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Date: Tue, 9 Oct 2018 07:56:04 -0500
From: Mauricio Vasquez <mauricio.vasquez@...ito.it>
To: Song Liu <liu.song.a23@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 3/6] bpf: add MAP_LOOKUP_AND_DELETE_ELEM syscall
On 10/08/2018 08:13 PM, Song Liu wrote:
> On Mon, Oct 8, 2018 at 12:12 PM Mauricio Vasquez B
> <mauricio.vasquez@...ito.it> wrote:
>> The following patch implements a bpf queue/stack maps that
>> provides the peek/pop/push functions. There is not a direct
>> relationship between those functions and the current maps
>> syscalls, hence a new MAP_LOOKUP_AND_DELETE_ELEM syscall is added,
>> this is mapped to the pop operation in the queue/stack maps
>> and it is still to implement in other kind of maps.
> Do we need this system call for other maps (non-stack/queue)?
> If not, maybe we can just call it POP, and only implement it for
> stack and queue?
>
Yes, this system call could also benefit other maps. The first idea was
to add pop/push/peek system calls as well, but them Alexei realized it
was too specific for queue/stack maps and we decided to go ahead with
this solution that is more general.
>> Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@...ito.it>
>> ---
>> include/linux/bpf.h | 1 +
>> include/uapi/linux/bpf.h | 1 +
>> kernel/bpf/syscall.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 83 insertions(+)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 027697b6a22f..98c7eeb6d138 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -39,6 +39,7 @@ struct bpf_map_ops {
>> void *(*map_lookup_elem)(struct bpf_map *map, void *key);
>> int (*map_update_elem)(struct bpf_map *map, void *key, void *value, u64 flags);
>> int (*map_delete_elem)(struct bpf_map *map, void *key);
>> + void *(*map_lookup_and_delete_elem)(struct bpf_map *map, void *key);
>>
>> /* funcs called by prog_array and perf_event_array map */
>> void *(*map_fd_get_ptr)(struct bpf_map *map, struct file *map_file,
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index f9187b41dff6..3bb94aa2d408 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -103,6 +103,7 @@ enum bpf_cmd {
>> BPF_BTF_LOAD,
>> BPF_BTF_GET_FD_BY_ID,
>> BPF_TASK_FD_QUERY,
>> + BPF_MAP_LOOKUP_AND_DELETE_ELEM,
>> };
>>
>> enum bpf_map_type {
>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index eb75e8af73ff..c33d9303f72f 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -975,6 +975,84 @@ static int map_get_next_key(union bpf_attr *attr)
>> return err;
>> }
>>
>> +#define BPF_MAP_LOOKUP_AND_DELETE_ELEM_LAST_FIELD value
>> +
>> +static int map_lookup_and_delete_elem(union bpf_attr *attr)
>> +{
>> + void __user *ukey = u64_to_user_ptr(attr->key);
>> + void __user *uvalue = u64_to_user_ptr(attr->value);
>> + int ufd = attr->map_fd;
>> + struct bpf_map *map;
>> + void *key, *value, *ptr;
>> + u32 value_size;
>> + struct fd f;
>> + int err;
>> +
>> + if (CHECK_ATTR(BPF_MAP_LOOKUP_AND_DELETE_ELEM))
>> + return -EINVAL;
>> +
>> + f = fdget(ufd);
>> + map = __bpf_map_get(f);
>> + if (IS_ERR(map))
>> + return PTR_ERR(map);
>> +
>> + if (!(f.file->f_mode & FMODE_CAN_WRITE)) {
>> + err = -EPERM;
>> + goto err_put;
>> + }
>> +
>> + key = __bpf_copy_key(ukey, map->key_size);
>> + if (IS_ERR(key)) {
>> + err = PTR_ERR(key);
>> + goto err_put;
>> + }
>> +
>> + value_size = map->value_size;
>> +
>> + err = -ENOMEM;
>> + value = kmalloc(value_size, GFP_USER | __GFP_NOWARN);
>> + if (!value)
>> + goto free_key;
>> +
>> + err = -EFAULT;
>> + if (copy_from_user(value, uvalue, value_size) != 0)
>> + goto free_value;
>> +
>> + /* must increment bpf_prog_active to avoid kprobe+bpf triggering from
>> + * inside bpf map update or delete otherwise deadlocks are possible
>> + */
>> + preempt_disable();
>> + __this_cpu_inc(bpf_prog_active);
>> + if (!map->ops->map_lookup_and_delete_elem) {
> Why do have have this check here? Shouldn't it be check much earlier?
> If we really need it here, we need at least add the following:
In this particular patch the check can be done much earlier, but in next
patch we need it on this position, so I leave it here to avoid moving
around on next patch.
> __this_cpu_dec(bpf_prog_active);
> preempt_enable();
You are right, I missed that. Will fix for next version.
>
>
>> + err = -ENOTSUPP;
>> + goto free_value;
>> + }
>> + rcu_read_lock();
>> + ptr = map->ops->map_lookup_and_delete_elem(map, key);
>> + if (ptr)
>> + memcpy(value, ptr, value_size);
>> + rcu_read_unlock();
>> + err = ptr ? 0 : -ENOENT;
>> + __this_cpu_dec(bpf_prog_active);
>> + preempt_enable();
>> +
>> + if (err)
>> + goto free_value;
>> +
>> + if (copy_to_user(uvalue, value, value_size) != 0)
>> + goto free_value;
>> +
>> + err = 0;
>> +
>> +free_value:
>> + kfree(value);
>> +free_key:
>> + kfree(key);
>> +err_put:
>> + fdput(f);
>> + return err;
>> +}
>> +
>> static const struct bpf_prog_ops * const bpf_prog_types[] = {
>> #define BPF_PROG_TYPE(_id, _name) \
>> [_id] = & _name ## _prog_ops,
>> @@ -2448,6 +2526,9 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>> case BPF_TASK_FD_QUERY:
>> err = bpf_task_fd_query(&attr, uattr);
>> break;
>> + case BPF_MAP_LOOKUP_AND_DELETE_ELEM:
>> + err = map_lookup_and_delete_elem(&attr);
>> + break;
>> default:
>> err = -EINVAL;
>> break;
>>
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