On Mon, May 3, 2021 at 11:32 AM Grant Seltzer Richman
<grantseltzer@xxxxxxxxx> wrote:
>
> On Wed, Apr 28, 2021 at 5:15 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Wed, Apr 28, 2021 at 1:53 PM Grant Seltzer Richman
> > <grantseltzer@xxxxxxxxx> wrote:
> > >
> > > Hi all,
> > >
> > > I'm working on enabling CO:RE in a project I work on, tracee, and am
> > > running into the dilemma of missing macros that we previously were
> > > able to import from their various header files. I understand that
> > > macros don't make their way into BTF and therefore the generated
> > > vmlinux.h won't have them. However I can't import the various header
> > > files because of multiple-definition issues.
> >
> > Sadly, copy/pasting has been the only way so far.
> >
> > >
> > > Do people typically redefine each of these macros for their project?
> > > If so is there anything I should be careful of, such as architectural
> > > differences. Does anyone have creative ideas, even if not developed
> > > fully yet that I can possibly contribute to libbpf?
> >
> > We've discussed adding Clang built-in to detect if a specific type is
> > already defined and doing something like this in vmlinux.h:
> >
> > #if !__builtin_is_type_defined(struct task_struct)
> > struct task_struct {
> > ...
> > }
> > #endif
> >
> > And just do that for every struct, union, typedef. That would allow
> > vmlinux.h to co-exist (somewhat) with other types.
> >
> > Another alternative is to not use vmlinux.h and use just linux
> > headers, but mark necessary types with
> > __attribute__((preserve_access_index)) to make them CO-RE relocatable.
> > You can add that to existing types with the same pragma that vmlinux.h
> > uses.
>
> I'm attempting to try doing the above. I'm just replacing
> bpf_probe_read with bpf_core_read and not importing vmlinux.h, just
> all the kernel headers I need.
Yes, that will work, bpf_core_read() uses preserve_access_index
built-in to achieve the same effect.
>
> When you say "Add that to existing types with the same pragma that
> vmlinux.h uses", Should I be able to add the following to my bpf
> source file before importing my headers?
>
> ifndef BPF_NO_PRESERVE_ACCESS_INDEX
> #pragma clang attribute push (__attribute__((preserve_access_index)),
> apply_to = record)
> #endif
>
> and then pop the attribute at the bottom of the file, or after the
> header includes.
Yeah, that's the idea and that's what vmlinux.h does for all its
structs. It doesn't add __attribute__((preserve_access_index)) after
each struct/union. So I wonder why you are getting those unknown
attribute errors. Can you paste an example?
Also check that you use Clang that supports preserve_access_index, of course.
>
> I've tried this and get a whole bunch of 'unknown attribute' warnings,
> leading me to believe that I either have something installed
> incorrectly or don't understand how to use clang attributes. Do I need
> to edit the types in the actual header files?
No, the whole idea is to not touch original headers.
>
> Thank you very very much for the help!
> - Grant
> >
> > >
> > > Thanks so much,
> > > Grant Seltzer
