Re: [PATCH v9 bpf-next 10/14] bpf: Add d_path helper

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Aug 04, 2020 at 11:35:53PM -0700, Andrii Nakryiko wrote:
> On Sat, Aug 1, 2020 at 10:04 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Adding d_path helper function that returns full path for
> > given 'struct path' object, which needs to be the kernel
> > BTF 'path' object. The path is returned in buffer provided
> > 'buf' of size 'sz' and is zero terminated.
> >
> >   bpf_d_path(&file->f_path, buf, size);
> >
> > The helper calls directly d_path function, so there's only
> > limited set of function it can be called from. Adding just
> > very modest set for the start.
> >
> > Updating also bpf.h tools uapi header and adding 'path' to
> > bpf_helpers_doc.py script.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  include/uapi/linux/bpf.h       | 13 +++++++++
> >  kernel/trace/bpf_trace.c       | 48 ++++++++++++++++++++++++++++++++++
> >  scripts/bpf_helpers_doc.py     |  2 ++
> >  tools/include/uapi/linux/bpf.h | 13 +++++++++
> >  4 files changed, 76 insertions(+)
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index eb5e0c38eb2c..a356ea1357bf 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3389,6 +3389,18 @@ union bpf_attr {
> >   *             A non-negative value equal to or less than *size* on success,
> >   *             or a negative error in case of failure.
> >   *
> > + * int bpf_d_path(struct path *path, char *buf, u32 sz)
> 
> nit: probably would be good to do `const struct path *` here, even if
> we don't do const-ification properly in all helpers.

ok

> 
> > + *     Description
> > + *             Return full path for given 'struct path' object, which
> > + *             needs to be the kernel BTF 'path' object. The path is
> > + *             returned in buffer provided 'buf' of size 'sz' and
> 
> typo: in the provided buffer 'buf' of size ... ?

ugh, sure

> 
> > + *             is zero terminated.
> > + *
> > + *     Return
> > + *             On success, the strictly positive length of the string,
> > + *             including the trailing NUL character. On error, a negative
> > + *             value.
> > + *
> >   */
> >  #define __BPF_FUNC_MAPPER(FN)          \
> >         FN(unspec),                     \
> > @@ -3533,6 +3545,7 @@ union bpf_attr {
> >         FN(skc_to_tcp_request_sock),    \
> >         FN(skc_to_udp6_sock),           \
> >         FN(get_task_stack),             \
> > +       FN(d_path),                     \
> >         /* */
> >
> >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index cb91ef902cc4..02a76e246545 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -1098,6 +1098,52 @@ static const struct bpf_func_proto bpf_send_signal_thread_proto = {
> >         .arg1_type      = ARG_ANYTHING,
> >  };
> >
> > +BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> > +{
> > +       int len;
> > +       char *p;
> > +
> > +       if (!sz)
> > +               return -ENAMETOOLONG;
> 
> if we are modeling this after bpf_probe_read_str(), sz == 0 returns
> success and just does nothing. I don't think anyone will ever handle
> or expect this error. I'd just return 0.

ook

thanks,
jirka