[lldb-dev] Are overlapping ELF sections problematic?

[Pavel Labath via lldb-dev]

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?
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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