On 4/3/24 13:52, Martin KaFai Lau wrote:
On 4/2/24 10:00 AM, Kui-Feng Lee wrote:
On 4/1/24 18:43, John Fastabend wrote:
Kui-Feng Lee wrote:
The verifier in the kernel checks the signatures of struct_ops
operators. Libbpf should not verify it in order to allow flexibility in
This description probably is not accurate. iirc, the verifier does not
check the function signature either. The verifier rejects only when the
struct_ops prog tries to access something invalid. e.g. reading a
function argument that does not exist in the running kernel.
Yes, kernel checks the behavior of programs. I will change the description.
loading different implementations of an operator with different
signatures
to try to comply with the kernel, even if the signature defined in
the BPF
programs does not match with the implementations and the kernel.
This feature enables user space applications to manage the variations
between different versions of the kernel by attempting various
implementations of an operator.
What is the utility of this? I'm missing what difference it would be
if libbpf rejected vs kernel rejecting it? For backwards compat the
kernel will fail or libbpf might throw an error and user will have to
fixup signature regardless right? Why not get the error as early as
possible.
The check described here is that libbpf compares BTF types of functions
and function pointers in struct_ops types in BPF programs, which may
differ from kernel definitions.
A scenario here is a struct_ops type that includes an operator op_A with
different versions depending on the kernel. All other fields in the
struct_ops type have the same types. The application has only one
definition for this struct_ops type, but the implementation of op_A is
done separately for each version.
The application can try variations by assigning implementations to the
op_A field until one is accepted by the kernel if libbpf doesn’t enforce
It probably would be clearer if the test actually does the retry. e.g.
Try to load a struct_ops prog which reads an extra arg that is not
supported by the running kernel and gets rejected by verifier. Then
assigns an older struct_ops prog to the skel->struct_ops...->fn and
loads successfully by the verifier.
