Re: [PATCH bpf-next v2 06/20] bpf: create anonymous bpf iterator

[Yonghong Song <yhs@fb.com>]

On 5/5/20 1:11 PM, Andrii Nakryiko wrote:
> On Sun, May 3, 2020 at 11:29 PM Yonghong Song <yhs@fb.com> wrote:
>>
>> A new bpf command BPF_ITER_CREATE is added.
>>
>> The anonymous bpf iterator is seq_file based.
>> The seq_file private data are referenced by targets.
>> The bpf_iter infrastructure allocated additional space
>> at seq_file->private before the space used by targets
>> to store some meta data, e.g.,
>>    prog:       prog to run
>>    session_id: an unique id for each opened seq_file
>>    seq_num:    how many times bpf programs are queried in this session
>>    do_stop:    an internal state to decide whether bpf program
>>                should be called in seq_ops->stop() or not
>>
>> Signed-off-by: Yonghong Song <yhs@fb.com>
>> ---
>>   include/linux/bpf.h            |   1 +
>>   include/uapi/linux/bpf.h       |   6 ++
>>   kernel/bpf/bpf_iter.c          | 128 +++++++++++++++++++++++++++++++++
>>   kernel/bpf/syscall.c           |  26 +++++++
>>   tools/include/uapi/linux/bpf.h |   6 ++
>>   5 files changed, 167 insertions(+)
>>
> 
> [...]
> 
>>   /* The description below is an attempt at providing documentation to eBPF
>> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
>> index 2674c9cbc3dc..2a9f939be6e6 100644
>> --- a/kernel/bpf/bpf_iter.c
>> +++ b/kernel/bpf/bpf_iter.c
>> @@ -2,6 +2,7 @@
>>   /* Copyright (c) 2020 Facebook */
>>
>>   #include <linux/fs.h>
>> +#include <linux/anon_inodes.h>
>>   #include <linux/filter.h>
>>   #include <linux/bpf.h>
>>
>> @@ -20,12 +21,26 @@ struct bpf_iter_link {
>>          struct bpf_iter_target_info *tinfo;
>>   };
>>
>> +struct bpf_iter_priv_data {
>> +       struct {
> 
> nit: anon struct seems unnecessary here? is it just for visual grouping?

Yes, this is just for virtual grouping. Not 100% sure whether this
is needed or not.

> 
>> +               struct bpf_iter_target_info *tinfo;
>> +               struct bpf_prog *prog;
>> +               u64 session_id;
>> +               u64 seq_num;
>> +               u64 do_stop;
>> +       };
>> +       u8 target_private[] __aligned(8);
>> +};
>> +
>>   static struct list_head targets = LIST_HEAD_INIT(targets);
>>   static DEFINE_MUTEX(targets_mutex);
>>
>>   /* protect bpf_iter_link changes */
>>   static DEFINE_MUTEX(link_mutex);
>>
>> +/* incremented on every opened seq_file */
>> +static atomic64_t session_id;
>> +
>>   /* bpf_seq_read, a customized and simpler version for bpf iterator.
>>    * no_llseek is assumed for this file.
>>    * The following are differences from seq_read():
>> @@ -154,6 +169,31 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
>>          goto Done;
>>   }
>>
>> +static int iter_release(struct inode *inode, struct file *file)
>> +{
>> +       struct bpf_iter_priv_data *iter_priv;
>> +       void *file_priv = file->private_data;
>> +       struct seq_file *seq;
>> +
>> +       seq = file_priv;
> 
> 
> seq might be NULL, if anon_inode_getfile succeeded, but then
> prepare_seq_file failed, so you need to handle that.

Thanks for catching this. Missed this case.

> 
> Also, file_priv is redundant, assign to seq directly from file->private_data?

Ack.

> 
>> +       iter_priv = container_of(seq->private, struct bpf_iter_priv_data,
>> +                                target_private);
>> +
>> +       if (iter_priv->tinfo->fini_seq_private)
>> +               iter_priv->tinfo->fini_seq_private(seq->private);
>> +
>> +       bpf_prog_put(iter_priv->prog);
>> +       seq->private = iter_priv;
>> +
>> +       return seq_release_private(inode, file);
>> +}
>> +
>> +static const struct file_operations bpf_iter_fops = {
>> +       .llseek         = no_llseek,
>> +       .read           = bpf_seq_read,
>> +       .release        = iter_release,
>> +};
>> +
>>   int bpf_iter_reg_target(struct bpf_iter_reg *reg_info)
>>   {
>>          struct bpf_iter_target_info *tinfo;
>> @@ -289,3 +329,91 @@ int bpf_iter_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>>
>>          return bpf_link_settle(&link_primer);
>>   }
>> +
>> +static void init_seq_meta(struct bpf_iter_priv_data *priv_data,
>> +                         struct bpf_iter_target_info *tinfo,
>> +                         struct bpf_prog *prog)
>> +{
>> +       priv_data->tinfo = tinfo;
>> +       priv_data->prog = prog;
>> +       priv_data->session_id = atomic64_add_return(1, &session_id);
> 
> nit: atomic64_inc_return?

Ack.

> 
>> +       priv_data->seq_num = 0;
>> +       priv_data->do_stop = 0;
>> +}
>> +
> 
> [...]
>