netdev - Re: [PATCH bpf-next v1 06/19] bpf: support bpf tracing/iter programs for BPF_LINK

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Date:   Tue, 28 Apr 2020 22:04:54 -0700
From:   Yonghong Song <yhs@...com>
To:     Martin KaFai Lau <kafai@...com>
CC:     Andrii Nakryiko <andriin@...com>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>
Subject: Re: [PATCH bpf-next v1 06/19] bpf: support bpf tracing/iter programs
 for BPF_LINK_UPDATE



On 4/28/20 6:32 PM, Martin KaFai Lau wrote:
> On Mon, Apr 27, 2020 at 01:12:41PM -0700, Yonghong Song wrote:
>> Added BPF_LINK_UPDATE support for tracing/iter programs.
>> This way, a file based bpf iterator, which holds a reference
>> to the link, can have its bpf program updated without
>> creating new files.
>>
>> Signed-off-by: Yonghong Song <yhs@...com>
>> ---
>>   include/linux/bpf.h   |  2 ++
>>   kernel/bpf/bpf_iter.c | 29 +++++++++++++++++++++++++++++
>>   kernel/bpf/syscall.c  |  5 +++++
>>   3 files changed, 36 insertions(+)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 60ecb73d8f6d..4fc39d9b5cd0 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -1131,6 +1131,8 @@ struct bpf_prog *bpf_iter_get_prog(struct seq_file *seq, u32 priv_data_size,
>>   				   u64 *session_id, u64 *seq_num, bool is_last);
>>   int bpf_iter_run_prog(struct bpf_prog *prog, void *ctx);
>>   int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
>> +int bpf_iter_link_replace(struct bpf_link *link, struct bpf_prog *old_prog,
>> +			  struct bpf_prog *new_prog);
>>   
>>   int bpf_percpu_hash_copy(struct bpf_map *map, void *key, void *value);
>>   int bpf_percpu_array_copy(struct bpf_map *map, void *key, void *value);
>> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
>> index 9532e7bcb8e1..fc1ce5ee5c3f 100644
>> --- a/kernel/bpf/bpf_iter.c
>> +++ b/kernel/bpf/bpf_iter.c
>> @@ -23,6 +23,9 @@ static struct list_head targets;
>>   static struct mutex targets_mutex;
>>   static bool bpf_iter_inited = false;
>>   
>> +/* protect bpf_iter_link.link->prog upddate */
>> +static struct mutex bpf_iter_mutex;
>> +
>>   int bpf_iter_reg_target(struct bpf_iter_reg *reg_info)
>>   {
>>   	struct bpf_iter_target_info *tinfo;
>> @@ -33,6 +36,7 @@ int bpf_iter_reg_target(struct bpf_iter_reg *reg_info)
>>   	if (!bpf_iter_inited) {
>>   		INIT_LIST_HEAD(&targets);
>>   		mutex_init(&targets_mutex);
>> +		mutex_init(&bpf_iter_mutex);
>>   		bpf_iter_inited = true;
>>   	}
>>   
>> @@ -121,3 +125,28 @@ int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>>   		kfree(link);
>>   	return err;
>>   }
>> +
>> +int bpf_iter_link_replace(struct bpf_link *link, struct bpf_prog *old_prog,
>> +			  struct bpf_prog *new_prog)
>> +{
>> +	int ret = 0;
>> +
>> +	mutex_lock(&bpf_iter_mutex);
>> +	if (old_prog && link->prog != old_prog) {
>> +		ret = -EPERM;
>> +		goto out_unlock;
>> +	}
>> +
>> +	if (link->prog->type != new_prog->type ||
>> +	    link->prog->expected_attach_type != new_prog->expected_attach_type ||
>> +	    strcmp(link->prog->aux->attach_func_name, new_prog->aux->attach_func_name)) {
> Can attach_btf_id be compared instead of strcmp()?

Yes, we can do it.

> 
>> +		ret = -EINVAL;
>> +		goto out_unlock;
>> +	}
>> +
>> +	link->prog = new_prog;
> Does the old link->prog need a bpf_prog_put()?

The old_prog is replaced in caller link_update (syscall.c):
static int link_update(union bpf_attr *attr)
{
         struct bpf_prog *old_prog = NULL, *new_prog;
         struct bpf_link *link;
         u32 flags;
         int ret;
...
         if (link->ops == &bpf_iter_link_lops) {
                 ret = bpf_iter_link_replace(link, old_prog, new_prog);
                 goto out_put_progs;
         }
         ret = -EINVAL;

out_put_progs:
         if (old_prog)
                 bpf_prog_put(old_prog);
...

> 
>> +
>> +out_unlock:
>> +	mutex_unlock(&bpf_iter_mutex);
>> +	return ret;
>> +}