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

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Message-ID: <20200618220511.jrwes44dfh7v52tt@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 18 Jun 2020 15:05:11 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        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>,
        Andrii Nakryiko <andriin@...com>,
        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 01:48:06PM +0200, 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)

the patch doesn't seem to be related to d_path. Unless I'm missing something.

Please add a selftest. bpftrace example is nice, but selftest is still mandatory.

> > > 
> > >   # 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));

extra () are unnecessary.

the function needs different name. May btf_type_is_struct_by_value() ?

> > 
> > 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
> 
> jirka
> 
> > 
> > > +}
> > > +
> > >  static bool __btf_type_is_struct(const struct btf_type *t)
> > >  {
> > >  	return BTF_INFO_KIND(t->info) == BTF_KIND_STRUCT;
> > > @@ -3768,7 +3776,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> > >  	/* skip modifiers */
> > >  	while (btf_type_is_modifier(t))
> > >  		t = btf_type_by_id(btf, t->type);
> > > -	if (btf_type_is_int(t) || btf_type_is_enum(t))
> > > +	if (btf_type_is_int(t) || btf_type_is_enum(t) || btf_type_is_struct_arg(t))
> > >  		/* accessing a scalar */
> > >  		return true;

It probably needs to be x86 gated?
I don't think all archs do that for small structs.

What kind of code clang generates for bpf prog?
I don't remember what we told clang to do for struct by value.
That has to be carefully defined and tested.