On Mon, Oct 5, 2020 at 6:04 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 10/5/20 6:41 AM, Luigi Rizzo wrote:
> > bpf_program__set_attach_target() will always fail with fd=0 (attach to a
> > kernel symbol) because obj->btf_vmlinux is NULL and there is no way to
> > set it.
> >
> > Fix this by explicitly calling libbpf_find_kernel_btf() in the function.
> >
> > Signed-off-by: Luigi Rizzo <lrizzo@xxxxxxxxxx>
> > ---
> > tools/lib/bpf/libbpf.c | 9 ++++++---
> > 1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index a4f55f8a460d..3a63db86666f 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -10352,10 +10352,13 @@ int bpf_program__set_attach_target(struct bpf_program *prog,
> > if (attach_prog_fd)
> > btf_id = libbpf_find_prog_btf_id(attach_func_name,
> > attach_prog_fd);
> > - else
> > - btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
> > - attach_func_name,
> > + else {
> > + struct btf *btf = libbpf_find_kernel_btf();
> > +
> > + btf_id = __find_vmlinux_btf_id(btf, attach_func_name,
> > prog->expected_attach_type);
> > + btf__free(btf);
> > + }
>
> Doesn't feel this is right fix. In libbpf, we have API function
> libbpf_find_vmlinux_btf_id
> just doing the above.
nice, that makes the fix even simpler, will post an update:
- btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
+ btf_id = libbpf_find_vmlinux_btf_id(attach_func_name,
>
> Also, could you point out why btf_vmlinux is NULL? Is this related
> to fd = 0? We probably
> should fix the problem there since what if other API functions
> like bpf_program__set_attach_target() wants btf_vmlinux. We should
> not duplicate this logic to other API functions.
As I mentioned in a previous post, I don't know how btf_vmlinux is
meant to be used. To me it looks more like temporary storage for use
in bpf_object__load_xattr() than a property of bpf_object
(in that case it should be released in bpf_object__close() ).
I'd probably fix the current bug now, and wait for the original
author to communicate the intent and decide on followup changes.
thanks
luigi
