libbpf: Add sane strncpy alternative and use it internally

strncpy() has a notoriously error-prone semantics which makes GCC
complain about it a lot (and quite often completely completely falsely
at that). Instead of pleasing GCC all the time (-Wno-stringop-truncation
is unfortunately only supported by GCC, so it's a bit too messy to just
enable it in Makefile), add libbpf-internal libbpf_strlcpy() helper
which follows what FreeBSD's strlcpy() does and what most people would
expect from strncpy(): copies up to N-1 first bytes from source string
into destination string and ensures zero-termination afterwards.

Replace all the relevant uses of strncpy/strncat/memcpy in libbpf with
libbpf_strlcpy().

This also fixes the issue reported by Emmanuel Deloget in xsk.c where
memcpy() could access source string beyond its end.

Fixes: 2f6324a3937f8 (libbpf: Support shared umems between queues and devices)
Reported-by: Emmanuel Deloget <emmanuel.deloget@eho.link>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211211004043.2374068-1-andrii@kernel.org

src/bpf.c

@@ -112,7 +112,7 @@ int bpf_map_create(enum bpf_map_type map_type,
 
 	attr.map_type = map_type;
 	if (map_name)
-		strncat(attr.map_name, map_name, sizeof(attr.map_name) - 1);
+		libbpf_strlcpy(attr.map_name, map_name, sizeof(attr.map_name));
 	attr.key_size = key_size;
 	attr.value_size = value_size;
 	attr.max_entries = max_entries;
@@ -271,7 +271,7 @@ int bpf_prog_load_v0_6_0(enum bpf_prog_type prog_type,
 	attr.kern_version = OPTS_GET(opts, kern_version, 0);
 
 	if (prog_name)
-		strncat(attr.prog_name, prog_name, sizeof(attr.prog_name) - 1);
+		libbpf_strlcpy(attr.prog_name, prog_name, sizeof(attr.prog_name));
 	attr.license = ptr_to_u64(license);
 
 	if (insn_cnt > UINT_MAX)

src/btf_dump.c

@@ -2321,8 +2321,8 @@ int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
 	if (!opts->indent_str)
 		d->typed_dump->indent_str[0] = '\t';
 	else
-		strncat(d->typed_dump->indent_str, opts->indent_str,
-			sizeof(d->typed_dump->indent_str) - 1);
+		libbpf_strlcpy(d->typed_dump->indent_str, opts->indent_str,
+			       sizeof(d->typed_dump->indent_str));
 
 	d->typed_dump->compact = OPTS_GET(opts, compact, false);
 	d->typed_dump->skip_names = OPTS_GET(opts, skip_names, false);

src/gen_loader.c

@@ -463,8 +463,7 @@ void bpf_gen__map_create(struct bpf_gen *gen,
 	attr.map_flags = map_attr->map_flags;
 	attr.map_extra = map_attr->map_extra;
 	if (map_name)
-		memcpy(attr.map_name, map_name,
-		       min((unsigned)strlen(map_name), BPF_OBJ_NAME_LEN - 1));
+		libbpf_strlcpy(attr.map_name, map_name, sizeof(attr.map_name));
 	attr.numa_node = map_attr->numa_node;
 	attr.map_ifindex = map_attr->map_ifindex;
 	attr.max_entries = max_entries;
@@ -970,8 +969,7 @@ void bpf_gen__prog_load(struct bpf_gen *gen,
 	core_relos = add_data(gen, gen->core_relos,
 			     attr.core_relo_cnt * attr.core_relo_rec_size);
 
-	memcpy(attr.prog_name, prog_name,
-	       min((unsigned)strlen(prog_name), BPF_OBJ_NAME_LEN - 1));
+	libbpf_strlcpy(attr.prog_name, prog_name, sizeof(attr.prog_name));
 	prog_load_attr = add_data(gen, &attr, attr_size);
 
 	/* populate union bpf_attr with a pointer to license */

src/libbpf.c

@@ -1201,12 +1201,10 @@ static struct bpf_object *bpf_object__new(const char *path,
 
 	strcpy(obj->path, path);
 	if (obj_name) {
-		strncpy(obj->name, obj_name, sizeof(obj->name) - 1);
-		obj->name[sizeof(obj->name) - 1] = 0;
+		libbpf_strlcpy(obj->name, obj_name, sizeof(obj->name));
 	} else {
 		/* Using basename() GNU version which doesn't modify arg. */
-		strncpy(obj->name, basename((void *)path),
-			sizeof(obj->name) - 1);
+		libbpf_strlcpy(obj->name, basename((void *)path), sizeof(obj->name));
 		end = strchr(obj->name, '.');
 		if (end)
 			*end = 0;
@@ -1358,7 +1356,7 @@ static int bpf_object__check_endianness(struct bpf_object *obj)
 static int
 bpf_object__init_license(struct bpf_object *obj, void *data, size_t size)
 {
-	memcpy(obj->license, data, min(size, sizeof(obj->license) - 1));
+	libbpf_strlcpy(obj->license, data, sizeof(obj->license));
 	pr_debug("license of %s is %s\n", obj->path, obj->license);
 	return 0;
 }

src/libbpf_internal.h

@@ -169,6 +169,25 @@ static inline void *libbpf_reallocarray(void *ptr, size_t nmemb, size_t size)
 	return realloc(ptr, total);
 }
 
+/* Copy up to sz - 1 bytes from zero-terminated src string and ensure that dst
+ * is zero-terminated string no matter what (unless sz == 0, in which case
+ * it's a no-op). It's conceptually close to FreeBSD's strlcpy(), but differs
+ * in what is returned. Given this is internal helper, it's trivial to extend
+ * this, when necessary. Use this instead of strncpy inside libbpf source code.
+ */
+static inline void libbpf_strlcpy(char *dst, const char *src, size_t sz)
+{
+	size_t i;
+
+	if (sz == 0)
+		return;
+
+	sz--;
+	for (i = 0; i < sz && src[i]; i++)
+		dst[i] = src[i];
+	dst[i] = '\0';
+}
+
 struct btf;
 struct btf_type;
 

src/xsk.c

@@ -548,8 +548,7 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
 		return -errno;
 
 	ifr.ifr_data = (void *)&channels;
-	memcpy(ifr.ifr_name, ctx->ifname, IFNAMSIZ - 1);
-	ifr.ifr_name[IFNAMSIZ - 1] = '\0';
+	libbpf_strlcpy(ifr.ifr_name, ctx->ifname, IFNAMSIZ);
 	err = ioctl(fd, SIOCETHTOOL, &ifr);
 	if (err && errno != EOPNOTSUPP) {
 		ret = -errno;
@@ -768,8 +767,7 @@ static int xsk_create_xsk_struct(int ifindex, struct xsk_socket *xsk)
 	}
 
 	ctx->ifindex = ifindex;
-	memcpy(ctx->ifname, ifname, IFNAMSIZ -1);
-	ctx->ifname[IFNAMSIZ - 1] = 0;
+	libbpf_strlcpy(ctx->ifname, ifname, IFNAMSIZ);
 
 	xsk->ctx = ctx;
 	xsk->ctx->has_bpf_link = xsk_probe_bpf_link();
@@ -951,8 +949,7 @@ static struct xsk_ctx *xsk_create_ctx(struct xsk_socket *xsk,
 	ctx->refcount = 1;
 	ctx->umem = umem;
 	ctx->queue_id = queue_id;
-	memcpy(ctx->ifname, ifname, IFNAMSIZ - 1);
-	ctx->ifname[IFNAMSIZ - 1] = '\0';
+	libbpf_strlcpy(ctx->ifname, ifname, IFNAMSIZ);
 
 	ctx->fill = fill;
 	ctx->comp = comp;