[PATCH bpf-next v6 1/2] 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: 71bb428fe2c1 ("tools: bpf: add bpftool")
Signed-off-by: Yuan Chen <[email protected]>
---
v6: clear errno before each fgets() call instead of tracking the
    too-long-line case with an explicit flag and checking ferror(), as
    suggested by Andrii Nakryiko

 tools/bpf/bpftool/main.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
index c91e1a6e1a1e..7a0c214f08a0 100644
--- a/tools/bpf/bpftool/main.c
+++ b/tools/bpf/bpftool/main.c
@@ -365,7 +365,11 @@
 
 	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 @@
 
 	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;
-- 
2.43.0