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

[John Fastabend <john.fastabend@gmail.com>]

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@kernel.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 ;)

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.