On 04/06/2019 11:21, Thomas Goodfellow wrote:
Hi Pavel
I can't say what's the situation in the rest of llvm, but right now lldb
has zero test coverage for the flow you are using, so the fact that this
has worked until now was pretty much an accident.
It was a pleasant surprise that it worked at all, since flat memory
maps have become near-ubiquitous. But it's good to at least know that
the conceptual ice hasn't become any thinner through the patch, i.e.
it refines the existing state rather than reflecting a more explicit
policy change.
Yes, I didn't mean to make anything drastic with this patch. However, I
would say that independently of this patch, in the past few years, lldb
has gotten more strict in accepting features/fixes which don't have test
coverage and/or are useful in only some peculiar downstream use case
(see removal of ocaml/go/java language support, etc.)..
In the mean time, I believe you can just patch out the part which drops
the overlapping sections from the section list and get behavior which
was more-or-less identical to the old one.
I think this also requires reverting the use of the IntervalMap as the
VM address container, since that relies upon non-overlapping
intervals? That smells like a bigger fork than I would want like to
keep indefinitely alive.
It sounds like you might be able to just skip adding some (all?) of the
sections into the interval map, which should result in all of them being
created, like they used to be.
Or maybe you could fudge their "file addresses" and remap them into
non-overlapping regions at this level too. It would break lookups by
file addresses for the remapped sections, but this is something that
didn't work already when the addresses overlapped. I'm not sure what
else could be broken by this.. We already do some fudging like this for
relocatable (.o) files, which have all addresses starting at zero, so it
seems like at least something can work here.
For my own education, would you be able to send me one of your files
with these overlapping sections (or maybe just the output of "readelf
-e" or something)? I don't know much about these more exotic platforms,
so being aware things like these might be of help when doing future changes.
Incidentally, I was just made aware that this change also breaks for
thread-local sections, which can appear to have overlapping file
addresses with other sections. So I will probably be revisiting this
piece of code soon. However, right not my thinking is to simply stop
putting thread-local section address range map while simultaneously
starting to ignore them for file address lookups (as thread-local
sections need to be handled in a more complex manner anyway). This won't
help your use case much...
I believe that a long term solution here would be to introduce some
concept of address spaces to lldb. Then these queries would no longer be
ambiguous as the function FindSectionContainingFileAddress would
(presumably) take an additional address-space identifier as an argument.
I know this is what some downstream users are doing to make things like
this work. However, this is a fairly invasive change, so doing something
like this upstream would require a lot of previous discussion.
Would this also extend the GDB remote protocol, where the single flat
address space seems the only current option? (at least the common
solution in various GDB discussions of DSP targets is address muxing
of the sort we're using)
I would say "hopefully yes", but I not very familiar with these kinds of
targets.
I imagine such changes are hampered by the lack of in-tree targets
that require them, both to motivate the change and to keep it testable
(the recent "removing magic numbers assuming 8-bit bytes" discussion
in llvm-dev features the same issue). Previously Embecosm was
attempting to upstream a LLVM target for its demonstration AAP
architecture (features multiple address spaces), e.g.
http://lists.llvm.org/pipermail/llvm-dev/2017-February/109776.html .
However their public forks on GitHub only reveal GDB support rather
than LLDB, and that implementation is by an address mux.
Unfortunately the architecture I'm working with is (yet another) poor
candidate for upstreaming, since it lacks general availability, but
hopefully one of the exotic architectures lurking in the LLVM shadows
someday steps forth with a commitment to keep it alive in-tree.
Yeah, the lack of in-tree targets is one of the causes (but also a
consequence of ?) the lack of address space support. I've been following
the non-8-bit thread from a distance, and FWIW, I would be fine with
having some kind of a mock target supporting these things in lldb. I
might even prefer debugging things against a simple mock instead of some
complicated-but-real target.
The other causes are the main contributors not knowing enough about
these architectures to help drive this, and just being generally busy
with other stuff. :/
cheers,
pavel
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