linux-kernel - Re: [PATCH bpf-next v4 06/12] bpf: selftests: Fix bpf_session

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Date: Mon, 17 Jun 2024 15:25:53 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Daniel Xu <dxu@...uu.xyz>, shuah@...nel.org, ast@...nel.org, andrii@...nel.org, 
	eddyz87@...il.com, daniel@...earbox.net, quentin@...valent.com, 
	alan.maguire@...cle.com, acme@...nel.org, mykolal@...com, 
	martin.lau@...ux.dev, song@...nel.org, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com, 
	haoluo@...gle.com, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH bpf-next v4 06/12] bpf: selftests: Fix bpf_session_cookie()
 kfunc prototype

On Mon, Jun 10, 2024 at 6:04 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Sat, Jun 08, 2024 at 03:16:02PM -0600, Daniel Xu wrote:
> > The prototype defined in bpf_kfuncs.h was not in line with how the
> > actual kfunc was defined. This causes compilation errors when kfunc
> > prototypes are generated from BTF.
> >
> > Fix by aligning with actual kfunc definition.
> >
> > Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> > ---
> >  tools/testing/selftests/bpf/bpf_kfuncs.h                        | 2 +-
> >  tools/testing/selftests/bpf/progs/kprobe_multi_session_cookie.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/testing/selftests/bpf/bpf_kfuncs.h b/tools/testing/selftests/bpf/bpf_kfuncs.h
> > index be91a6919315..3b6675ab4086 100644
> > --- a/tools/testing/selftests/bpf/bpf_kfuncs.h
> > +++ b/tools/testing/selftests/bpf/bpf_kfuncs.h
> > @@ -77,5 +77,5 @@ extern int bpf_verify_pkcs7_signature(struct bpf_dynptr *data_ptr,
> >                                     struct bpf_key *trusted_keyring) __ksym;
> >
> >  extern bool bpf_session_is_return(void) __ksym __weak;
> > -extern long *bpf_session_cookie(void) __ksym __weak;
> > +extern __u64 *bpf_session_cookie(void) __ksym __weak;
>
> the original intent was to expose long instead of __u64 :-\
>

Cookies internally are always u64 (8 byte values). Marking them
internally in the kernel as long could lead to problems on 32-bit
architectures, potentially (it still needs to be 64-bit value
according to BPF contract, but we'll allocate only 4 bytes for them).

It seems better and safer to be explicit with __u64/u64 for cookies everywhere.

What am I missing?

> could we rather change the bpf_session_cookie function to return long?
> should be just return value type change
>
> thanks,
> jirka
>
>
> >  #endif
> > diff --git a/tools/testing/selftests/bpf/progs/kprobe_multi_session_cookie.c b/tools/testing/selftests/bpf/progs/kprobe_multi_session_cookie.c
> > index d49070803e22..0835b5edf685 100644
> > --- a/tools/testing/selftests/bpf/progs/kprobe_multi_session_cookie.c
> > +++ b/tools/testing/selftests/bpf/progs/kprobe_multi_session_cookie.c
> > @@ -25,7 +25,7 @@ int BPF_PROG(trigger)
> >
> >  static int check_cookie(__u64 val, __u64 *result)
> >  {
> > -     long *cookie;
> > +     __u64 *cookie;
> >
> >       if (bpf_get_current_pid_tgid() >> 32 != pid)
> >               return 1;
> > --
> > 2.44.0
> >