bpf: Fix pos computation for bpf_iter seq_ops->start()

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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200722195156.4029817-1-yhs@fb.com

diff --git a/kernel/bpf/map_iter.c b/kernel/bpf/map_iter.c
index 8a7af11b411f189b2a4dbbf2e248d83ad2f986df..5926c76d854edba0feb9609bf526f4bc501eb2ee 100644 (file)
--- 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 2feecf095609029eeaad91994731ca9bd936d592..1039e52ebd8bec26f45ca590ebc9be27a034c1ce 100644 (file)
--- 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;