selftests/bpf: cgroup_helpers.c: Fix strncpy() fortify warning

Copy libbpf_strlcpy() from libbpf_internal.h to bpf_util.h, and rename it
to bpf_strlcpy(), then replace selftests strncpy()/libbpf_strlcpy() with
bpf_strlcpy(), fix compile warning.

The libbpf_internal.h header cannot be used directly here, because
references to cgroup_helpers.c in samples/bpf will generate compilation
errors. We also can't add libbpf_strlcpy() directly to bpf_util.h,
because the definition of libbpf_strlcpy() in libbpf_internal.h is
duplicated. In order not to modify the libbpf code, add a new function
bpf_strlcpy() to selftests bpf_util.h.

How to reproduce this compilation warning:

$ make -C samples/bpf
cgroup_helpers.c: In function ‘__enable_controllers’:
cgroup_helpers.c:80:17: warning: ‘strncpy’ specified bound 4097 equals destination size [-Wstringop-truncation]
   80 |                 strncpy(enable, controllers, sizeof(enable));
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/tencent_469D8AF32BD56816A29981BED06E96D22506@qq.com

tools/testing/selftests/bpf/bpf_util.h

@@ -20,6 +20,25 @@ static inline unsigned int bpf_num_possible_cpus(void)
 	return possible_cpus;
 }
 
+/* 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 bpf_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';
+}
+
 #define __bpf_percpu_val_align	__attribute__((__aligned__(8)))
 
 #define BPF_DECLARE_PERCPU(type, name)				\

tools/testing/selftests/bpf/cgroup_helpers.c

@@ -13,6 +13,7 @@
 #include <ftw.h>
 
 #include "cgroup_helpers.h"
+#include "bpf_util.h"
 
 /*
  * To avoid relying on the system setup, when setup_cgroup_env is called
@@ -77,7 +78,7 @@ static int __enable_controllers(const char *cgroup_path, const char *controllers
 		enable[len] = 0;
 		close(fd);
 	} else {
-		strncpy(enable, controllers, sizeof(enable));
+		bpf_strlcpy(enable, controllers, sizeof(enable));
 	}
 
 	snprintf(path, sizeof(path), "%s/cgroup.subtree_control", cgroup_path);

tools/testing/selftests/bpf/xsk.c

@@ -33,6 +33,7 @@
 #include <bpf/bpf.h>
 #include <bpf/libbpf.h>
 #include "xsk.h"
+#include "bpf_util.h"
 
 #ifndef SOL_XDP
  #define SOL_XDP 283
@@ -521,25 +522,6 @@ static int xsk_create_bpf_link(struct xsk_socket *xsk)
 	return 0;
 }
 
-/* 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';
-}
-
 static int xsk_get_max_queues(struct xsk_socket *xsk)
 {
 	struct ethtool_channels channels = { .cmd = ETHTOOL_GCHANNELS };
@@ -552,7 +534,7 @@ static int xsk_get_max_queues(struct xsk_socket *xsk)
 		return -errno;
 
 	ifr.ifr_data = (void *)&channels;
-	libbpf_strlcpy(ifr.ifr_name, ctx->ifname, IFNAMSIZ);
+	bpf_strlcpy(ifr.ifr_name, ctx->ifname, IFNAMSIZ);
 	err = ioctl(fd, SIOCETHTOOL, &ifr);
 	if (err && errno != EOPNOTSUPP) {
 		ret = -errno;
@@ -771,7 +753,7 @@ static int xsk_create_xsk_struct(int ifindex, struct xsk_socket *xsk)
 	}
 
 	ctx->ifindex = ifindex;
-	libbpf_strlcpy(ctx->ifname, ifname, IFNAMSIZ);
+	bpf_strlcpy(ctx->ifname, ifname, IFNAMSIZ);
 
 	xsk->ctx = ctx;
 	xsk->ctx->has_bpf_link = xsk_probe_bpf_link();
@@ -958,7 +940,7 @@ static struct xsk_ctx *xsk_create_ctx(struct xsk_socket *xsk,
 	ctx->refcount = 1;
 	ctx->umem = umem;
 	ctx->queue_id = queue_id;
-	libbpf_strlcpy(ctx->ifname, ifname, IFNAMSIZ);
+	bpf_strlcpy(ctx->ifname, ifname, IFNAMSIZ);
 
 	ctx->fill = fill;
 	ctx->comp = comp;