Re: [PATCH dwarves] dwarf_loader: handle subprogram ret type with abstract_origin properly

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On Thu, Apr 1, 2021 at 2:41 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 4/1/21 2:36 PM, Yonghong Song wrote:
> > With latest bpf-next built with clang lto (thin or full), I hit one test
> > failures:
> >    $ ./test_progs -t tcp
> >    ...
> >    libbpf: extern (func ksym) 'tcp_slow_start': func_proto [23] incompatible with kernel [115303]
> >    libbpf: failed to load object 'bpf_cubic'
> >    libbpf: failed to load BPF skeleton 'bpf_cubic': -22
> >    test_cubic:FAIL:bpf_cubic__open_and_load failed
> >    #9/2 cubic:FAIL
> >    ...
> >
> > The reason of the failure is due to bpf program 'tcp_slow_start'
> > func signature is different from vmlinux BTF. bpf program uses
> > the following signature:
> >    extern __u32 tcp_slow_start(struct tcp_sock *tp, __u32 acked);
> > which is identical to the kernel definition in linux:include/net/tcp.h:
> >    u32 tcp_slow_start(struct tcp_sock *tp, u32 acked);
> > While vmlinux BTF definition like:
> >    [115303] FUNC_PROTO '(anon)' ret_type_id=0 vlen=2
> >            'tp' type_id=39373
> >            'acked' type_id=18
> >    [115304] FUNC 'tcp_slow_start' type_id=115303 linkage=static
> > The above is dumped with `bpftool btf dump file vmlinux`.
> > You can see the ret_type_id is 0 and this caused the problem.
> >
> > Looking at dwarf, we have:
> >
> > 0x11f2ec67:   DW_TAG_subprogram
> >                  DW_AT_low_pc    (0xffffffff81ed2330)
> >                  DW_AT_high_pc   (0xffffffff81ed235c)
> >                  DW_AT_frame_base        ()
> >                  DW_AT_GNU_all_call_sites        (true)
> >                  DW_AT_abstract_origin   (0x11f2ed66 "tcp_slow_start")
> > ...
> > 0x11f2ed66:   DW_TAG_subprogram
> >                  DW_AT_name      ("tcp_slow_start")
> >                  DW_AT_decl_file ("/home/yhs/work/bpf-next/net/ipv4/tcp_cong.c")
> >                  DW_AT_decl_line (392)
> >                  DW_AT_prototyped        (true)
> >                  DW_AT_type      (0x11f130c2 "u32")
> >                  DW_AT_external  (true)
> >                  DW_AT_inline    (DW_INL_inlined)
>
> David,
>
> Could you help confirm whether DW_AT_abstract_origin at a
> DW_TAG_subprogram always points to another DW_TAG_subprogram,
> or there are possible other cases?

That's correct, so far as I understand the spec, specifically DWARFv5
3.3.8.3 says:

"The root entry for a concrete out-of-line instance of a given inlined
subroutine has the same tag as does its associated (abstract) inlined
subroutine entry (that is, tag DW_TAG_subprogram rather than
DW_TAG_inlined_subroutine)."

Though people may come up with novel uses of DWARF features. What
would happen if this constraint were violated/what's your motivation
for asking (I don't quite understand the connection between test_progs
failure description, and this question)

- David

>
> Thanks,
>
> >
> > We have a subprogram which has an abstract_origin pointing to
> > the subprogram prototype with return type. Current one pass
> > recoding cannot easily resolve this easily since
> > at the time recoding for 0x11f2ec67, the return type in
> > 0x11f2ed66 has not been resolved.
> >
> > To simplify implementation, I just added another pass to
> > go through all functions after recoding pass. This should
> > resolve the above issue.
> >
> > With this patch, among total 250999 functions in vmlinux,
> > 4821 functions needs return type adjustment from type id 0
> > to correct values. The above failed bpf selftest passed too.
> >
> > Signed-off-by: Yonghong Song <yhs@xxxxxx>
> > ---
> >   dwarf_loader.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 46 insertions(+)
> >
> > Arnaldo, this is the last known pahole bug in my hand w.r.t. clang
> > LTO. With this, all self tests are passed except ones due
> > to global function inlining, static variable promotion etc, which
> > are not related to pahole.
> >
> > diff --git a/dwarf_loader.c b/dwarf_loader.c
> > index 026d137..367ac06 100644
> > --- a/dwarf_loader.c
> > +++ b/dwarf_loader.c
> > @@ -2198,6 +2198,42 @@ out:
> >       return 0;
> >   }
> >
> > +static int cu__resolve_func_ret_types(struct cu *cu)
> > +{
> > +     struct ptr_table *pt = &cu->functions_table;
> > +     uint32_t i;
> > +
> > +     for (i = 0; i < pt->nr_entries; ++i) {
> > +             struct tag *tag = pt->entries[i];
> > +
> > +             if (tag == NULL || tag->type != 0)
> > +                     continue;
> > +
> > +             struct function *fn = tag__function(tag);
> > +             if (!fn->abstract_origin)
> > +                     continue;
> > +
> > +             struct dwarf_tag *dtag = tag->priv;
> > +             struct dwarf_tag *dfunc;
> > +             dfunc = dwarf_cu__find_tag_by_ref(cu->priv, &dtag->abstract_origin);
> > +             if (dfunc == NULL) {
> > +                     tag__print_abstract_origin_not_found(tag);
> > +                     return -1;
> > +             }
> > +
> > +             /*
> > +              * Based on what I see it should be a subprogram,
> > +              * but double check anyway to ensure I won't mess up
> > +              * now and in the future.
> > +              */
> > +             if (dfunc->tag->tag != DW_TAG_subprogram)
> > +                     continue;
> > +
> > +             tag->type = dfunc->tag->type;
> > +     }
> > +     return 0;
> > +}
> > +
> >   static int cu__recode_dwarf_types_table(struct cu *cu,
> >                                       struct ptr_table *pt,
> >                                       uint32_t i)
> > @@ -2637,6 +2673,16 @@ static int cus__merge_and_process_cu(struct cus *cus, struct conf_load *conf,
> >       /* process merged cu */
> >       if (cu__recode_dwarf_types(cu) != LSK__KEEPIT)
> >               return DWARF_CB_ABORT;
> > +
> > +     /*
> > +      * for lto build, the function return type may not be
> > +      * resolved due to the return type of a subprogram is
> > +      * encoded in another subprogram through abstract_origin
> > +      * tag. Let us visit all subprograms again to resolve this.
> > +      */
> > +     if (cu__resolve_func_ret_types(cu) != LSK__KEEPIT)
> > +             return DWARF_CB_ABORT;
> > +
> >       if (finalize_cu_immediately(cus, cu, dcu, conf)
> >           == LSK__STOP_LOADING)
> >               return DWARF_CB_ABORT;
> >



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