On Fri, Jan 13, 2023 at 3:41 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Jan 12, 2023 at 2:18 PM Hao Luo <haoluo@xxxxxxxxxx> wrote:
> >
> > Hi all,
> >
> > Feature request:
> >
> > To support checking the type of a specific field directly.
> >
> > Background:
> >
> > Currently, As far as I know, CORE is able to check a field’s
> > existence, offset, size and signedness, but not the field’s type
> > directly.
> >
> > There are changes that convert a field from a scalar type to a struct
> > type, without changing the field’s name, offset or size. In that case,
> > it is currently difficult to use CORE to check such changes. For a
> > concrete example,
> >
> > Commit 94a9717b3c (“locking/rwsem: Make rwsem->owner an atomic_long_t”)
> >
> > Changed the type of rw_semaphore::owner from tast_struct * to
> > atomic_long_t. In that change, the field name, offset and size remain
> > the same. But the BPF program code used to extract the value is
> > different. For the kernel where the field is a pointer, we can write:
> >
> > sem->owner
> >
> > While in the kernel where the field is an atomic, we need to write:
> >
> > sem->owner.counter.
> >
> > It would be great to be able to check a field’s type directly.
> > Probably something like:
> >
> > #include “vmlinux.h”
> >
> > struct rw_semaphore__old {
> > struct task_struct *owner;
> > };
> >
> > struct rw_semaphore__new {
> > atomic_long_t owner;
> > };
> >
> > u64 owner;
> > if (bpf_core_field_type_is(sem->owner, struct task_struct *)) {
> > struct rw_semaphore__old *old = (struct rw_semaphore__old *)sem;
> > owner = (u64)sem->owner;
> > } else if (bpf_core_field_type_is(sem->owner, atomic_long_t)) {
> > struct rw_semaphore__new *new = (struct rw_semaphore__new *)sem;
> > owner = new->owner.counter;
> > }
> >
>
> 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
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.
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.
> [0] https://github.com/libbpf/libbpf/blob/master/src/relo_core.c#L1517-L1543
>
> > Hao
