On Fri, Jan 13, 2023 at 5:14 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Fri, Jan 13, 2023 at 5:06 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > On Fri, Jan 13, 2023 at 3:41 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> > >
<...>
> > >
> > > Have you tried bpf_core_type_matches()? It seems like exactly what you
> > > are looking for? See [0] for logic of what constitutes "a match".
> > >
> >
> > It seems bpf_core_type_matches() is for the userspace code. I'm
>
> It's in the same family as bpf_type_{exists,size}() and
> bpf_field_{exists,size,offset}(). It's purely BPF-side. Please grep
> for bpf_core_type_matches() in selftests/bpf.
>
> > looking for type checking in the BPF code. We probably don't need to
> > check type equivalence, just comparing the btf_id of the field's type
> > and the btf_id of a target type may be sufficient.
>
> With the example above something like below should work:
>
> struct rw_semaphore__old {
> struct task_struct *owner;
> };
>
> struct rw_semaphore__new {
> atomic_long_t owner;
> };
>
> u64 owner;
> if (bpf_core_type_matches(struct rw_semaphore__old) /* owner is
> task_struct pointer */) {
> struct rw_semaphore__old *old = (struct rw_semaphore__old *)sem;
> owner = (u64)sem->owner;
> } else if (bpf_core_type_matches(struct rw_semaphore__old) /* owner
> field is atomic_long_t */) {
> struct rw_semaphore__new *new = (struct rw_semaphore__new *)sem;
> owner = new->owner.counter;
> }
>
> >
> > The commit 94a9717b3c (“locking/rwsem: Make rwsem->owner an
> > atomic_long_t”) is rare, but the 'owner' field is useful for tracking
> > the owner of a kernel lock.
>
> We implemented bpf_core_type_matches() to detect tracepoint changes,
> which is equivalent (if not harder) use case. Give it a try.
>
Thanks Andrii for the pointer. It's still not working. I got the
following error when loading:
libbpf: prog 'on_contention_begin': relo #1: parsing [43] struct
rw_semaphore__old + 0 failed: -22
libbpf: prog 'on_contention_begin': relo #1: failed to relocate: -22
libbpf: failed to perform CO-RE relocations: -22
I'll dig a little more next week.
