[PATCH net-next v2 1/2] libbpf: parse maps sections of varying size

[Craig Gallek <kraigatgoog@gmail.com>]

From: Craig Gallek <kraig@google.com>

This library previously assumed a fixed-size map options structure.
Any new options were ignored.  In order to allow the options structure
to grow and to support parsing older programs, this patch updates
the maps section parsing to handle varying sizes.

Object files with maps sections smaller than expected will have the new
fields initialized to zero.  Object files which have larger than expected
maps sections will be rejected unless all of the unrecognized data is zero.

This change still assumes that each map definition in the maps section
is the same size.

Signed-off-by: Craig Gallek <kraig@google.com>
---
 tools/lib/bpf/libbpf.c | 54 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 46 insertions(+), 8 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 4f402dcdf372..28b300868ad7 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -580,7 +580,7 @@ bpf_object__init_kversion(struct bpf_object *obj,
 }
 
 static int
-bpf_object__validate_maps(struct bpf_object *obj)
+bpf_object__validate_maps(struct bpf_object *obj, int map_def_sz)
 {
 	int i;
 
@@ -595,9 +595,11 @@ bpf_object__validate_maps(struct bpf_object *obj)
 		const struct bpf_map *a = &obj->maps[i - 1];
 		const struct bpf_map *b = &obj->maps[i];
 
-		if (b->offset - a->offset < sizeof(struct bpf_map_def)) {
-			pr_warning("corrupted map section in %s: map \"%s\" too small\n",
-				   obj->path, a->name);
+		if (b->offset - a->offset < map_def_sz) {
+			pr_warning("corrupted map section in %s: map \"%s\" too small "
+				   "(%zd vs %d)\n",
+				   obj->path, a->name, b->offset - a->offset,
+				   map_def_sz);
 			return -EINVAL;
 		}
 	}
@@ -615,7 +617,7 @@ static int compare_bpf_map(const void *_a, const void *_b)
 static int
 bpf_object__init_maps(struct bpf_object *obj)
 {
-	int i, map_idx, nr_maps = 0;
+	int i, map_idx, map_def_sz, nr_maps = 0;
 	Elf_Scn *scn;
 	Elf_Data *data;
 	Elf_Data *symbols = obj->efile.symbols;
@@ -658,6 +660,15 @@ bpf_object__init_maps(struct bpf_object *obj)
 	if (!nr_maps)
 		return 0;
 
+	/* Assume equally sized map definitions */
+	map_def_sz = data->d_size / nr_maps;
+	if (!data->d_size || (data->d_size % nr_maps) != 0) {
+		pr_warning("unable to determine map definition size "
+			   "section %s, %d maps in %zd bytes\n",
+			   obj->path, nr_maps, data->d_size);
+		return -EINVAL;
+	}
+
 	obj->maps = calloc(nr_maps, sizeof(obj->maps[0]));
 	if (!obj->maps) {
 		pr_warning("alloc maps for object failed\n");
@@ -690,7 +701,7 @@ bpf_object__init_maps(struct bpf_object *obj)
 				      obj->efile.strtabidx,
 				      sym.st_name);
 		obj->maps[map_idx].offset = sym.st_value;
-		if (sym.st_value + sizeof(struct bpf_map_def) > data->d_size) {
+		if (sym.st_value + map_def_sz > data->d_size) {
 			pr_warning("corrupted maps section in %s: last map \"%s\" too small\n",
 				   obj->path, map_name);
 			return -EINVAL;
@@ -704,12 +715,39 @@ bpf_object__init_maps(struct bpf_object *obj)
 		pr_debug("map %d is \"%s\"\n", map_idx,
 			 obj->maps[map_idx].name);
 		def = (struct bpf_map_def *)(data->d_buf + sym.st_value);
-		obj->maps[map_idx].def = *def;
+		/*
+		 * If the definition of the map in the object file fits in
+		 * bpf_map_def, copy it.  Any extra fields in our version
+		 * of bpf_map_def will default to zero as a result of the
+		 * calloc above.
+		 */
+		if (map_def_sz <= sizeof(struct bpf_map_def)) {
+			memcpy(&obj->maps[map_idx].def, def, map_def_sz);
+		} else {
+			/*
+			 * Here the map structure being read is bigger than what
+			 * we expect, truncate if the excess bits are all zero.
+			 * If they are not zero, reject this map as
+			 * incompatible.
+			 */
+			char *b;
+			for (b = ((char *)def) + sizeof(struct bpf_map_def);
+			     b < ((char *)def) + map_def_sz; b++) {
+				if (*b != 0) {
+					pr_warning("maps section in %s: \"%s\" "
+						   "has unrecognized, non-zero "
+						   "options\n",
+						   obj->path, map_name);
+					return -EINVAL;
+				}
+			}
+			obj->maps[map_idx].def = *def;
+		}
 		map_idx++;
 	}
 
 	qsort(obj->maps, obj->nr_maps, sizeof(obj->maps[0]), compare_bpf_map);
-	return bpf_object__validate_maps(obj);
+	return bpf_object__validate_maps(obj, map_def_sz);
 }
 
 static int bpf_object__elf_collect(struct bpf_object *obj)
-- 
2.14.2.822.g60be5d43e6-goog