[llvm-dev] Extracting LocList address ranges from DWO .debug_info

Alexander Yermolovich via llvm-dev
2021-02-22T18:50:23Z

Hello David.

My apologies, let me provide some context. I am helping with BOLT binary
optimizer (soon to be upstreamed). As part of its functionality it updates debug
information to reflect the changes it had made to the binary. Moving functions
around, extracting cold blocks, ICF, etc.
Right now, it works with monolithic Debug information, but not with Fission one.

It completely re-writes debug line, ranges/aranges, and patches relevant DIEs
entries to point to new offsets within those sections. Which means finding what
current addresses are in DIE, mapping them to new addresses and from that new
offsets within sections. For debug fission it also will need to re-write
.debug_addr and update indices that point to it.

I looked at llvm-symbolizer and this seems a bit high level. So usage model is
closer to 1) I think.

Right now there is no link, but one solution would be to add it, when
getNonSkeletonUnitDIE/parseDWO is called. This reflects the code in
getAddrOffsetSection that tries to parse normal CUs current DWARFUnit is DWO. I
don't know what original intent of that code was, but as it stands, I
don't think it works because it parses none relocated skeleton CU in A.o.
Rough idea: *
https://reviews.llvm.org/D96826

Alternative, that whole code can be skipped entirely. *
https://reviews.llvm.org/D96827

This works because in parseDWO we set AddrOffsetSectionBase, and
AddrOffsetSection from .debug_addr in binary. Then in getAddrOffsetSectionItem
we have all the information to get addresses from indices. One weird part is
that DWARFDataExtractor is created with A.o file, while AddrOffsetSection is
from A binary.

The getAddrOffsetSectionItem is an important low level API. For example, it is
also used by DWARFUnit::getLowandHighPC, along with DWARFDie::getLocations,
DWARFUnit::findLocationLIstFromOffset. So, making a fix at that level, would
make other more high-level APIs work for DWO contents.

*Diffs are same ones as previously mentioned.

Alex

"
Sorry I'm not really following all these pieces.

There's two basic ways these APIs are predominantly used:

1) llvm-dwarfdump: This opens one file/context at a time, and generally
doesn't open other files - such as dwos or o/exe for skeleton. (indeed,
there's no reliable way to find a skeleton, given a dwo - only to find dwos
given skeletons)
2) llvm-symbolizer: this opens executable files (or .o files) and from
there can load dwo/dwp/dsym related files as needed

What sort of use case do you have? I guess it can/should look something
like (2) so can you use the LLVM debug info APIs in a similar manner to
llvm-symbolizer to achieve your goals?

"
________________________________
From: David Blaikie <dblaikie at gmail.com>
Sent: Monday, February 15, 2021 10:09 PM
To: Alexander Yermolovich <ayermolo at fb.com>; Pavel Labath <pavel at
labath.sk>
Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Extracting LocList address ranges from DWO .debug_info

This stuff is a bit ad-hoc at best.

I believe some of these APIs have been generalized enough to be usable
for your use-case, but it might be at a lower level - specifically I
think the loclist infrastructure is used by lldb when parsing DWARFv5.
But it might be used without some of the LLVM DWARF Unit abstractions
you're using. (those abstractions are used in llvm-dwarfdump - which
often isn't dealing with both .o and .dwo, but only dumping one of the
files & doing what it can (or sometimes dumping one file containing
both sets of sections, in which case it can do some address lookup,
etc, more conveniently))

On Fri, Feb 12, 2021 at 6:07 PM Alexander Yermolovich via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello
>
> I am wondering if this is a bug, or more likely something I am doing
wrong/using wrong APIs.
> I have binary A, and object file A.o, compiled with Clang debug fission
single mode. So .dwo sections are in the object file. Although with split mode
it would bre the same behavior.
> Relevant parts of the code:
> for (const auto &CU : DwCtx->compile_units()) {
>       auto *const DwarfUnit = CU.get();
>       if (llvm::Optional<uint64_t> DWOId = DwarfUnit->getDWOId())
{
>         auto *CUDWO =
static_cast<DWARFCompileUnit*>(DwarfUnit->getNonSkeletonUnitDIE(false).getDwarfUnit());
>         ...
>       }
>   }
>
> Later in the code I iterate over DIEs for .debug_info.dwo and call
> DIE.getLocations(dwarf::DW_AT_location);
>
> Alternatively can manually extract offset and call
> CUnit->findLoclistFromOffset(Offset);
>
> It fails because it tries to look up address using DWARFUnit in NormalUnits
that it extracts from A.o.
> Under the hood vistAsoluteLocationList is called with
getAddrOffsetSectionItem passed in.
> Since this DWARFUnit is DWO, it invokes Context.info_section_units(). Which
uses A.o to create DW_SECT_INFO and DW_SECT_EXT_TYPES.
> Then calls itself, but from the newly constructed Debug DWARFUnit. The
skeleton CU that is in A.o.
>
> Since the way it's constructed the AddrOffsetSectionBase is never set,
so getAddrOffsetSectionItem returns None. Eventually error is returned from high
level API call.
>
> I ended up doing this to get address ranges:
>           DWARFLocationExpressionsVector LocEVector;
>           auto CallBack = [&](const DWARFLocationEntry &Entry)
-> bool {
>             auto StartAddress >
BaseUnit->getAddrOffsetSectionItem(Entry.Value0);
>             if (!StartAddress) {
>               //TODO: Handle Error
>               return false;
>             }
>
LocEVector.emplace_back(DWARFLocationExpression{DWARFAddressRange{
>                 (*StartAddress).Address, (*StartAddress).Address +
Entry.Value1,
>                 Entry.SectionIndex}, Entry.Loc});
>             return true;
>           };
>
>           if(Unit->getLocationTable().visitLocationList(&Offset,
CallBack))
>             ...
>
>
> But back to original API calls. Are they just not designed to work with DWO
CUs, or am I missing something?
>
> Even if AddrOffsetSectionBase was set to 0, the address section it is
accessing is in A.o and is not relocated. One would still need to get base
address from the address from Skeleton CU to get fully resolved address ranges,
or what I did to use index to access binary .debug_addr section directly (with
appropriate AddrOffsetSectionBase).
>
> Thank You
> Alex
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