From: Daniel Borkmann [mailto:daniel@iogearbox.net] Sent: Monday, May 30, 2022 11:55 PM On 5/30/22 10:45 AM, Roberto Sassu wrote:
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));- }
But then what about bpf_obj_get() API in libbpf? attr.file_flags has similar purpose as attr.open_flags in this case.
Ok, I missed it.
The other issue is that this could have upgrade implications, e.g. where an application bailed out before, it is now passing wrt bpf_map_get_fd_by_id(), but then suddenly failing during map update calls.
Good point.
Imho, it might be better to be explicit about user intent w/o the lib doing guess work upon failure cases (... or have the BPF LSM set the attr.open_flags to BPF_F_RDONLY from within the BPF prog).
Uhm, I don't like that the users should be aware of permissions assigned to maps that they don't own.
Maybe, better the original idea, request read-only permission for the list and dump operations.
Roberto
HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Li Peng, Zhong Ronghua