Re: [RFC PATCH v3 21/37] bpf tools: Create eBPF maps defined in an object file

Wangnan (F)
2015-05-25T09:26:20Z

On 2015/5/19 2:48, Alexei Starovoitov wrote:
> On 5/17/15 3:56 AM, Wang Nan wrote:
>> This patch creates maps based on 'map' section in object file using
>> bpf_create_map(), and store the fds into an array in
>> 'struct bpf_object'. Since the byte order of the object may differ
>> from the host, swap map definition before processing.
>>
>> This is the first patch in 'loading' phase. Previous patches parse ELF
>> object file and create needed data structure, but doesnnt play with
>> kernel. They belong to 'opening' phase.
>>
>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> ...
>> + for (j = 0; j < i; j++) {
>> + close(obj->maps_fds[j]);
>> + obj->maps_fds[j] = -1;
>
> this line is unnecessary, since you're freeing the whole array
> at the line below:
>
>> + }
>> + free(obj->maps_fds);
>> + obj->maps_fds = NULL;
>
> ...
>
>> void bpf_close_object(struct bpf_object *obj)
>> {
>> if (!obj)
>> return;
>>
>> bpf_obj_clear_elf(obj);
>> + bpf_unload_object(obj);
>
> just realized that this API keeps elf file open for the whole
> life time. I think that should be split up.
> bpf_open_object() can do elf parsing, creation of maps and
> bpf loading.
> bpf_close_object() can unload programs, maps. That's fine,
> but closing of elf file and freeing memory associated
> with parsing sections should be a separate call.
>

Looks like I didn't describe the API of libbpf very clearly in patch 0/37.

There are two phases of the API: opening and loading. Opening phase
does ELF parsing, and records all related information info 'struct
bpf_object'.
After that, ELF file itself is closed by bpf_obj_elf_finish(). See patch
19/37.
After that, caller will have a chance to adjust programs and maps
(currently not
implemented). Caller should call bpf_load_object() to do real useful
things with
kernel. bpf_unload_object() remove map and programs in the object from
kernel. It
also calls bpf_obj_elf_finish(). However, if it has been called once,
the 2nd call
doesn't take effect at all. Just in case.

Thank you.