Subject: Re: [PATCH 1/9] bpf: Add d_path helper

From: Jiri Olsa <[email-masked]>
Date: Fri, 15 May 2020 16:59:07 +0200

On Thu, May 14, 2020 at 03:06:01PM -0700, Andrii Nakryiko wrote:
> On Wed, May 6, 2020 at 6:30 AM Jiri Olsa <[email-masked]> wrote:
> >
> > Adding d_path helper function that returns full path
> > for give 'struct path' object, which needs to be the
> > kernel BTF 'path' object.
> >
> > The helper calls directly d_path function.
> >
> > Updating also bpf.h tools uapi header and adding
> > 'path' to bpf_helpers_doc.py script.
> >
> > Signed-off-by: Jiri Olsa <[email-masked]>
> > ---
> >  include/uapi/linux/bpf.h       | 14 +++++++++++++-
> >  kernel/trace/bpf_trace.c       | 31 +++++++++++++++++++++++++++++++
> >  scripts/bpf_helpers_doc.py     |  2 ++
> >  tools/include/uapi/linux/bpf.h | 14 +++++++++++++-
> >  4 files changed, 59 insertions(+), 2 deletions(-)
> >
> 
> [...]
> 
> >
> > +BPF_CALL_3(bpf_d_path, struct path *, path, char *, buf, u32, sz)
> > +{
> > +       char *p = d_path(path, buf, sz - 1);
> > +       int len;
> > +
> > +       if (IS_ERR(p)) {
> > +               len = PTR_ERR(p);
> > +       } else {
> > +               len = strlen(p);
> > +               if (len && p != buf) {
> > +                       memmove(buf, p, len);
> > +                       buf[len] = 0;
> > +               }
> > +       }
> > +
> > +       return len;
> > +}
> > +
> > +static u32 bpf_d_path_btf_ids[3];
> 
> Using shorter than 5 element array is "unconventional", but seems like
> btf_distill_func_proto will never access elements that are not
> ARG_PTR_TO_BTF_ID, so it's fine. But than again, if we are saving
> space, why not just 1-element array? :)

right, that can be actualy just 1 element array ;-)

> 
> 
> > +static const struct bpf_func_proto bpf_d_path_proto = {
> > +       .func           = bpf_d_path,
> > +       .gpl_only       = true,
> > +       .ret_type       = RET_INTEGER,
> > +       .arg1_type      = ARG_PTR_TO_BTF_ID,
> > +       .arg2_type      = ARG_PTR_TO_MEM,
> > +       .arg3_type      = ARG_CONST_SIZE,
> > +       .btf_id         = bpf_d_path_btf_ids,
> > +};
> > +
> 
> [...]
> 
> > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> > index b3643e27e264..bc13cad27872 100644
> > --- a/tools/include/uapi/linux/bpf.h
> > +++ b/tools/include/uapi/linux/bpf.h
> > @@ -3068,6 +3068,17 @@ union bpf_attr {
> >   *             See: clock_gettime(CLOCK_BOOTTIME)
> >   *     Return
> >   *             Current *ktime*.
> > + *
> > + * int bpf_d_path(struct path *path, char *buf, u32 sz)
> > + *     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'.
> > + *
> 
> Please specify if it's always zero-terminated string (especially on truncation).

ok, will mention the zero termination in here

thanks,
jirka