On 4/28/20 11:44 PM, Yonghong Song wrote:
On 4/28/20 11:40 PM, Andrii Nakryiko wrote:
On Tue, Apr 28, 2020 at 11:30 PM Alexei Starovoitov <ast@xxxxxx> wrote:
On 4/28/20 11:20 PM, Yonghong Song wrote:
On 4/28/20 11:08 PM, Andrii Nakryiko wrote:
On Tue, Apr 28, 2020 at 10:10 PM Yonghong Song <yhs@xxxxxx> wrote:
On 4/28/20 7:44 PM, Alexei Starovoitov wrote:
On 4/28/20 6:15 PM, Yonghong Song wrote:
On 4/28/20 5:48 PM, Alexei Starovoitov wrote:
On 4/28/20 5:37 PM, Martin KaFai Lau wrote:
+ prog = bpf_iter_get_prog(seq, sizeof(struct
bpf_iter_seq_map_info),
+ &meta.session_id, &meta.seq_num,
+ v == (void *)0);
From looking at seq_file.c, when will show() be called with
"v ==
NULL"?
that v == NULL here and the whole verifier change just to allow
NULL...
may be use seq_num as an indicator of the last elem instead?
Like seq_num with upper bit set to indicate that it's last?
We could. But then verifier won't have an easy way to verify that.
For example, the above is expected:
int prog(struct bpf_map *map, u64 seq_num) {
if (seq_num >> 63)
return 0;
... map->id ...
... map->user_cnt ...
}
But if user writes
int prog(struct bpf_map *map, u64 seq_num) {
... map->id ...
... map->user_cnt ...
}
verifier won't be easy to conclude inproper map pointer tracing
here and in the above map->id, map->user_cnt will cause
exceptions and they will silently get value 0.
I mean always pass valid object pointer into the prog.
In above case 'map' will always be valid.
Consider prog that iterating all map elements.
It's weird that the prog would always need to do
if (map == 0)
goto out;
even if it doesn't care about finding last.
All progs would have to have such extra 'if'.
If we always pass valid object than there is no need
for such extra checks inside the prog.
First and last element can be indicated via seq_num
or via another flag or via helper call like is_this_last_elem()
or something.
Okay, I see what you mean now. Basically this means
seq_ops->next() should try to get/maintain next two elements,
What about the case when there are no elements to iterate to begin
with? In that case, we still need to call bpf_prog for (empty)
post-aggregation, but we have no valid element... For bpf_map
iteration we could have fake empty bpf_map that would be passed, but
I'm not sure it's applicable for any time of object (e.g., having a
fake task_struct is probably quite a bit more problematic?)...
Oh, yes, thanks for reminding me of this. I put a call to
bpf_prog in seq_ops->stop() especially to handle no object
case. In that case, seq_ops->start() will return NULL,
seq_ops->next() won't be called, and then seq_ops->stop()
is called. My earlier attempt tries to hook with next()
and then find it not working in all cases.
wait a sec. seq_ops->stop() is not the end.
With lseek of seq_file it can be called multiple times.
Yes, I have taken care of this. when the object is NULL,
bpf program will be called. When the object is NULL again,
it won't be called. The private data remembers it has
been called with NULL.
Even without lseek stop() will be called multiple times.
If I read seq_file.c correctly it will be called before
every copy_to_user(). Which means that for a lot of text
(or if read() is done with small buffer) there will be
plenty of start,show,show,stop sequences.
