Messages in this thread | | | Subject | Re: [PATCH 1/2] libbpf: Retry map access with read-only permission | From | Daniel Borkmann <> | Date | Mon, 30 May 2022 23:55:10 +0200 |
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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.
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); > } > >
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