linux-kernel - Re: [PATCH v2 02/13] bpf tools: Extract and collect map names from BPF object file

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Message-ID: <565BD7FE.50405@huawei.com>
Date:	Mon, 30 Nov 2015 13:00:46 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	<acme@...nel.org>, <ast@...nel.org>,
	<linux-kernel@...r.kernel.org>, <masami.hiramatsu.pt@...achi.com>,
	<lizefan@...wei.com>, <pi3orama@....com>,
	He Kuang <hekuang@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH v2 02/13] bpf tools: Extract and collect map names from
 BPF object file



On 2015/11/30 0:14, Namhyung Kim wrote:
> Hi Wang,
>
> On Fri, Nov 27, 2015 at 08:47:36AM +0000, Wang Nan wrote:
>> This patch collects name of maps in BPF object files and saves them into
>> 'maps' field in 'struct bpf_object'. 'bpf_object__get_map_by_name' is
>> introduced to retrive fd and definitions of a map through its name.
>>
>> Signed-off-by: Wang Nan <wangnan0@...wei.com>
>> Signed-off-by: He Kuang <hekuang@...wei.com>
>> Cc: Alexei Starovoitov <ast@...nel.org>
>> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
>> Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
>> Cc: Namhyung Kim <namhyung@...nel.org>
>> Cc: Zefan Li <lizefan@...wei.com>
>> Cc: pi3orama@....com
>> ---
>>   tools/lib/bpf/libbpf.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++---
>>   tools/lib/bpf/libbpf.h |  3 +++
>>   2 files changed, 65 insertions(+), 3 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index f509825..a298614 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -165,6 +165,7 @@ struct bpf_program {
>>   
>>   struct bpf_map {
>>   	int fd;
>> +	char *name;
>>   	struct bpf_map_def def;
>>   	void *priv;
>>   	bpf_map_clear_priv_t clear_priv;
>> @@ -526,12 +527,46 @@ bpf_object__init_maps(struct bpf_object *obj, void *data,
>>   	return 0;
>>   }
>>   
>> +static void
>> +bpf_object__init_maps_name(struct bpf_object *obj, int maps_shndx)
>> +{
>> +	int i;
>> +	Elf_Data *symbols = obj->efile.symbols;
>> +
>> +	if (!symbols || maps_shndx < 0)
>> +		return;
>> +
>> +	for (i = 0; i < symbols->d_size / sizeof(GElf_Sym); i++) {
>> +		GElf_Sym sym;
>> +		size_t map_idx;
>> +		const char *map_name;
>> +
>> +		if (!gelf_getsym(symbols, i, &sym))
>> +			continue;
>> +		if (sym.st_shndx != maps_shndx)
>> +			continue;
>> +
>> +		map_name = elf_strptr(obj->efile.elf,
>> +				      obj->efile.ehdr.e_shstrndx,
>> +				      sym.st_name);
> It means that each map name is saved in section header string table?

According to elf format specification:

For an symbol table entry, the st_name field "holds an index
into the object file’s symbol string table, which holds the
character representations of the symbol names. If the value
is non-zero, it represents a string table index that gives
the symbol name. Otherwise, the symbol table entry has no
name."

And so called "object file’s symbol string table" is a
section in the object file which index is stored into
ehdr and be loaded during gelf_getehdr(), and its index
would be set to ehdr->e_shstrndx. So I think for each map
its name should be saved in that string table.

>
>> +		map_idx = sym.st_value / sizeof(struct bpf_map_def);
>> +		if (map_idx >= obj->nr_maps) {
>> +			pr_warning("index of map \"%s\" is buggy: %zu > %zu\n",
>> +				   map_name, map_idx, obj->nr_maps);
>> +			continue;
>> +		}
>> +		obj->maps[map_idx].name = strdup(map_name);
> You need to check the return value.

Will send a patch for it.

Thank you.

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