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

Alexander Yermolovich via llvm-dev
2021-02-17T02:39:03Z

I was hoping you will see this. 🙂

Digging some more this is also affected for DW_TAG_subprogram
DWARFDie::getLowAndHighPC

Under the hood it eventually calls
DWARFUnit::getAddrOffsetSectionItem

I played/hacked around with things some more, and would like opinions on
potential fixes.
One is to add SkeletonCU to the DWO CU when it is created.
https://reviews.llvm.org/D96826

It kind of follows logic in getAddrOffsetSectionItem in a sense that if Unit is
DWO, it looks in to NormalUnits, and uses that to get the address.

Alternatively there is just adding a check in getAddrOffsetSectionItem itself.
https://reviews.llvm.org/D96827

Reason this works is that when parseDWO, by way of getNonSkeletonUnitDIE, is
invoked the AddrOffsetSection and AddrOffsetSectionBase get set correctly. The
AddrOffsetSection points to the base of .debug_addr in the binary A, and
AddrOffsetSectionBase is proper offset for this unit within it.

So when getAddrOffsetSectionItem is invoked address is looked up correctly in
the section. Kind of weird part is that Obj is A.o since we are using the DWO
unit Context.

I am just digging into these APIs, so maybe I am missing something, and there is
a better option.

Thank You
Alex
________________________________
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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