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

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Date: Tue, 18 Jun 2024 09:58:23 -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 Tue, Jun 18, 2024 at 5:43 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> On Mon, Jun 17, 2024 at 03:25:53PM -0700, Andrii Nakryiko wrote:
> > 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.
>
> hum, I based that on what we did for kprobe session,
> but I guess it makes sense just for bpf side:

yep, exactly, long is 64-bit only for BPF "architecture", but
internally it will be 4 bytes for 32-bit architectures, which will
potentially lead to problems. With recent kfunc vmlinux.h generation,
it's probably better to stick to explicitly sized types.

>
>   https://lore.kernel.org/bpf/CAEf4BzbyQpKvZS-mUECLRq3gyBJbsqQghOKyAbutoB76mJM8xw@mail.gmail.com/
>
> jirka
>
> >
> > 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
> > > >