Re: [PATCH bpf-next v5] bpf: add new helper get_file_path for mapping a file descriptor to a pathname

[Wenbo Zhang <ethercflow@gmail.com>]

> nit: I'd avoid unnecessary goto end (and end label itself) by having
> two explicit returns:

>    return 0;
> error:
>    memset(...);

Agree, I'll send a new patch with explicit returns.  BTW Yonghong has
pointed this on v1 too, but I didn't get the whole point at that time,
 just made a part of change. Thank you both.

> This seems like a rather useful helper not just in tracing context. So
> if maintainers are ok with that, maybe you can follow up with patch
> that adds it in more BPF program types.

Glad to hear this, I'll keep following up with it.

Andrii Nakryiko <andrii.nakryiko@gmail.com> 于2019年11月2日周六 下午1:37写道：
>
> On Fri, Nov 1, 2019 at 5:57 AM Wenbo Zhang <ethercflow@gmail.com> wrote:
> >
> > When people want to identify which file system files are being opened,
> > read, and written to, they can use this helper with file descriptor as
> > input to achieve this goal. Other pseudo filesystems are also supported.
> >
> > This requirement is mainly discussed here:
> >
> >   https://github.com/iovisor/bcc/issues/237
> >
> > v4->v5: addressed Andrii and Daniel's feedback
> > - rename bpf_fd2path to bpf_get_file_path to be consistent with other
> > helper's names
> > - when fdget_raw fails, set ret to -EBADF instead of -EINVAL
> > - remove fdput from fdget_raw's error path
> > - use IS_ERR instead of IS_ERR_OR_NULL as d_path ether returns a pointer
> > into the buffer or an error code if the path was too long
> > - modify the normal path's return value to return copied string lengh
> > including NUL
> > - update this helper description's Return bits.
> >
> > v3->v4: addressed Daniel's feedback
> > - fix missing fdput()
> > - move fd2path from kernel/bpf/trace.c to kernel/trace/bpf_trace.c
> > - move fd2path's test code to another patch
> > - add comment to explain why use fdget_raw instead of fdget
> >
> > v2->v3: addressed Yonghong's feedback
> > - remove unnecessary LOCKDOWN_BPF_READ
> > - refactor error handling section for enhanced readability
> > - provide a test case in tools/testing/selftests/bpf
> >
> > v1->v2: addressed Daniel's feedback
> > - fix backward compatibility
> > - add this helper description
> > - fix signed-off name
> > ---
>
> See nit below, but I'm fine with the current state as well.
>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
>
> >  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> >   * function eBPF program intends to call
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index f50bf19f7a05..fc9f577e65f5 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -683,6 +683,53 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
> >         .arg1_type      = ARG_ANYTHING,
> >  };
> >
> > +BPF_CALL_3(bpf_get_file_path, char *, dst, u32, size, int, fd)
> > +{
> > +       struct fd f;
> > +       char *p;
> > +       int ret = -EBADF;
> > +
> > +       /* Use fdget_raw instead of fdget to support O_PATH, and
> > +        * fdget_raw doesn't have any sleepable code, so it's ok
> > +        * to be here.
> > +        */
> > +       f = fdget_raw(fd);
> > +       if (!f.file)
> > +               goto error;
> > +
> > +       /* d_path doesn't have any sleepable code, so it's ok to
> > +        * be here. But it uses the current macro to get fs_struct
> > +        * (current->fs). So this helper shouldn't be called in
> > +        * interrupt context.
> > +        */
> > +       p = d_path(&f.file->f_path, dst, size);
> > +       if (IS_ERR(p)) {
> > +               ret = PTR_ERR(p);
> > +               fdput(f);
> > +               goto error;
> > +       }
> > +
> > +       ret = strlen(p);
> > +       memmove(dst, p, ret);
> > +       dst[ret++] = '\0';
> > +       fdput(f);
> > +       goto end;
> > +
> > +error:
> > +       memset(dst, '0', size);
> > +end:
> > +       return ret;
>
> nit: I'd avoid unnecessary goto end (and end label itself) by having
> two explicit returns:
>
>     return 0;
> error:
>     memset(...);
>     return ret;
>
> > +}
> > +
> > +static const struct bpf_func_proto bpf_get_file_path_proto = {
> > +       .func       = bpf_get_file_path,
> > +       .gpl_only   = true,
> > +       .ret_type   = RET_INTEGER,
> > +       .arg1_type  = ARG_PTR_TO_UNINIT_MEM,
> > +       .arg2_type  = ARG_CONST_SIZE,
> > +       .arg3_type  = ARG_ANYTHING,
> > +};
> > +
> >  static const struct bpf_func_proto *
> >  tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >  {
> > @@ -735,6 +782,8 @@ tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> >  #endif
> >         case BPF_FUNC_send_signal:
> >                 return &bpf_send_signal_proto;
> > +       case BPF_FUNC_get_file_path:
> > +               return &bpf_get_file_path_proto;
>
> This seems like a rather useful helper not just in tracing context. So
> if maintainers are ok with that, maybe you can follow up with patch
> that adds it in more BPF program types.
>
> >         default:
> >                 return NULL;
> >         }
>
> [...]