Re: [PATCH dwarves 0/3] dwarves: detect BTF kinds supported by kernel

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2023-09-13 at 15:26 +0100, Alan Maguire wrote:
> > > > When a newer pahole is run on an older kernel, it often knows about BTF
> > > > kinds that the kernel does not support, and adds them to the BTF
> > > > representation.  This is a problem because the BTF generated is then
> > > > embedded in the kernel image.  When it is later read - possibly by
> > > > a different older toolchain or by the kernel directly - it is not usable.
> > > > 
> > > > The scripts/pahole-flags.sh script enumerates the various pahole options
> > > > available associated with various versions of pahole, but in the case
> > > > of an older kernel is the set of BTF kinds the _kernel_ can handle that
> > > > is of more importance.
> > > > 
> > > > Because recent features such as BTF_KIND_ENUM64 are added by default
> > > > (and only skipped if --skip_encoding_btf_* is set), BTF will be
> > > > created with these newer kinds that the older kernel cannot read.
> > > > This can be (and has been) fixed by stable-backporting --skip options,
> > > > but this is cumbersome and would have to be done every time a new BTF kind
> > > > is introduced.
> > > > 
> > > > So this series attempts to detect the BTF kinds supported by the
> > > > kernel/modules so that this can inform BTF encoding for older
> > > > kernels.  We look for BTF_KIND_MAX - either as an enumerated value
> > > > in vmlinux DWARF (patch 1) or as an enumerated value in base vmlinux
> > > > BTF (patch 3).  Knowing this prior to encoding BTF allows us to specify
> > > > skip_encoding options to avoid having BTF with kinds the kernel itself
> > > > will not understand.
> > > > 
> > > > The aim is to minimize pain for older stable kernels when new BTF
> > > > kinds are introduced.  Kind encoding [1] can solve the parsing problem
> > > > with BTF, but this approach is intended to ensure generated BTF is
> > > > usable when newer pahole runs on older kernels.
> > > > 
> > > > This approach requires BTF kinds to be defined via an enumerated type,
> > > > which happened for 5.16 and later.  Older kernels than this used #defines
> > > > so the approach will only work for 5.16 stable kernels and later currently.
> > > > 
> > > > With this change in hand, adding new BTF kinds becomes a bit simpler,
> > > > at least for the user of pahole.  All that needs to be done is to add
> > > > internal "skip_new_kind" booleans to struct conf_load and set them
> > > > in dwarves__set_btf_kind_max() if the detected maximum kind is less
> > > > than the kind in question - in other words, if the kernel does not know
> > > > about that kind.  In that case, we will not use it in encoding.
> > > > 
> > > > The approach was tested on Linux 5.16 as released, i.e. prior to the
> > > > backports adding --skip_encoding logic, and the BTF generated did not
> > > > contain kinds > BTF_KIND_MAX for the kernel (corresponding to
> > > > BTF_KIND_DECL_TAG in that case).

Hi Alan,

I tested this patch (by tweaking BTF_KIND_MAX value and comparing generated BTF)
and it seems to work as intended. Left a few nitpicks.

Thanks,
Eduard


> > > > 
> > > > Changes since RFC [2]:
> > > >  - added --skip_autodetect_btf_kind_max to disable kind autodetection
> > > >    (Jiri, patch 2)
> > > > 
> > > > [1] https://lore.kernel.org/bpf/20230616171728.530116-1-alan.maguire@oracle.com/
> > > > [2] https://lore.kernel.org/bpf/20230720201443.224040-1-alan.maguire@oracle.com/
> > > > 
> > > > Alan Maguire (3):
> > > >   dwarves: auto-detect maximum kind supported by vmlinux
> > > >   pahole: add --skip_autodetect_btf_kind_max to disable kind autodetect
> > > >   btf_encoder: learn BTF_KIND_MAX value from base BTF when generating
> > > >     split BTF
> > > > 
> > > >  btf_encoder.c      | 37 +++++++++++++++++++++++++++++++++
> > > >  btf_encoder.h      |  2 ++
> > > >  dwarf_loader.c     | 52 ++++++++++++++++++++++++++++++++++++++++++++++
> > > >  dwarves.h          |  3 +++
> > > >  man-pages/pahole.1 |  4 ++++
> > > >  pahole.c           | 10 +++++++++
> > > >  6 files changed, 108 insertions(+)
> > > >