From: David Blaikie <dblaikie@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
dwarves@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
Bill Wendling <morbo@google.com>, bpf <bpf@vger.kernel.org>,
kernel-team@fb.com, Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH dwarves] dwarf_loader: handle subprogram ret type with abstract_origin properly
Date: Thu, 1 Apr 2021 17:00:46 -0700 [thread overview]
Message-ID: <CAENS6EsiRsY1JptWJqu2wH=m4fkSiR+zD8JDD5DYke=ZnJOMrg@mail.gmail.com> (raw)
In-Reply-To: <1ef31dd8-2385-1da1-2c95-54429c895d8a@fb.com>
On Thu, Apr 1, 2021 at 4:41 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 4/1/21 3:27 PM, David Blaikie wrote:
> > On Thu, Apr 1, 2021 at 2:41 PM Yonghong Song <yhs@fb.com
> > <mailto:yhs@fb.com>> 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
> > <http://dwarfstd.org/doc/DWARF5.pdf>
> > 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)."
>
> Thanks. That means that some of my codes in the patch is
> dead code.
>
> >
> > 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)
>
> I have some codes to check the tag associated with abstract_origin
> for a subprogram must be a subprogram. Through experiment, I didn't
> see a violation, so I wonder that I can get confirmation from you
> and then I may delete that code.
>
> The test_progs failure exposed the bug, that is all.
>
> pahole cannot handle all weird usages of dwarf, so I think pahole
> is fine only to support well-formed dwarf.
Sounds good. Thanks for the context!
>
> >
> > - 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@fb.com <mailto:yhs@fb.com>>
> > > > ---
> > > > 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;
> > > >
next prev parent reply other threads:[~2021-04-02 0:00 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 21:36 [PATCH dwarves] dwarf_loader: handle subprogram ret type with abstract_origin properly Yonghong Song
2021-04-01 21:41 ` Yonghong Song
2021-04-01 22:30 ` David Blaikie
[not found] ` <CAENS6EsZ5OX9o=Cn5L1jmx8ucR9siEWbGYiYHCUWuZjLyP3E7Q@mail.gmail.com>
2021-04-01 23:40 ` Yonghong Song
2021-04-02 0:00 ` David Blaikie [this message]
2021-04-02 14:04 ` Arnaldo Carvalho de Melo
2021-04-02 14:57 ` Arnaldo Carvalho de Melo
2021-04-02 17:23 ` Yonghong Song
2021-04-02 17:42 ` Yonghong Song
2021-04-02 18:08 ` Arnaldo
2021-04-02 19:01 ` Jiri Olsa
2021-04-02 19:41 ` Yonghong Song
2021-04-03 17:08 ` Arnaldo Carvalho de Melo
2021-04-04 17:22 ` Jiri Olsa
2021-04-07 14:54 ` Yonghong Song
2021-04-07 15:37 ` Arnaldo Carvalho de Melo
2021-04-07 15:40 ` [RFT] prepping up pahole 1.21, wanna test it? was " Arnaldo Carvalho de Melo
2021-04-07 22:53 ` Yonghong Song
2021-04-09 13:52 ` Arnaldo Carvalho de Melo
2021-04-08 21:23 ` Jiri Olsa
2021-04-09 13:51 ` Arnaldo Carvalho de Melo
2021-04-07 22:51 ` Yonghong Song
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