ELF: allow non allocated sections to go into allocated sections

[andrewrk]

Summary

For context, see https://bugs.llvm.org/show_bug.cgi?id=39862

The use case is embedded / OS programming where the kernel wants
access to its own debug info via mapped dwarf info. I have a proof of
concept of this working, using this 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 = .;
}

Thanks Rui for writing the test for me.

[ruiu]

LGTM.

Please add this as a test.

$ cat lld/test/ELF/linkerscript/merge-nonalloc.s
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
# RUN: echo "SECTIONS { .text : { *(.text) *(.nonalloc) } }" > %t.script
# RUN: ld.lld -shared -o %t.exe %t.script %t.o
# RUN: llvm-objdump -syms %t.exe | FileCheck %s

# CHECK: .text 00000000 nonalloc_start

_start:
  nop

.section .nonalloc,"",@progbits
nonalloc_start:
  .long 0xcafe

[andrewrk]

Added Rui's test case. Thank you for helping me with that. Please note that I do not have commit privileges and so someone will have to merge this on my behalf.

lld/ELF/OutputSections.cpp

if (!Live) {
     // If IS is the first section to be added to this section,
     // initialize Type, Entsize and flags from IS.
     Live = true;
     Type = IS->Type;
     Entsize = IS->Entsize;
     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 +
             ": 0x" + utohexstr(Flags));
 
     if (Type != IS->Type) {
       if (!canMergeToProgbits(Type) || !canMergeToProgbits(IS->Type))
         error("section type mismatch for " + IS->Name + "\n>>> " +

lld/test/ELF/linkerscript/merge-nonalloc.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS { .text : { *(.text) *(.nonalloc) } }" > %t.script
+# RUN: ld.lld -shared -o %t.exe %t.script %t.o
+# RUN: llvm-objdump -syms %t.exe | FileCheck %s
+
+# CHECK: .text 00000000 nonalloc_start
+
+_start:
+  nop
+
+.section .nonalloc,"",@progbits
+nonalloc_start:
+  .long 0xcafe