linux-kernel - Re: [PATCH v3 1/3] bpf: Add new bpf map type to store the pointer to struct perf

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Message-ID: <55B1A159.2090106@huawei.com>
Date:	Fri, 24 Jul 2015 10:22:17 +0800
From:	xiakaixu <xiakaixu@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
CC:	<davem@...emloft.net>, <acme@...nel.org>, <mingo@...hat.com>,
	<a.p.zijlstra@...llo.nl>, <masami.hiramatsu.pt@...achi.com>,
	<jolsa@...nel.org>, <wangnan0@...wei.com>,
	<linux-kernel@...r.kernel.org>, <pi3orama@....com>,
	<hekuang@...wei.com>
Subject: Re: [PATCH v3 1/3] bpf: Add new bpf map type to store the pointer
 to struct perf_event

于 2015/7/24 6:54, Alexei Starovoitov 写道:
> On 7/23/15 2:42 AM, Kaixu Xia wrote:
>> Introduce a new bpf map type 'BPF_MAP_TYPE_PERF_EVENT_ARRAY'.
>> This map only stores the pointer to struct perf_event. The
>> user space event FDs from perf_event_open() syscall are converted
>> to the pointer to struct perf_event and stored in map.
> ...
>> +static struct bpf_map *perf_event_array_map_alloc(union bpf_attr *attr)
>> +{
>> +    /* only the pointer to struct perf_event can be stored in
>> +     * perf_event_array map
>> +     */
>> +    if (attr->value_size != sizeof(u32))
>> +        return ERR_PTR(-EINVAL);
>> +
>> +    return array_map_alloc(attr);
>> +}
> 
> since it's exactly the same as prog_array_map_alloc(),
> just rename it to something like 'fd_array_map_alloc'
> and use for both types.
> 
>> +static int perf_event_array_map_get_next_key(struct bpf_map *map, void *key,
>> +                         void *next_key)
>> +{
>> +    return -EINVAL;
>> +}
>> +
>> +static void *perf_event_array_map_lookup_elem(struct bpf_map *map, void *key)
>> +{
>> +    return NULL;
>> +}
> 
> same for the above two.
> rename prog_array_map_* into fd_array_map_* and use for both map types.
> 
>> +static struct perf_event *convert_map_with_perf_event(void *value)
>> +{
>> +    struct perf_event *event;
>> +    u32 fd;
>> +
>> +    fd = *(u32 *)value;
>> +
>> +    event = perf_event_get(fd);
>> +    if (IS_ERR(event))
>> +        return NULL;
> 
> don't lose error code, do 'return event' instead.
> 
>> +
>> +    /* limit the event type to PERF_TYPE_RAW
>> +     * and PERF_TYPE_HARDWARE.
>> +     */
>> +    if (event->attr.type != PERF_TYPE_RAW &&
>> +        event->attr.type != PERF_TYPE_HARDWARE)
>> +        return NULL;
> 
> perf_event refcnt leak? need to do put_event.
> and return ERR_PTR(-EINVAL)
> 
>> +
>> +    return event;
>> +}
>> +
>> +/* only called from syscall */
>> +static int perf_event_array_map_update_elem(struct bpf_map *map, void *key,
>> +                        void *value, u64 map_flags)
>> +{
>> +    struct bpf_array *array = container_of(map, struct bpf_array, map);
>> +    struct perf_event *event;
>> +    u32 index = *(u32 *)key;
>> +
>> +    if (map_flags != BPF_ANY)
>> +        return -EINVAL;
>> +
>> +    if (index >= array->map.max_entries)
>> +        return -E2BIG;
>> +
>> +    /* check if the value is already stored */
>> +    if (array->events[index])
>> +        return -EINVAL;
>> +
>> +    /* convert the fd to the pointer to struct perf_event */
>> +    event = convert_map_with_perf_event(value);
> 
> imo helper name is misleading and it's too short to be separate
> function. Just inline it and you can reuse 'index' variable.
> 
>> +    if (!event)
>> +        return -EBADF;
>> +
>> +    xchg(array->events + index, event);
> 
> refcnt leak of old event! Please think it through.
> This type of bugs I shouldn't be finding.

Maybe the commit message is not elaborate. Here I prevent
user space from updating the existed event, so the return
value of xchg() is NULL and no refcnt leak of old event.
I will do the same as prog_array in next version.
> 
>> +static int perf_event_array_map_delete_elem(struct bpf_map *map, void *key)
>> +{
>> +    return -EINVAL;
>> +}
> 
> no way to dec refcnt of perf_event from user space?
> why not to do the same as prog_array_delete?

Will follow them in V4.
> 
> 
> .
> 


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