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

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Date:	Mon, 30 Nov 2015 17:51:30 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
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 Mon, Nov 30, 2015 at 01:00:46PM +0800, Wangnan (F) wrote:
> 
> 
> 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.

AFAIK there're two symbol string tables in a ELF file.  One for
section headers (.shstrtab) and another for normal symbols (.strtab).
And ehdr->e_shstrndx is the index of section header string table so
your code assumes map names are saved in the section header string
table, right?

Thanks,
Namhyung


> 
> >
> >>+		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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