Commit 6ae003ad authored by Alexei Starovoitov's avatar Alexei Starovoitov
Browse files

Merge branch 'bpf-fix-softlock-condition-in-bpf-hashmap-interation' 



Brandon Kammerdiener says:

====================
This patchset fixes an endless loop condition that can occur in
bpf_for_each_hash_elem, causing the core to softlock. My understanding is
that a combination of RCU list deletion and insertion introduces the new
element after the iteration cursor and that there is a chance that an RCU
reader may in fact use this new element in iteration. The patch uses a
_safe variant of the macro which gets the next element to iterate before
executing the loop body for the current element.

I have also added a subtest in the for_each selftest that can trigger this
condition without the fix.

Changes since v2:
- Renaming and additional checks in selftests/bpf/prog_tests/for_each.c

Changes since v1:
- Added missing Signed-off-by lines to both patches
====================

Acked-by: default avatarHou Tao <houtao1@huawei.com>
Link: https://patch.msgid.link/20250424153246.141677-1-brandon.kammerdiener@intel.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parents f2858f30 3d9c463f

kernel/bpf/hashtab.c

+1 −1
Original line number Diff line number Diff line
@@ -2189,7 +2189,7 @@ static long bpf_for_each_hash_elem(struct bpf_map *map, bpf_callback_t callback_ 
		b = &htab->buckets[i];

		rcu_read_lock();

		head = &b->head;

		hlist_nulls_for_each_entry_rcu(elem, n, head, hash_node) {

		hlist_nulls_for_each_entry_safe(elem, n, head, hash_node) {

			key = elem->key;

			if (is_percpu) {

				/* current cpu value for percpu map */

tools/testing/selftests/bpf/prog_tests/for_each.c

+37 −0
Original line number Diff line number Diff line
@@ -6,6 +6,7 @@ 
#include "for_each_array_map_elem.skel.h"

#include "for_each_map_elem_write_key.skel.h"

#include "for_each_multi_maps.skel.h"

#include "for_each_hash_modify.skel.h"



static unsigned int duration;
@@ -203,6 +204,40 @@ static void test_multi_maps(void) 
	for_each_multi_maps__destroy(skel);

}



static void test_hash_modify(void)

{

	struct for_each_hash_modify *skel;

	int max_entries, i, err;

	__u64 key, val;



	LIBBPF_OPTS(bpf_test_run_opts, topts,

		.data_in = &pkt_v4,

		.data_size_in = sizeof(pkt_v4),

		.repeat = 1

	);



	skel = for_each_hash_modify__open_and_load();

	if (!ASSERT_OK_PTR(skel, "for_each_hash_modify__open_and_load"))

		return;



	max_entries = bpf_map__max_entries(skel->maps.hashmap);

	for (i = 0; i < max_entries; i++) {

		key = i;

		val = i;

		err = bpf_map__update_elem(skel->maps.hashmap, &key, sizeof(key),

					   &val, sizeof(val), BPF_ANY);

		if (!ASSERT_OK(err, "map_update"))

			goto out;

	}



	err = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.test_pkt_access), &topts);

	ASSERT_OK(err, "bpf_prog_test_run_opts");

	ASSERT_OK(topts.retval, "retval");



out:

	for_each_hash_modify__destroy(skel);

}



void test_for_each(void)

{

	if (test__start_subtest("hash_map"))
@@ -213,4 +248,6 @@ void test_for_each(void) 
		test_write_map_key();

	if (test__start_subtest("multi_maps"))

		test_multi_maps();

	if (test__start_subtest("hash_modify"))

		test_hash_modify();

}

tools/testing/selftests/bpf/progs/for_each_hash_modify.c

0 → 100644
+30 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0

/* Copyright (c) 2025 Intel Corporation */

#include "vmlinux.h"

#include <bpf/bpf_helpers.h>



char _license[] SEC("license") = "GPL";



struct {

	__uint(type, BPF_MAP_TYPE_HASH);

	__uint(max_entries, 128);

	__type(key, __u64);

	__type(value, __u64);

} hashmap SEC(".maps");



static int cb(struct bpf_map *map, __u64 *key, __u64 *val, void *arg)

{

	bpf_map_delete_elem(map, key);

	bpf_map_update_elem(map, key, val, 0);

	return 0;

}



SEC("tc")

int test_pkt_access(struct __sk_buff *skb)

{

	(void)skb;



	bpf_for_each_map_elem(&hashmap, cb, NULL, 0);



	return 0;

}