Tombstone values in debug sections (was: Range lists, zero-length functions, linker gc)

[David Blaikie]

On Fri, Jun 19, 2020 at 1:04 PM Mark Wielaard <mark@klomp.org> wrote:
>
> Hi,
>
> On Tue, 2020-06-09 at 13:24 -0700, Fangrui Song via Elfutils-devel wrote:
> > I want to revive the thread, but focus on whether a tombstone value
> > (-1/-2) in .debug_* can cause trouble to various DWARF consumers (gdb,
> > debug related tools in elfutils and other utilities I don't know about).
> >
> > Paul Robinson has proposed that DWARF v6 should reserve a tombstone
> > value  (the value a relocation referencing a discarded symbol in a
> > .debug_* section should be resolved to)
> > http://www.dwarfstd.org/ShowIssue.php?issue=200609.1
>
> I would appreciate having a clear "not valid" marker instead of getting
> a possibly bogus (but valid) address. -1 seems a reasonable value.
> Although I have seen (and written) code that simply assumes zero is
> that value.

Yep - and zero seemed like a good one - except in debug_ranges and
debug_loc where that would produce a premature list termination
(bfd.ld gets around this by using 1 in debug_ranges) - or on
architectures for which 0 is a valid address.
if you use the zero+addend approach that gold uses (and lld did
use/maybe still does, but is going to move away from) then you
/almost/ avoid the need to special case debug_ranges and debug_loc,
until you hit a zero-length function (you can create zero-length
functions from code like "int f1() { }" or "void f2() {
__builtin_unreachable(); }") - then you get the early list termination
again
Also zero+addend might trip up in a case like: "void f1() { }
__attribute__((nodebug)) void f2() { } void f3() { }" - now f3's
starting address has a non-zero addend, so it's indistinguishable from
valid code at a very low address


> Would such an invalid address marker in an DW_AT_low_pc make the whole
> program scope under a DIE invalid? What about (addr, loc, rng) base
> addresses? Can they contain an invalid marker, does that make the whole
> table/range invalid?

That would be my intent, yes - any pointer derived from an invalid
address would be invalid. Take the f1/f2/f3 nodebug example above -
f3's starting address could be described by "invalid address + offset"
(currently DWARF has no way of describing this - well, it sort of
does, you could use an exprloc with an OP_addrx and the arithmetic
necessary to add to that - though I doubt many consumers could handle
an exprloc there - but I would like to champion that to enable reuse
of address pool entries to reduce the size of .o debug info
contributions when using Split DWARF - or just reduce the number of
relocations/.o file size when using non-split DWARF), so it'd be
important for that to be special cased in pointer arithmetic so the
tombstone value propagates through arithmetic.

> I must admit that as a DWARF consumer I am slightly worried that having
> a sanctioned "invalid marker" will cause DWARF producers to just not
> coordinate and simply assume they can always invalidate anything they
> emit.

At least in my experience (8 years or so working on LLVM's DWARF
emission) we've got a pretty strong incentive to reduce DWARF size
already - I don't think any producers are being particularly cavalier
about producing excess DWARF on the basis that it can be marked
invalid.

> Even if there could be a real solution by coordinating between
> compiler/linker who is responsible for producing the valid DWARF
> entries (especially when LTO is involved).

A lot of engineering work went into restructuring LLVM's debug info IR
representation for LTO to ensure LLVM doesn't produce DWARF for
functions deduplicated or dropped by LTO.

- Dave

>
> > Some comments about the proposal:
> >
> > > - deduplicating different functions with identical content; GNU
> > > refers
> > >   to this as ICF (Identical Code Folding);
> >
> > ICF (gold --icf={safe,all}) can cause DW_TAG_subprogram with
> > different DW_AT_name to have the same range.
>
> Cary Coutant wrote up a general Two-Level Line Number Table proposal to
> address the issue of having a single machine instruction corresponds to
> more than one source statement:
> http://wiki.dwarfstd.org/index.php?title=TwoLevelLineTables
>
> Which seems useful in these kind of situations. But I don't know the
> current status of the proposal.

This was motivated by a desire to be able to do symbolized stack
traces including inline stack frames with a smaller representation
than is currently possible in DWARF - it allows the line table itself
to describe inlining, to some degree, rather than relying on the DIE
tree (in part this was motivated by a desire to be able to symbolized
backtraces with inlining in-process when Split DWARF is used and the
.dwo/.dwp files are not available).

I don't think it extends to dealing with the case of deduplication
like this - nor addresses the possibility of two CUs having
overlapping instruction ranges. (it's semantically roughly equivalent
to the inlined_subroutines of a subprogram - not so much related to
two copies of a function being deduplicated & then being shared by
CUs)
- Dave