[llvm-dev] workaround to force LLD to make dwarf info sections mappable/loadable?

[Andrew Kelley via llvm-dev]

On 12/3/18 5:30 PM, Andrew Kelley via llvm-dev wrote:
> One more thing - this project as it currently exists is a proof of
> concept of how a bare bones embedded system executable running on bare
> metal can still have the same advanced debugging capabilities as when
> using, e.g. Linux. If the debug info can be directly mapped, and thus
> eliminate a dependency on a file system or on a special bootloader, it's
> a great demo.

Here's a working proof of concept.

Source:
https://github.com/andrewrk/clashos/tree/bf8e57ac220715d0698ab910d337ea590c4b4e33

Screenshot: https://i.imgur.com/8C1tYys.png

Copy pasted text output:
[nix-shell:~/dev/clashos]$ zig build qemu
ClashOS 0.0

!KERNEL PANIC!
integer overflow
/home/andy/dev/clashos/src/serial.zig:99:7: 0x1b10 in ??? (clashos)
    x += 1;
      ^
/home/andy/dev/clashos/src/main.zig:58:16: 0x1110 in ??? (clashos)
    serial.boom();
               ^
/home/andy/dev/clashos/src/main.zig:67:18: 0xecc in ??? (clashos)
    some_function();
                 ^
???:?:?: 0x1c in ??? (???)

linker script snippet:

    .rodata : ALIGN(4K) {
        *(.rodata)
        __debug_info_start = .;
        KEEP(*(.debug_info))
        __debug_info_end = .;
        __debug_abbrev_start = .;
        KEEP(*(.debug_abbrev))
        __debug_abbrev_end = .;
        __debug_str_start = .;
        KEEP(*(.debug_str))
        __debug_str_end = .;
        __debug_line_start = .;
        KEEP(*(.debug_line))
        __debug_line_end = .;
        __debug_ranges_start = .;
        KEEP(*(.debug_ranges))
        __debug_ranges_end = .;
    }

LLD patch (submitted upstream at https://reviews.llvm.org/D55276):

--- a/deps/lld/ELF/OutputSections.cpp
+++ b/deps/lld/ELF/OutputSections.cpp
@@ -95,7 +95,7 @@ void OutputSection::addSection(InputSection *IS) {
     Flags = IS->Flags;
   } else {
     // Otherwise, check if new type or flags are compatible with
existing ones.
-    unsigned Mask = SHF_ALLOC | SHF_TLS | SHF_LINK_ORDER;
+    unsigned Mask = SHF_TLS | SHF_LINK_ORDER;
     if ((Flags & Mask) != (IS->Flags & Mask))
       error("incompatible section flags for " + Name + "\n>>> " +
toString(IS) +
             ": 0x" + utohexstr(IS->Flags) + "\n>>> output section " +
Name +


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