Re: [PATCH RFC bpf-next v2 08/18] bpf: Implement iterator APIs for resizable hashtab

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 4/14/26 6:49 PM, Emil Tsalapatis wrote:
> On Wed Apr 8, 2026 at 11:10 AM EDT, Mykyta Yatsenko wrote:
>> From: Mykyta Yatsenko <yatsenko@meta.com>
>>
>> Wire up seq_file BPF iterator for BPF_MAP_TYPE_RHASH so that
>> bpf_iter and bpftool map dump work with resizable hash maps.
>>
>> Use rhashtable_walk_enter_from() with a saved last_key to resume
>> iteration across read() calls without linear skip from the
>> beginning on each seq_start.
>>
>> Also implement rhtab_map_seq_show_elem() for bpftool map dump
>> in non-iterator mode.
>>
>> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> 
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> 
>> ---
>>   kernel/bpf/hashtab.c | 98 +++++++++++++++++++++++++++++++++++++++++++++++++---
>>   1 file changed, 94 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
>> index a79d434dc626..492c6a9154b6 100644
>> --- a/kernel/bpf/hashtab.c
>> +++ b/kernel/bpf/hashtab.c
>> @@ -3008,6 +3008,19 @@ static int rhtab_map_get_next_key(struct bpf_map *map, void *key, void *next_key
>>   
>>   static void rhtab_map_seq_show_elem(struct bpf_map *map, void *key, struct seq_file *m)
>>   {
>> +	void *value;
>> +
>> +	/* Guarantee that hashtab value is not freed */
>> +	guard(rcu)();
>> +
>> +	value = rhtab_map_lookup_elem(map, key);
>> +	if (!value)
>> +		return;
>> +
>> +	btf_type_seq_show(map->btf, map->btf_key_type_id, key, m);
>> +	seq_puts(m, ": ");
>> +	btf_type_seq_show(map->btf, map->btf_value_type_id, value, m);
>> +	seq_putc(m, '\n');
>>   }
>>   
>>   static long bpf_each_rhash_elem(struct bpf_map *map, bpf_callback_t callback_fn,
>> @@ -3201,36 +3214,113 @@ struct bpf_iter_seq_rhash_map_info {
>>   	struct bpf_map *map;
>>   	struct bpf_rhtab *rhtab;
>>   	struct rhashtable_iter iter;
>> -	u32 skip_elems;
>> +	void *last_key;
> 
> Nit: Could we avoid adding skip_elems to this in the first place since
> it's never used?
> 

Definitely yes, thanks for noticing.

>>   	bool iter_active;
>>   };
>>   
> 
> Question: Would it make sense/be worth annotating the seq functions with
> __acquires/__releases(RCU)?
> 

I guess it won't hurt.

>>   static void *bpf_rhash_map_seq_start(struct seq_file *seq, loff_t *pos)
>>   {
>> -	return NULL;
>> +	struct bpf_iter_seq_rhash_map_info *info = seq->private;
>> +	struct rhtab_elem *elem;
>> +	void *key = *pos > 0 ? info->last_key : NULL;
>> +
>> +	scoped_guard(rcu) {
>> +		rhashtable_walk_enter_from(&info->rhtab->ht, &info->iter,
>> +					   key, info->rhtab->params);
>> +		rhashtable_walk_start(&info->iter);
>> +	}
>> +	info->iter_active = true;
>> +
>> +	elem = rhtab_iter_next(&info->iter);
>> +	if (!elem)
>> +		return NULL;
>> +	/*
>> +	 * if *pos is not 0, previously iteration failed on this elem,
>> +	 * so we are restarting it. That's why no need to increment *pos.
>> +	 */
>> +	if (*pos == 0)
>> +		++*pos;
>> +	return elem;
>>   }
>>   
>>   static void *bpf_rhash_map_seq_next(struct seq_file *seq, void *v, loff_t *pos)
>>   {
>> -	return NULL;
>> +	struct bpf_iter_seq_rhash_map_info *info = seq->private;
>> +	struct rhtab_elem *elem = v;
>> +
>> +	/* Save current key for O(1) resume in next seq_start */
>> +	memcpy(info->last_key, elem->data, info->map->key_size);
>> +
>> +	++*pos;
>> +
>> +	return rhtab_iter_next(&info->iter);
>> +}
>> +
>> +static int __bpf_rhash_map_seq_show(struct seq_file *seq,
>> +				    struct rhtab_elem *elem)
>> +{
>> +	struct bpf_iter_seq_rhash_map_info *info = seq->private;
>> +	struct bpf_iter__bpf_map_elem ctx = {};
>> +	struct bpf_iter_meta meta;
>> +	struct bpf_prog *prog;
>> +	int ret = 0;
>> +
>> +	meta.seq = seq;
>> +	prog = bpf_iter_get_info(&meta, elem == NULL);
>> +	if (prog) {
>> +		ctx.meta = &meta;
>> +		ctx.map = info->map;
>> +		if (elem) {
>> +			ctx.key = elem->data;
>> +			ctx.value = rhtab_elem_value(elem, info->map->key_size);
>> +		}
>> +		ret = bpf_iter_run_prog(prog, &ctx);
>> +	}
>> +
>> +	return ret;
>>   }
>>   
>>   static int bpf_rhash_map_seq_show(struct seq_file *seq, void *v)
>>   {
>> -	return 0;
>> +	return __bpf_rhash_map_seq_show(seq, v);
>>   }
>>   
>>   static void bpf_rhash_map_seq_stop(struct seq_file *seq, void *v)
>>   {
>> +	struct bpf_iter_seq_rhash_map_info *info = seq->private;
>> +
>> +	if (!v)
>> +		(void)__bpf_rhash_map_seq_show(seq, NULL);
>> +
>> +	if (info->iter_active) {
>> +		rhashtable_walk_stop(&info->iter);
>> +		rhashtable_walk_exit(&info->iter);
>> +		info->iter_active = false;
>> +	}
>>   }
>>   
>>   static int bpf_iter_init_rhash_map(void *priv_data, struct bpf_iter_aux_info *aux)
>>   {
>> +	struct bpf_iter_seq_rhash_map_info *info = priv_data;
>> +	struct bpf_map *map = aux->map;
>> +
>> +	info->last_key = kmalloc(map->key_size, GFP_USER);
>> +	if (!info->last_key)
>> +		return -ENOMEM;
>> +
>> +	bpf_map_inc_with_uref(map);
>> +	info->map = map;
>> +	info->rhtab = container_of(map, struct bpf_rhtab, map);
>> +	info->iter_active = false;
>>   	return 0;
>>   }
>>   
>>   static void bpf_iter_fini_rhash_map(void *priv_data)
>>   {
>> +	struct bpf_iter_seq_rhash_map_info *info = priv_data;
>> +
>> +	kfree(info->last_key);
>> +	bpf_map_put_with_uref(info->map);
>>   }
>>   
>>   static const struct seq_operations bpf_rhash_map_seq_ops = {
>