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

[Wangnan (F)]

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@xxxxxxxxxx>
> > Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
> > Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> > Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> > Cc: Zefan Li <lizefan@xxxxxxxxxx>
> > Cc: pi3orama@xxxxxxx
> > ---
> >   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 fileâs 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 fileâs 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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