Re: [PATCH bpf-next 1/2] libbpf: accommodate DWARF/compiler bug with duplicated structs

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Nov 24, 2021 at 12:39:00PM +0100, Jiri Olsa wrote:
> On Thu, Nov 18, 2021 at 02:49:35PM -0800, Andrii Nakryiko wrote:
> > On Thu, Nov 18, 2021 at 6:27 AM Jiri Olsa <jolsa@redhat.com> wrote:
> > >
> > > On Wed, Nov 17, 2021 at 11:42:58AM -0800, Andrii Nakryiko wrote:
> > > > On Wed, Nov 17, 2021 at 11:41 AM Andrii Nakryiko <andrii@kernel.org> wrote:
> > > > >
> > > > > According to [0], compilers sometimes might produce duplicate DWARF
> > > > > definitions for exactly the same struct/union within the same
> > > > > compilation unit (CU). We've had similar issues with identical arrays
> > > > > and handled them with a similar workaround in 6b6e6b1d09aa ("libbpf:
> > > > > Accomodate DWARF/compiler bug with duplicated identical arrays"). Do the
> > > > > same for struct/union by ensuring that two structs/unions are exactly
> > > > > the same, down to the integer values of field referenced type IDs.
> > > >
> > > > Jiri, can you please try this in your setup and see if that handles
> > > > all situations or there are more complicated ones still. We'll need a
> > > > test for more complicated ones in that case :( Thanks.
> > >
> > > it seems to help largely, but I still see few modules (67 out of 780)
> > > that keep 'struct module' for some reason.. their struct module looks
> > > completely the same as is in vmlinux
> > 
> > Curious, what's the size of all the module BTFs now?
> 
> sorry for delay, I was waiting for s390x server
> 
> so with 'current' fedora kernel rawhide I'm getting slightly different
> total size number than before, so something has changed after the merge
> window..
> 
> however the increase with BTF enabled in modules is now from 16M to 18M,
> so the BTF data adds just about 2M, which I think we can live with
> 
> > And yes, please
> > try to narrow down what is causing the bloat this time. I think this
> 
> I'm on it

I'm seeing vmlinux BTF having just FWD record for sctp_mib struct,
while the kernel module has the full definition

kernel:

        [2798] STRUCT 'netns_sctp' size=296 vlen=46
                'sctp_statistics' type_id=2800 bits_offset=0

        [2799] FWD 'sctp_mib' fwd_kind=struct
        [2800] PTR '(anon)' type_id=2799


module before dedup:

        [78928] STRUCT 'netns_sctp' size=296 vlen=46
                'sctp_statistics' type_id=78930 bits_offset=0

        [78929] STRUCT 'sctp_mib' size=272 vlen=1
                'mibs' type_id=80518 bits_offset=0
        [78930] PTR '(anon)' type_id=78929


this field is referenced from within 'struct module' so it won't
match its kernel version and as a result extra 'struct module'
stays in the module's BTF

I'll need to check debuginfo/pahole if that FWD is correct, but
I guess it's normal that some structs might end up unwinded only
in modules and not necessarily in vmlinux

jirka