Re: [PATCH bpf-next v1 2/3] bpf: btf: Ensure natural alignment of .BTF_ids section

[Jiri Olsa <olsajiri@gmail.com>]

On Sat, Sep 21, 2024 at 02:46:17AM -0700, Eduard Zingerman wrote:
> On Fri, 2024-09-20 at 00:49 -0700, Tony Ambardar wrote:
> > While building of vmlinux employs a linker script to align the .BTF_ids
> > section to 4 bytes, other usage leaves .BTF_ids unaligned and may lead to
> > problems (e.g. [1]). Post-processing and libelf-based endian translation by
> > resolve_btfids may also potentially suffer from misalignment.
> > 
> > Update encoding macros in btf_ids.h to always align BTF ID data to 4 bytes.
> > 
> > [1]: 3effc06a4dde ("selftests/bpf: Fix alignment of .BTF_ids")
> > 
> > Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
> > ---
> >  include/linux/btf_ids.h | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> > index c0e3e1426a82..c10b163dc340 100644
> > --- a/include/linux/btf_ids.h
> > +++ b/include/linux/btf_ids.h
> > @@ -89,6 +89,7 @@ word							\
> >  #define __BTF_ID_LIST(name, scope)			\
> >  asm(							\
> >  ".pushsection " BTF_IDS_SECTION ",\"a\";       \n"	\
> > +".balign 4, 0;                                 \n"	\
> >  "." #scope " " #name ";                        \n"	\
> >  #name ":;                                      \n"	\
> >  ".popsection;                                  \n");
> 
> This forces all id list symbols to be aligned on 4 bytes.
> Should the same be done for __BTF_SET_START?

it seems all the set macros use __BTF_ID_LIST, so it should be taken
care of by that

> 
> Also, is it guaranteed that all btf ids are organized in lists and sets?
> Grepping through the code it seems they are, but it looks like resolve_btfids
> does not really enforce this, simply looking for symbols matching a special name
> __BTF_ID__<type>__<symbol>[__<id>] .

yes, you need the BTF_ID to be part of list or set to be able to access it

resolve_btfids does not enforce some loose BTF_ID definition without list/set,
but that does not seem to be a problem

thanks,
jirka