[lldb-dev] Are overlapping ELF sections problematic?

Tue Jun 4 02:21:48 PDT 2019

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.

> 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.

> 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 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.

Cheers,
Tom

On Mon, 3 Jun 2019 at 13:19, Pavel Labath <pavel at labath.sk> wrote:
>
> On 03/06/2019 10:19, Thomas Goodfellow via lldb-dev wrote:
> > I'm working with an embedded platform that segregates memory between
> > executable code, RAM, and constant values. The three kinds occupy
> > three separate address spaces, accessed by specific instructions (e.g.
> > "load from RAM address #0" vs "load from constant ROM address #0")
> > with fairly small ranges for literal address values. So necessarily
> > all three address spaces all start at zero.
> >
> > We're using the LLVM toolchain with ELF32 files, mapping the three
> > spaces as.text, .data, and .crom sections, with a linker script
> > setting the address for all three sections to zero and so producing a
> > non-relocatable executable image (the .text section becomes a ROM for
> > an embedded device so final addresses are required). To support
> > debugging with LLDB (where the GDB server protocol presumes a single
> > flat memory space) the sections are mapped to address ranges in a
> > larger space (using the top two bits) and the debugger stub of the
> > platform then demuxes the memory accesses to the appropriate address
> > spaces).
> >
> > Until recently this was done by loading the ELF file in LLDB, e.g:
> > "target modules load --file test.elf .data 0 .crom 0x40000000 .text
> > 0x80000000". However the changes introduced through
> > https://reviews.llvm.org/D55998 removed support for overlapping
> > sections, with a remark "I don't anticipate running into this
> > situation in the real world. However, if we do run into it, and the
> > current behavior is not suitable for some reason, we can implement
> > this logic differently."
> >
> > Our immediate coping strategy was implementing the remapping in the
> > file parsing of ObjectFileELF, but this LLDB change makes us
> > apprehensive that we may start encountering similar issues elsewhere
> > in the LLVM tooling. Are ELF sections with overlapping addresses so
> > rare (or even actually invalid) that ongoing support will be fragile?
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
> >
>
> Hi Thomas,
>
> 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.
>
> The reason I chose to disallow the overlapping sections in the patch you
> quote was because it was very hard to say what will be the meaning of
> this to the upper layers of lldb. For instance, a lot things in lldb
> work with "file addresses" (that is, virtual address, as they are known
> in the file, without any remapping). This means that the overlapping
> sections become ambiguous even though you have remapped them to
> non-overlapping "load addresses" with the "target modules load" command.
> For instance, the result of a query like
> "SectionList::FindSectionContainingFileAddress(lldb::addr_t)" would
> depend on how exactly was the search algorithm implemented.
>
> 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.
>
> 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. However, I can't guarantee
> that nothing else will break in this scenario. I also wouldn't be
> opposed to making some change to this logic upstream too, if we can come
> up with some consistent story as to what exactly this means.
>
> regards,
> pl