf_iter_seq_map_info *info = seq->private;
-	struct bpf_map *map;
 
 	++*pos;
-	++info->mid;
+	++info->map_id;
 	bpf_map_put((struct bpf_map *)v);
-	map = bpf_map_get_curr_or_next(&info->mid);
-	if (!map)
-		return NULL;
-
-	return map;
+	return bpf_map_get_curr_or_next(&info->map_id);
 }
 
 struct bpf_iter__bpf_map {
diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 2feecf095609..1039e52ebd8b 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -51,7 +51,8 @@ static void *task_seq_start(struct seq_file *seq, loff_t *pos)
 	if (!task)
 		return NULL;
 
-	++*pos;
+	if (*pos == 0)
+		++*pos;
 	return task;
 }
 
@@ -210,7 +211,8 @@ static void *task_file_seq_start(struct seq_file *seq, loff_t *pos)
 		return NULL;
 	}
 
-	++*pos;
+	if (*pos == 0)
+		++*pos;
 	info->task = task;
 	info->files = files;
 
-- 
2.24.1