linux-kernel - [PATCH bpf-next] libbpf: fix errno is overwritten after being closed.

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Date:   Fri, 23 Dec 2022 21:36:18 +0800
From:   Xin Liu <liuxin350@...wei.com>
To:     <andrii@...nel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <martin.lau@...ux.dev>, <song@...nel.org>, <yhs@...com>,
        <john.fastabend@...il.com>, <kpsingh@...nel.org>, <sdf@...gle.com>,
        <haoluo@...gle.com>, <jolsa@...nel.org>
CC:     <bpf@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yanan@...wei.com>, <wuchangye@...wei.com>,
        <xiesongyang@...wei.com>, <kongweibin2@...wei.com>,
        <liuxin350@...wei.com>, <zhangmingyi5@...wei.com>
Subject: [PATCH bpf-next] libbpf: fix errno is overwritten after being closed.

In the ensure_good_fd function, if the fcntl function succeeds but
the close function fails, ensure_good_fd returns a normal fd and
sets errno, which may cause users to misunderstand. The close
failure is not a serious problem, and the correct FD has been
handed over to the upper-layer application. Let's restore errno here.

Signed-off-by: Xin Liu <liuxin350@...wei.com>
---
 tools/lib/bpf/libbpf_internal.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 377642ff51fc..98333a6c38e9 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -543,10 +543,9 @@ static inline int ensure_good_fd(int fd)
 		fd = fcntl(fd, F_DUPFD_CLOEXEC, 3);
 		saved_errno = errno;
 		close(old_fd);
-		if (fd < 0) {
+		errno = saved_errno;
+		if (fd < 0)
 			pr_warn("failed to dup FD %d to FD > 2: %d\n", old_fd, -saved_errno);
-			errno = saved_errno;
-		}
 	}
 	return fd;
 }
-- 
2.33.0