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

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Date:	Mon, 25 May 2015 17:23:54 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Alexei Starovoitov <ast@...mgrid.com>, <paulus@...ba.org>,
	<a.p.zijlstra@...llo.nl>, <mingo@...hat.com>, <acme@...nel.org>,
	<namhyung@...nel.org>, <jolsa@...nel.org>, <dsahern@...il.com>,
	<daniel@...earbox.net>, <brendan.d.gregg@...il.com>,
	<masami.hiramatsu.pt@...achi.com>
CC:	<lizefan@...wei.com>, <linux-kernel@...r.kernel.org>,
	<pi3orama@....com>
Subject: Re: [RFC PATCH v3 21/37] bpf tools: Create eBPF maps defined in an
 object file



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@...wei.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.


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