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From:   Roberto Sassu <roberto.sassu@huawei.com>
To:     <ast@kernel.org>, <daniel@iogearbox.net>, <andrii@kernel.org>,
        <kpsingh@kernel.org>
CC:     <bpf@vger.kernel.org>, <netdev@vger.kernel.org>,
        <linux-kselftest@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Roberto Sassu <roberto.sassu@huawei.com>
Subject: [PATCH 1/2] libbpf: Retry map access with read-only permission
Date:   Mon, 30 May 2022 10:45:13 +0200
Message-ID: <20220530084514.10170-2-roberto.sassu@huawei.com>
In-Reply-To: <20220530084514.10170-1-roberto.sassu@huawei.com>
References: <20220530084514.10170-1-roberto.sassu@huawei.com>
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Retry map access with read-only permission, if access was denied when all
permissions were requested (open_flags is set to zero). Write access might
have been denied by the bpf_map security hook.

Some operations, such as show and dump, don't need write permissions, so
there is a good chance of success with retrying.

Prefer this solution to extending the API, as otherwise a new mechanism
would need to be implemented to determine the right permissions for an
operation.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 tools/lib/bpf/bpf.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index 240186aac8e6..b4eec39021a4 100644
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c
@@ -1056,6 +1056,11 @@ int bpf_map_get_fd_by_id(__u32 id)
 	attr.map_id = id;
 
 	fd = sys_bpf_fd(BPF_MAP_GET_FD_BY_ID, &attr, sizeof(attr));
+	if (fd < 0) {
+		attr.open_flags = BPF_F_RDONLY;
+		fd = sys_bpf_fd(BPF_MAP_GET_FD_BY_ID, &attr, sizeof(attr));
+	}
+
 	return libbpf_err_errno(fd);
 }
 
-- 
2.25.1