Re: [PATCH 1/6] bpf: Allow ctx access for pointers to scalar

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Jan 22, 2020 at 02:33:32AM +0000, Yonghong Song wrote:
> 
> 
> On 1/21/20 5:51 PM, Alexei Starovoitov wrote:
> > On Tue, Jan 21, 2020 at 4:05 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >>
> >> When accessing the context we allow access to arguments with
> >> scalar type and pointer to struct. But we omit pointer to scalar
> >> type, which is the case for many functions and same case as
> >> when accessing scalar.
> >>
> >> Adding the check if the pointer is to scalar type and allow it.
> >>
> >> Acked-by: John Fastabend <john.fastabend@gmail.com>
> >> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> >> ---
> >>   kernel/bpf/btf.c | 13 ++++++++++++-
> >>   1 file changed, 12 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> >> index 832b5d7fd892..207ae554e0ce 100644
> >> --- a/kernel/bpf/btf.c
> >> +++ b/kernel/bpf/btf.c
> >> @@ -3668,7 +3668,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> >>                      const struct bpf_prog *prog,
> >>                      struct bpf_insn_access_aux *info)
> >>   {
> >> -       const struct btf_type *t = prog->aux->attach_func_proto;
> >> +       const struct btf_type *tp, *t = prog->aux->attach_func_proto;
> >>          struct bpf_prog *tgt_prog = prog->aux->linked_prog;
> >>          struct btf *btf = bpf_prog_get_target_btf(prog);
> >>          const char *tname = prog->aux->attach_func_name;
> >> @@ -3730,6 +3730,17 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> >>                   */
> >>                  return true;
> >>
> >> +       tp = btf_type_by_id(btf, t->type);
> >> +       /* skip modifiers */
> >> +       while (btf_type_is_modifier(tp))
> >> +               tp = btf_type_by_id(btf, tp->type);
> >> +
> >> +       if (btf_type_is_int(tp) || btf_type_is_enum(tp))
> >> +               /* This is a pointer scalar.
> >> +                * It is the same as scalar from the verifier safety pov.
> >> +                */
> >> +               return true;
> > 
> > The reason I didn't do it earlier is I was thinking to represent it
> > as PTR_TO_BTF_ID as well, so that corresponding u8..u64
> > access into this memory would still be possible.
> > I'm trying to analyze the situation that returning a scalar now
> > and converting to PTR_TO_BTF_ID in the future will keep progs
> > passing the verifier. Is it really the case?
> > Could you give a specific example that needs this support?
> > It will help me understand this backward compatibility concern.
> > What prog is doing with that 'u32 *' that is seen as scalar ?
> > It cannot dereference it. Use it as what?
> 
> If this is from original bcc code, it will use bpf_probe_read for 
> dereference. This is what I understand when I first reviewed this patch.
> But it will be good to get Jiri's confirmation.

it blocked me from accessing 'filename' argument when I probed
do_sys_open via trampoline in bcc, like:

	KRETFUNC_PROBE(do_sys_open)
	{
	    const char *filename = (const char *) args[1];

AFAICS the current code does not allow for trampoline arguments
being other pointers than to void or struct, the patch should
detect that the argument is pointer to scalar type and let it
pass

jirka