bpftool: Fix error message of strerror

strerror() expects a positive errno, however variable err will never be
positive when an error occurs. This causes bpftool to output too many
"unknown error", even a simple "file not exist" error can not get an
accurate message.

This patch fixed all "strerror(err)" patterns in bpftool.
Specially in btf.c#L823, hashmap__append() is an internal function of
libbpf and will not change errno, so there's a little difference.
Some libbpf_get_error() calls are kept for return values.

Changes since v1: https://lore.kernel.org/bpf/SY4P282MB1084B61CD8671DFA395AA8579D539@SY4P282MB1084.AUSP282.PROD.OUTLOOK.COM/
Check directly for NULL values instead of calling libbpf_get_error().

Signed-off-by: Tianyi Liu <i.pear@outlook.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/SY4P282MB1084AD9CD84A920F08DF83E29D549@SY4P282MB1084.AUSP282.PROD.OUTLOOK.COM

tools/bpf/bpftool/btf.c

@@ -635,10 +635,9 @@ static int do_dump(int argc, char **argv)
 
 		btf = btf__parse_split(*argv, base ?: base_btf);
 		err = libbpf_get_error(btf);
-		if (err) {
-			btf = NULL;
+		if (!btf) {
 			p_err("failed to load BTF from %s: %s",
-			      *argv, strerror(err));
+			      *argv, strerror(errno));
 			goto done;
 		}
 		NEXT_ARG();
@@ -683,8 +682,8 @@ static int do_dump(int argc, char **argv)
 
 		btf = btf__load_from_kernel_by_id_split(btf_id, base_btf);
 		err = libbpf_get_error(btf);
-		if (err) {
-			p_err("get btf by id (%u): %s", btf_id, strerror(err));
+		if (!btf) {
+			p_err("get btf by id (%u): %s", btf_id, strerror(errno));
 			goto done;
 		}
 	}
@@ -820,7 +819,7 @@ build_btf_type_table(struct hashmap *tab, enum bpf_obj_type type,
 				      u32_as_hash_field(id));
 		if (err) {
 			p_err("failed to append entry to hashmap for BTF ID %u, object ID %u: %s",
-			      btf_id, id, strerror(errno));
+			      btf_id, id, strerror(-err));
 			goto err_free;
 		}
 	}

tools/bpf/bpftool/gen.c

@@ -1594,14 +1594,14 @@ static int do_object(int argc, char **argv)
 
 		err = bpf_linker__add_file(linker, file, NULL);
 		if (err) {
-			p_err("failed to link '%s': %s (%d)", file, strerror(err), err);
+			p_err("failed to link '%s': %s (%d)", file, strerror(errno), errno);
 			goto out;
 		}
 	}
 
 	err = bpf_linker__finalize(linker);
 	if (err) {
-		p_err("failed to finalize ELF file: %s (%d)", strerror(err), err);
+		p_err("failed to finalize ELF file: %s (%d)", strerror(errno), errno);
 		goto out;
 	}
 

tools/bpf/bpftool/map_perf_ring.c

@@ -188,10 +188,9 @@ int do_event_pipe(int argc, char **argv)
 	opts.map_keys = &ctx.idx;
 	pb = perf_buffer__new_raw(map_fd, MMAP_PAGE_CNT, &perf_attr,
 				  print_bpf_output, &ctx, &opts);
-	err = libbpf_get_error(pb);
-	if (err) {
+	if (!pb) {
 		p_err("failed to create perf buffer: %s (%d)",
-		      strerror(err), err);
+		      strerror(errno), errno);
 		goto err_close_map;
 	}
 
@@ -206,7 +205,7 @@ int do_event_pipe(int argc, char **argv)
 		err = perf_buffer__poll(pb, 200);
 		if (err < 0 && err != -EINTR) {
 			p_err("perf buffer polling failed: %s (%d)",
-			      strerror(err), err);
+			      strerror(errno), errno);
 			goto err_close_pb;
 		}
 	}