Re: [lldb-dev] Are overlapping ELF sections problematic?
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 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 0x4000 .text > > 0x8000". 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@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 t
Re: [lldb-dev] Are overlapping ELF sections problematic?
Hi Zdenek
In an ideal world LLVM and LLDB would support a common approach for
address spaces. Currently our LLVM backend doesn't yet support address
spaces anyway, e.g. access to a variable declared as constant data:
const int my_val __attribute__((section (".crom"))) = { 42 };
is only possible from assembler code. Since current code already
features a blend of C and assembler this limitation is cumbersome
rather than catastrophic, but of course we expect to add proper
lowering for such addresses, so we're certainly interested in this
domain.
The work already done in coupling LLDB more closely to LLVM is obvious
(e.g. migrating from duplicated utility code) but the backends still
seem a little disjoint, e.g. some targets specify the same
architectural attributes such as registers in both projects. It would
be nice if a new (?) feature like address spaces was added in a way
that minimised redundancy.
Cheers,
Tom
On Mon, 3 Jun 2019 at 15:08, Zdenek Prikryl wrote:
>
> Hi Pavel, Thomas,
>
> Just a note that this topic is repeating now and then. It'd be nice to
> have a concept at least. We can go with an additional argument, or
> enhance addr_t, or enhance Address, or create a new type for it. So,
> some sort of discussion that would clarify the concept a little bit is
> welcome, I think.
>
> Best regards.
>
> On 6/3/19 1:21 PM, Pavel Labath via lldb-dev 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 0x4000 .text
> >> 0x8000". 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@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 up
[lldb-dev] June LLVM bay-area social is this Thursday!
We'll be at Tied House as usual, starting on Thursday the 6th at 7pm! If you can, help us plan and RSVP here: https://www.meetup.com/LLVM-Bay-Area-Social/events/kncsjlyzjbjb/ See everyone there! ___ lldb-dev mailing list lldb-dev@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
[lldb-dev] [Bug 41965] Compile LLDB with coverage results in "LLVM ERROR: out of memory"
https://bugs.llvm.org/show_bug.cgi?id=41965 sguel...@redhat.com changed: What|Removed |Added Resolution|--- |FIXED Status|CONFIRMED |RESOLVED --- Comment #6 from sguel...@redhat.com --- Fixed by https://reviews.llvm.org/rG4cd07dbeec9811a98efbc0ed3518e78dc38c0f53 -- You are receiving this mail because: You are the assignee for the bug.___ lldb-dev mailing list lldb-dev@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
