Commit 3f9969f2 authored by Yonghong Song's avatar Yonghong Song Committed by Alexei Starovoitov
Browse files

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: 6086d29d ("bpf: Add bpf_map iterator")
Fixes: eaaacd23 ("bpf: Add task and task/file iterator targets")
Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200722195156.4029817-1-yhs@fb.com
parent 86176a18

kernel/bpf/map_iter.c

+6 −10
Original line number Diff line number Diff line
@@ -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)
@@ -15,10 +15,11 @@ 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;



	if (*pos == 0)

		++*pos;

	return map;

}
@@ -26,16 +27,11 @@ static void *bpf_map_seq_start(struct seq_file *seq, loff_t *pos) 
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 {

kernel/bpf/task_iter.c

+4 −2
Original line number Diff line number Diff line
@@ -51,6 +51,7 @@ static void *task_seq_start(struct seq_file *seq, loff_t *pos) 
	if (!task)

		return NULL;



	if (*pos == 0)

		++*pos;

	return task;

}
@@ -210,6 +211,7 @@ static void *task_file_seq_start(struct seq_file *seq, loff_t *pos) 
		return NULL;

	}



	if (*pos == 0)

		++*pos;

	info->task = task;

	info->files = files;