Re: [PATCH v2 dwarves 1/5] dwarves: help dwarf loader spot functions with optimized-out parameters

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Feb 01, 2023 at 08:49:07AM -0800, Alexei Starovoitov escreveu:
> On Wed, Feb 1, 2023 at 7:19 AM David Vernet <void@manifault.com> wrote:
> >
> > On Wed, Feb 01, 2023 at 12:02:07PM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Wed, Feb 01, 2023 at 01:59:30PM +0000, Alan Maguire escreveu:
> > > > On 01/02/2023 03:02, David Vernet wrote:
> > > > > On Tue, Jan 31, 2023 at 04:14:13PM -0800, Alexei Starovoitov wrote:
> > > > >> On Tue, Jan 31, 2023 at 3:59 PM David Vernet <void@manifault.com> wrote:
> > > > >>>
> > > > >>> On Tue, Jan 31, 2023 at 11:45:29PM +0000, Alan Maguire wrote:
> > > > >>>> On 31/01/2023 18:16, Alexei Starovoitov wrote:
> > > > >>>>> On Tue, Jan 31, 2023 at 9:43 AM Alexei Starovoitov
> > > > >>>>> <alexei.starovoitov@gmail.com> wrote:
> > > > >>>>>>
> > > > >>>>>> On Tue, Jan 31, 2023 at 4:14 AM Alan Maguire <alan.maguire@oracle.com> wrote:
> > > > >>>>>>>
> > > > >>>>>>> On 31/01/2023 01:04, Arnaldo Carvalho de Melo wrote:
> > > > >>>>>>>> Em Mon, Jan 30, 2023 at 09:25:17PM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > >>>>>>>>> Em Mon, Jan 30, 2023 at 10:37:56PM +0000, Alan Maguire escreveu:
> > > > >>>>>>>>>> On 30/01/2023 20:23, Arnaldo Carvalho de Melo wrote:
> > > > >>>>>>>>>>> Em Mon, Jan 30, 2023 at 05:10:51PM -0300, Arnaldo Carvalho de Melo escreveu:
> > > > >>>>>>>>>>>> +++ b/dwarves.h
> > > > >>>>>>>>>>>> @@ -262,6 +262,7 @@ struct cu {
> > > > >>>>>>>>>>>>   uint8_t          has_addr_info:1;
> > > > >>>>>>>>>>>>   uint8_t          uses_global_strings:1;
> > > > >>>>>>>>>>>>   uint8_t          little_endian:1;
> > > > >>>>>>>>>>>> + uint8_t          nr_register_params;
> > > > >>>>>>>>>>>>   uint16_t         language;
> > > > >>>>>>>>>>>>   unsigned long    nr_inline_expansions;
> > > > >>>>>>>>>>>>   size_t           size_inline_expansions;
> > > > >>>>>>>>>>>
> > > > >>>>>>>>>
> > > > >>>>>>>>>> Thanks for this, never thought of cross-builds to be honest!
> > > > >>>>>>>>>
> > > > >>>>>>>>>> Tested just now on x86_64 and aarch64 at my end, just ran
> > > > >>>>>>>>>> into one small thing on one system; turns out EM_RISCV isn't
> > > > >>>>>>>>>> defined if using a very old elf.h; below works around this
> > > > >>>>>>>>>> (dwarves otherwise builds fine on this system).
> > > > >>>>>>>>>
> > > > >>>>>>>>> Ok, will add it and will test with containers for older distros too.
> > > > >>>>>>>>
> > > > >>>>>>>> Its on the 'next' branch, so that it gets tested in the libbpf github
> > > > >>>>>>>> repo at:
> > > > >>>>>>>>
> > > > >>>>>>>> https://github.com/libbpf/libbpf/actions/workflows/pahole.yml
> > > > >>>>>>>>
> > > > >>>>>>>> It failed yesterday and today due to problems with the installation of
> > > > >>>>>>>> llvm, probably tomorrow it'll be back working as I saw some
> > > > >>>>>>>> notifications floating by.
> > > > >>>>>>>>
> > > > >>>>>>>> I added the conditional EM_RISCV definition as well as removed the dup
> > > > >>>>>>>> iterator that Jiri noticed.
> > > > >>>>>>>>
> > > > >>>>>>>
> > > > >>>>>>> Thanks again Arnaldo! I've hit an issue with this series in
> > > > >>>>>>> BTF encoding of kfuncs; specifically we see some kfuncs missing
> > > > >>>>>>> from the BTF representation, and as a result:
> > > > >>>>>>>
> > > > >>>>>>> WARN: resolve_btfids: unresolved symbol bpf_xdp_metadata_rx_hash
> > > > >>>>>>> WARN: resolve_btfids: unresolved symbol bpf_task_kptr_get
> > > > >>>>>>> WARN: resolve_btfids: unresolved symbol bpf_ct_change_status
> > > > >>>>>>>
> > > > >>>>>>> Not sure why I didn't notice this previously.
> > > > >>>>>>>
> > > > >>>>>>> The problem is the DWARF - and therefore BTF - generated for a function like
> > > > >>>>>>>
> > > > >>>>>>> int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash)
> > > > >>>>>>> {
> > > > >>>>>>>         return -EOPNOTSUPP;
> > > > >>>>>>> }
> > > > >>>>>>>
> > > > >>>>>>> looks like this:
> > > > >>>>>>>
> > > > >>>>>>>    <8af83a2>   DW_AT_external    : 1
> > > > >>>>>>>     <8af83a2>   DW_AT_name        : (indirect string, offset: 0x358bdc): bpf_xdp_metadata_rx_hash
> > > > >>>>>>>     <8af83a6>   DW_AT_decl_file   : 5
> > > > >>>>>>>     <8af83a7>   DW_AT_decl_line   : 737
> > > > >>>>>>>     <8af83a9>   DW_AT_decl_column : 5
> > > > >>>>>>>     <8af83aa>   DW_AT_prototyped  : 1
> > > > >>>>>>>     <8af83aa>   DW_AT_type        : <0x8ad8547>
> > > > >>>>>>>     <8af83ae>   DW_AT_sibling     : <0x8af83cd>
> > > > >>>>>>>  <2><8af83b2>: Abbrev Number: 38 (DW_TAG_formal_parameter)
> > > > >>>>>>>     <8af83b3>   DW_AT_name        : ctx
> > > > >>>>>>>     <8af83b7>   DW_AT_decl_file   : 5
> > > > >>>>>>>     <8af83b8>   DW_AT_decl_line   : 737
> > > > >>>>>>>     <8af83ba>   DW_AT_decl_column : 51
> > > > >>>>>>>     <8af83bb>   DW_AT_type        : <0x8af421d>
> > > > >>>>>>>  <2><8af83bf>: Abbrev Number: 35 (DW_TAG_formal_parameter)
> > > > >>>>>>>     <8af83c0>   DW_AT_name        : (indirect string, offset: 0x27f6a2): hash
> > > > >>>>>>>     <8af83c4>   DW_AT_decl_file   : 5
> > > > >>>>>>>     <8af83c5>   DW_AT_decl_line   : 737
> > > > >>>>>>>     <8af83c7>   DW_AT_decl_column : 61
> > > > >>>>>>>     <8af83c8>   DW_AT_type        : <0x8adc424>
> > > > >>>>>>>
> > > > >>>>>>> ...and because there are no further abstract origin references
> > > > >>>>>>> with location information either, we classify it as lacking
> > > > >>>>>>> locations for (some of) the parameters, and as a result
> > > > >>>>>>> we skip BTF encoding. We can work around that by doing this:
> > > > >>>>>>>
> > > > >>>>>>> __attribute__ ((optimize("O0"))) int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash)
> > > > >>>>>>
> > > > >>>>>> replied in the other thread. This attr is broken and discouraged by gcc.
> > > > >>>>>>
> > > > >>>>>> For kfuncs where aregs are unused, please try __used and __may_unused
> > > > >>>>>> applied to arguments.
> > > > >>>>>> If that won't work, please add barrier_var(arg) to the body of kfunc
> > > > >>>>>> the way we do in selftests.
> > > > >>>>>
> > > > >>>>> There is also
> > > > >>>>> # define __visible __attribute__((__externally_visible__))
> > > > >>>>> that probably fits the best here.
> > > > >>>>>
> > > > >>>>
> > > > >>>> testing thus for seems to show that for x86_64, David's series
> > > > >>>> (using __used noinline in the BPF_KFUNC() wrapper and extended
> > > > >>>> to cover recently-arrived kfuncs like cpumask) is sufficient
> > > > >>>> to avoid resolve_btfids warnings.
> > > > >>>
> > > > >>> Nice. Alexei -- lmk how you want to proceed. I think using the
> > > > >>> __bpf_kfunc macro in the short term (with __used and noinline) is
> > > > >>> probably the least controversial way to unblock this, but am open to
> > > > >>> other suggestions.
> > > > >>
> > > > >> Sounds good to me, but sounds like __used and noinline are not
> > > > >> enough to address the issues on aarch64?
> > > > >
> > > > > Indeed, we'll have to make sure that's also addressed. Alan -- did you
> > > > > try Alexei's suggestion to use __weak? Does that fix the issue for
> > > > > aarch64? I'm still confused as to why it's only complaining for a small
> > > > > subset of kfuncs, which include those that have external linkage.
> > > > >
> > > >
> > > > I finally got to the bottom of the aarch64 issues; there was a 1-line bug
> > > > in the changes I made to the DWARF handling code which leads to BTF generation;
> > > > it was excluding a bunch of functions incorrectly, marking them as optimized out.
> > > > The fix is:
> > > >
> > > > diff --git a/dwarf_loader.c b/dwarf_loader.c
> > > > index dba2d37..8364e17 100644
> > > > --- a/dwarf_loader.c
> > > > +++ b/dwarf_loader.c
> > > > @@ -1074,7 +1074,7 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
> > > >                         Dwarf_Op *expr = loc.expr;
> > > >
> > > >                         switch (expr->atom) {
> > > > -                       case DW_OP_reg1 ... DW_OP_reg31:
> > > > +                       case DW_OP_reg0 ... DW_OP_reg31:
> > > >                         case DW_OP_breg0 ... DW_OP_breg31:
> > > >                                 break;
> > > >                         default:
> > > >
> > > > ..and because reg0 is the first parameter for aarch64, we were
> > > > incorrectly landing in the "default:" of the switch statement
> > > > and marking a bunch of functions as optimized out
> > > > because we thought the first argument was. Sorry about this,
> > > > and thanks for all the suggestions!
> >
> > Great, so inline and __used with __bpf_kfunc sounds like the way forward
> > in the short term. Arnaldo / Alexei -- how do you want to resolve the
> > dependency here? Going through bpf-next is probably a good idea so that
> > we get proper CI coverage, and any kfuncs added to bpf-next after this
> > can use the macro. Does that work for you?
> 
> It feels fixed pahole should be done under some flag
> otherwise when people update the pahole the existing and older
> kernels might stop building with warns:
> WARN: resolve_btfids: unresolved symbol bpf_xdp_metadata_rx_hash
> WARN: resolve_btfids: unresolved symbol bpf_task_kptr_get
> ...
> 
> Arnaldo, could you check what warns do you see with this fixed pahole
> in bpf tree ?

Sure.

> If there are only few warns then we can manually add __used noinline
> to these places, push to bpf tree and push to stable.
> 
> Then in bpf-next we can clean up everything with __bpf_kfunc.

-- 

- Arnaldo