On 1/30/24 17:01, Martin KaFai Lau wrote:
On 1/30/24 9:49 AM, Kui-Feng Lee wrote:
2. Returning a kptr from a program and treating it as releasing the
reference.
e.g. for dequeue:
struct Qdisc_ops {
/* ... */
struct sk_buff * (*dequeue)(struct Qdisc *);
};
Right now the verifier should complain on check_reference_leak() if
the struct_ops bpf prog is returning a referenced kptr.
Unlike an argument, the return type of a function does not have a
name to tag. It is the first case that a struct_ops bpf_prog returning a
We may tag the stub functions instead, right?
What is the suggestion on how to tag the return type?
I was suggesting it doesn't need to tag and it should by default require
a trusted ptr for the pointer returned by struct_ops. The pointer
argument and the return pointer of a struct_ops should be a trusted ptr.
That make sense to me. Should we also allow operators to return a null
pointer?
Is the purpose here to return a referenced pointer from a struct_ops
operator without verifier complaining?
Yes, basically need to teach the verifier the kernel will do the
reference release.
pointer. One idea is to assume it must be a trusted pointer
(PTR_TRUSTED) and the verifier should check it is indeed with
PTR_TRUSTED flag.
May be release_reference_state() can be called to assume the kernel
will release it as long as the return pointer type is PTR_TRUSTED and
the type matches the return type of the ops. Take a look at
check_return_code().
