[PATCH bpf-next] libbpf: handle ENOTSUPP errno in libbpf_strerror()

From: Pedro Tammela <pctammela@gmail.com>
To: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	netdev@vger.kernel.org, bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org
Cc: Pedro Tammela <pctammela@mojatatu.com>
Subject: [PATCH bpf-next] libbpf: handle ENOTSUPP errno in libbpf_strerror()
Date: Sat, 24 Apr 2021 19:16:48 -0300	[thread overview]
Message-ID: <20210424221648.809525-1-pctammela@mojatatu.com> (raw)

The 'bpf()' syscall is leaking the ENOTSUPP errno that is internal to the kernel[1].
More recent code is already using the correct EOPNOTSUPP, but changing
older return codes is not possible due to dependency concerns, so handle ENOTSUPP
in libbpf_strerror().

[1] https://lore.kernel.org/netdev/20200511165319.2251678-1-kuba@kernel.org/

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
---
 tools/lib/bpf/libbpf_errno.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/lib/bpf/libbpf_errno.c b/tools/lib/bpf/libbpf_errno.c
index 0afb51f7a919..7de8bbc34a37 100644
--- a/tools/lib/bpf/libbpf_errno.c
+++ b/tools/lib/bpf/libbpf_errno.c
@@ -13,6 +13,9 @@
 
 #include "libbpf.h"
 
+/* This errno is internal to the kernel but leaks in the bpf() syscall. */
+#define ENOTSUPP 524
+
 /* make sure libbpf doesn't use kernel-only integer typedefs */
 #pragma GCC poison u8 u16 u32 u64 s8 s16 s32 s64
 
@@ -43,6 +46,12 @@ int libbpf_strerror(int err, char *buf, size_t size)
 
 	err = err > 0 ? err : -err;
 
+	if (err == ENOTSUPP) {
+		snprintf(buf, size, "Operation not supported");
+		buf[size - 1] = '\0';
+		return 0;
+	}
+
 	if (err < __LIBBPF_ERRNO__START) {
 		int ret;
 
-- 
2.25.1

