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@xxxxxxxxxx>
---
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.
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.
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).
return libbpf_err_errno(fd);
}
