bpftool: Fix spurious batch file read error

do_batch() checks errno after the read loop to detect read failures,
but fgets() does not clear errno on success, so a stale errno left by
a previously executed command (e.g. map dump's EBADF from a double
close) makes bpftool report a batch file read failure and exit with an
error even though every command succeeded.

Clear errno before each fgets() call, so the post-loop check only
sees the outcome of the last read: zero on success or EOF, E2BIG for
an overlong line, and a genuine errno when fgets() fails.

Since errno is now reset before every read in batch mode, drop the
USE_LIBCAP errno reset in main() that existed only to keep errno clean
for the batch mode.

Fixes: 71bb428fe2 ("tools: bpf: add bpftool")
Signed-off-by: Yuan Chen <chenyuan@kylinos.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260824092657.1789956-2-chenyuan_fl@163.com

tools/bpf/bpftool/main.c

@@ -365,7 +365,11 @@ static int do_batch(int argc, char **argv)
 
 	if (json_output)
 		jsonw_start_array(json_wtr);
-	while (fgets(buf, sizeof(buf), fp)) {
+	for (;;) {
+		errno = 0;
+		if (!fgets(buf, sizeof(buf), fp))
+			break;
+
 		cp = strchr(buf, '#');
 		if (cp)
 			*cp = '\0';
@@ -467,16 +471,6 @@ int main(int argc, char **argv)
 
 	setlinebuf(stdout);
 
-#ifdef USE_LIBCAP
-	/* Libcap < 2.63 hooks before main() to compute the number of
-	 * capabilities of the running kernel, and doing so it calls prctl()
-	 * which may fail and set errno to non-zero.
-	 * Let's reset errno to make sure this does not interfere with the
-	 * batch mode.
-	 */
-	errno = 0;
-#endif
-
 	last_do_help = do_help;
 	pretty_output = false;
 	json_output = false;