netdev - Re: [PATCH] bpf: Allow small structs to be type of function argument

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Date:   Thu, 18 Jun 2020 16:59:16 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Jiri Olsa <jolsa@...hat.com>, Andrii Nakryiko <andriin@...com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...hat.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        KP Singh <kpsingh@...omium.org>,
        Masanori Misono <m.misono760@...il.com>
Subject: Re: [PATCH] bpf: Allow small structs to be type of function argument

On Thu, Jun 18, 2020 at 3:50 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Jiri Olsa wrote:
> > On Wed, Jun 17, 2020 at 04:20:54PM -0700, John Fastabend wrote:
> > > Jiri Olsa wrote:
> > > > This way we can have trampoline on function
> > > > that has arguments with types like:
> > > >
> > > >   kuid_t uid
> > > >   kgid_t gid
> > > >
> > > > which unwind into small structs like:
> > > >
> > > >   typedef struct {
> > > >         uid_t val;
> > > >   } kuid_t;
> > > >
> > > >   typedef struct {
> > > >         gid_t val;
> > > >   } kgid_t;
> > > >
> > > > And we can use them in bpftrace like:
> > > > (assuming d_path changes are in)
> > > >
> > > >   # bpftrace -e 'lsm:path_chown { printf("uid %d, gid %d\n", args->uid, args->gid) }'
> > > >   Attaching 1 probe...
> > > >   uid 0, gid 0
> > > >   uid 1000, gid 1000
> > > >   ...
> > > >
> > > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > > > ---
> > > >  kernel/bpf/btf.c | 12 +++++++++++-
> > > >  1 file changed, 11 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > > index 58c9af1d4808..f8fee5833684 100644
> > > > --- a/kernel/bpf/btf.c
> > > > +++ b/kernel/bpf/btf.c
> > > > @@ -362,6 +362,14 @@ static bool btf_type_is_struct(const struct btf_type *t)
> > > >   return kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION;
> > > >  }
> > > >
> > > > +/* type is struct and its size is within 8 bytes
> > > > + * and it can be value of function argument
> > > > + */
> > > > +static bool btf_type_is_struct_arg(const struct btf_type *t)
> > > > +{
> > > > + return btf_type_is_struct(t) && (t->size <= sizeof(u64));
> > >
> > > Can you comment on why sizeof(u64) here? The int types can be larger
> > > than 64 for example and don't have a similar check, maybe the should
> > > as well?
> > >
> > > Here is an example from some made up program I ran through clang and
> > > bpftool.
> > >
> > > [2] INT '__int128' size=16 bits_offset=0 nr_bits=128 encoding=SIGNED
> > >
> > > We also have btf_type_int_is_regular to decide if the int is of some
> > > "regular" size but I don't see it used in these paths.
> >
> > so this small structs are passed as scalars via function arguments,
> > so the size limit is to fit teir value into register size which holds
> > the argument
> >
> > I'm not sure how 128bit numbers are passed to function as argument,
> > but I think we can treat them separately if there's a need
> >
>
> Moving Andrii up to the TO field ;)

I've got an upgrade, thanks :)

>
> Andrii, do we also need a guard on the int type with sizeof(u64)?
> Otherwise the arg calculation might be incorrect? wdyt did I follow
> along correctly.

Yes, we probably do. I actually never used __int128 in practice, but
decided to look at what Clang does for a function accepting __int128.
Turns out it passed it in two consecutive registers. So:

__weak int bla(__int128 x) { return (int)(x + 1); }

The assembly is:

      38:       b7 01 00 00 fe ff ff ff r1 = -2
      39:       b7 02 00 00 ff ff ff ff r2 = -1
      40:       85 10 00 00 ff ff ff ff call -1
      41:       bc 01 00 00 00 00 00 00 w1 = w0

So low 64-bits go into r1, high 64-bits into r2.

Which means the 1:1 mapping between registers and input arguments
breaks with __int128, at least for target BPF. I'm too lazy to check
for x86-64, though.