Subject: [PATCH 7/8] tools lib bpf: fix maps resolution

From: Eric Leblond <[email-masked]>
Date: Sun, 16 Oct 2016 23:18:33 +0200

It is not correct to assimilate the elf data of the maps section
to an array of map definition. In fact the sizes differ. The
offset provided in the symbol section has to be used instead.

This patch fixes a bug causing a elf with two maps not to load
correctly.

Signed-off-by: Eric Leblond <[email-masked]>
---
 tools/lib/bpf/libbpf.c | 50 +++++++++++++++++++++++++++++++++++---------------
 1 file changed, 35 insertions(+), 15 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1fe4532..f72628b 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -186,6 +186,7 @@ struct bpf_program {
 struct bpf_map {
 	int fd;
 	char *name;
+	size_t offset;
 	struct bpf_map_def def;
 	void *priv;
 	bpf_map_clear_priv_t clear_priv;
@@ -529,13 +530,6 @@ bpf_object__init_maps(struct bpf_object *obj, void *data,
 
 	pr_debug("maps in %s: %zd bytes\n", obj->path, size);
 
-	obj->maps = calloc(nr_maps, sizeof(obj->maps[0]));
-	if (!obj->maps) {
-		pr_warning("alloc maps for object failed\n");
-		return -ENOMEM;
-	}
-	obj->nr_maps = nr_maps;
-
 	for (i = 0; i < nr_maps; i++) {
 		struct bpf_map_def *def = &obj->maps[i].def;
 
@@ -547,23 +541,42 @@ bpf_object__init_maps(struct bpf_object *obj, void *data,
 		obj->maps[i].fd = -1;
 
 		/* Save map definition into obj->maps */
-		*def = ((struct bpf_map_def *)data)[i];
+		*def = *(struct bpf_map_def *)(data + obj->maps[i].offset);
 	}
 	return 0;
 }
 
 static int
-bpf_object__init_maps_name(struct bpf_object *obj)
+bpf_object__init_maps_symbol(struct bpf_object *obj)
 {
 	int i;
+	int nr_maps = 0;
 	Elf_Data *symbols = obj->efile.symbols;
+	size_t map_idx = 0;
 
 	if (!symbols || obj->efile.maps_shndx < 0)
 		return -EINVAL;
 
+	/* get the number of maps */
+	for (i = 0; i < symbols->d_size / sizeof(GElf_Sym); i++) {
+		GElf_Sym sym;
+
+		if (!gelf_getsym(symbols, i, &sym))
+			continue;
+		if (sym.st_shndx != obj->efile.maps_shndx)
+			continue;
+		nr_maps++;
+	}
+
+	obj->maps = calloc(nr_maps, sizeof(obj->maps[0]));
+	if (!obj->maps) {
+		pr_warning("alloc maps for object failed\n");
+		return -ENOMEM;
+	}
+	obj->nr_maps = nr_maps;
+
 	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))
@@ -574,12 +587,12 @@ bpf_object__init_maps_name(struct bpf_object *obj)
 		map_name = elf_strptr(obj->efile.elf,
 				      obj->efile.strtabidx,
 				      sym.st_name);
-		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].offset = sym.st_value;
 		obj->maps[map_idx].name = strdup(map_name);
 		if (!obj->maps[map_idx].name) {
 			pr_warning("failed to alloc map name\n");
@@ -587,6 +600,7 @@ bpf_object__init_maps_name(struct bpf_object *obj)
 		}
 		pr_debug("map %zu is \"%s\"\n", map_idx,
 			 obj->maps[map_idx].name);
+		map_idx++;
 	}
 	return 0;
 }
@@ -647,8 +661,6 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 							data->d_buf,
 							data->d_size);
 		else if (strcmp(name, "maps") == 0) {
-			err = bpf_object__init_maps(obj, data->d_buf,
-						    data->d_size);
 			obj->efile.maps_shndx = idx;
 		} else if (sh.sh_type == SHT_SYMTAB) {
 			if (obj->efile.symbols) {
@@ -698,8 +710,16 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 		pr_warning("Corrupted ELF file: index of strtab invalid\n");
 		return LIBBPF_ERRNO__FORMAT;
 	}
-	if (obj->efile.maps_shndx >= 0)
-		err = bpf_object__init_maps_name(obj);
+	if (obj->efile.maps_shndx >= 0) {
+		Elf_Data *data;
+		err = bpf_object__init_maps_symbol(obj);
+		if (err)
+			goto out;
+
+		scn = elf_getscn(elf, obj->efile.maps_shndx);
+		data = elf_getdata(scn, 0);
+		err = bpf_object__init_maps(obj, data->d_buf, data->d_size);
+	}
 out:
 	return err;
 }
-- 
2.9.3