bpf: fix build error in libbpf with EXTRA_CFLAGS="-Wp, -D_FORTIFY_SOURCE=2 -O2"

Commit 531b014e ("tools: bpf: make use of reallocarray") causes
a compiler error when building the perf tool in the linux-next tree.
Compile file tools/lib/bpf/libbpf.c on a FEDORA 28 installation with
gcc compiler version: gcc (GCC) 8.0.1 20180324 (Red Hat 8.0.1-0.20)
shows this error message:

  [root@p23lp27] # make V=1 EXTRA_CFLAGS="-Wp,-D_FORTIFY_SOURCE=2 -O2"
  [...]
  make -f /home6/tmricht/linux-next/tools/build/Makefile.build
  	dir=./util/scripting-engines obj=libperf
  libbpf.c: In function ‘bpf_object__elf_collect’:
  libbpf.c:811:15: error: ignoring return value of ‘strerror_r’,
  		declared with attribute warn_unused_result [-Werror=unused-result]
       strerror_r(-err, errmsg, sizeof(errmsg));
                 ^
  cc1: all warnings being treated as errors
  mv: cannot stat './.libbpf.o.tmp': No such file or directory
  /home6/tmricht/linux-next/tools/build/Makefile.build:96: recipe for target 'libbpf.o' failed

Replace all occurrences of strerror() by calls to strerror_r(). To
keep the compiler quiet also use the return value from strerror_r()
otherwise a 'variable set but not use' warning which is treated as
error terminates the compile.

Fixes: 531b014e ("tools: bpf: make use of reallocarray")
Suggested-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Suggested-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NThomas Richter <tmricht@linux.ibm.com>
Reviewed-by: NHendrik Brueckner <brueckner@linux.ibm.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -467,8 +467,10 @@ static int bpf_object__elf_init(struct bpf_object *obj)
 	} else {
 		obj->efile.fd = open(obj->path, O_RDONLY);
 		if (obj->efile.fd < 0) {
-			pr_warning("failed to open %s: %s\n", obj->path,
-					strerror(errno));
+			char errmsg[STRERR_BUFSIZE];
+			char *cp = strerror_r(errno, errmsg, sizeof(errmsg));
+
+			pr_warning("failed to open %s: %s\n", obj->path, cp);
 			return -errno;
 		}
 
@@ -807,10 +809,11 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 						      data->d_size, name, idx);
 			if (err) {
 				char errmsg[STRERR_BUFSIZE];
+				char *cp = strerror_r(-err, errmsg,
+						      sizeof(errmsg));
 
-				strerror_r(-err, errmsg, sizeof(errmsg));
 				pr_warning("failed to alloc program %s (%s): %s",
-					   name, obj->path, errmsg);
+					   name, obj->path, cp);
 			}
 		} else if (sh.sh_type == SHT_REL) {
 			void *reloc = obj->efile.reloc;
@@ -1104,6 +1107,7 @@ bpf_object__create_maps(struct bpf_object *obj)
 	for (i = 0; i < obj->nr_maps; i++) {
 		struct bpf_map *map = &obj->maps[i];
 		struct bpf_map_def *def = &map->def;
+		char *cp, errmsg[STRERR_BUFSIZE];
 		int *pfd = &map->fd;
 
 		if (map->fd >= 0) {
@@ -1131,8 +1135,9 @@ bpf_object__create_maps(struct bpf_object *obj)
 
 		*pfd = bpf_create_map_xattr(&create_attr);
 		if (*pfd < 0 && create_attr.btf_key_type_id) {
+			cp = strerror_r(errno, errmsg, sizeof(errmsg));
 			pr_warning("Error in bpf_create_map_xattr(%s):%s(%d). Retrying without BTF.\n",
-				   map->name, strerror(errno), errno);
+				   map->name, cp, errno);
 			create_attr.btf_fd = 0;
 			create_attr.btf_key_type_id = 0;
 			create_attr.btf_value_type_id = 0;
@@ -1145,9 +1150,9 @@ bpf_object__create_maps(struct bpf_object *obj)
 			size_t j;
 
 			err = *pfd;
+			cp = strerror_r(errno, errmsg, sizeof(errmsg));
 			pr_warning("failed to create map (name: '%s'): %s\n",
-				   map->name,
-				   strerror(errno));
+				   map->name, cp);
 			for (j = 0; j < i; j++)
 				zclose(obj->maps[j].fd);
 			return err;
@@ -1299,6 +1304,7 @@ load_program(enum bpf_prog_type type, enum bpf_attach_type expected_attach_type,
 	     char *license, u32 kern_version, int *pfd, int prog_ifindex)
 {
 	struct bpf_load_program_attr load_attr;
+	char *cp, errmsg[STRERR_BUFSIZE];
 	char *log_buf;
 	int ret;
 
@@ -1328,7 +1334,8 @@ load_program(enum bpf_prog_type type, enum bpf_attach_type expected_attach_type,
 	}
 
 	ret = -LIBBPF_ERRNO__LOAD;
-	pr_warning("load bpf program failed: %s\n", strerror(errno));
+	cp = strerror_r(errno, errmsg, sizeof(errmsg));
+	pr_warning("load bpf program failed: %s\n", cp);
 
 	if (log_buf && log_buf[0] != '\0') {
 		ret = -LIBBPF_ERRNO__VERIFY;
@@ -1627,6 +1634,7 @@ int bpf_object__load(struct bpf_object *obj)
 
 static int check_path(const char *path)
 {
+	char *cp, errmsg[STRERR_BUFSIZE];
 	struct statfs st_fs;
 	char *dname, *dir;
 	int err = 0;
@@ -1640,7 +1648,8 @@ static int check_path(const char *path)
 
 	dir = dirname(dname);
 	if (statfs(dir, &st_fs)) {
-		pr_warning("failed to statfs %s: %s\n", dir, strerror(errno));
+		cp = strerror_r(errno, errmsg, sizeof(errmsg));
+		pr_warning("failed to statfs %s: %s\n", dir, cp);
 		err = -errno;
 	}
 	free(dname);
@@ -1656,6 +1665,7 @@ static int check_path(const char *path)
 int bpf_program__pin_instance(struct bpf_program *prog, const char *path,
 			      int instance)
 {
+	char *cp, errmsg[STRERR_BUFSIZE];
 	int err;
 
 	err = check_path(path);
@@ -1674,7 +1684,8 @@ int bpf_program__pin_instance(struct bpf_program *prog, const char *path,
 	}
 
 	if (bpf_obj_pin(prog->instances.fds[instance], path)) {
-		pr_warning("failed to pin program: %s\n", strerror(errno));
+		cp = strerror_r(errno, errmsg, sizeof(errmsg));
+		pr_warning("failed to pin program: %s\n", cp);
 		return -errno;
 	}
 	pr_debug("pinned program '%s'\n", path);
@@ -1684,13 +1695,16 @@ int bpf_program__pin_instance(struct bpf_program *prog, const char *path,
 
 static int make_dir(const char *path)
 {
+	char *cp, errmsg[STRERR_BUFSIZE];
 	int err = 0;
 
 	if (mkdir(path, 0700) && errno != EEXIST)
 		err = -errno;
 
-	if (err)
-		pr_warning("failed to mkdir %s: %s\n", path, strerror(-err));
+	if (err) {
+		cp = strerror_r(-err, errmsg, sizeof(errmsg));
+		pr_warning("failed to mkdir %s: %s\n", path, cp);
+	}
 	return err;
 }
 
@@ -1737,6 +1751,7 @@ int bpf_program__pin(struct bpf_program *prog, const char *path)
 
 int bpf_map__pin(struct bpf_map *map, const char *path)
 {
+	char *cp, errmsg[STRERR_BUFSIZE];
 	int err;
 
 	err = check_path(path);
@@ -1749,7 +1764,8 @@ int bpf_map__pin(struct bpf_map *map, const char *path)
 	}
 
 	if (bpf_obj_pin(map->fd, path)) {
-		pr_warning("failed to pin map: %s\n", strerror(errno));
+		cp = strerror_r(errno, errmsg, sizeof(errmsg));
+		pr_warning("failed to pin map: %s\n", cp);
 		return -errno;
 	}