[LLD] Crash when relocations proceed relocated section in Orphan Sections Initialization

[mykouHW]

When the relocation section is placed before the relocated section and the relocated section is not defined in the linker script, an error will occur during the linking process.

Issue Cause:
In a.ro, .rela.text precedes its relocated InputSection .text. addOrphanSections doesn't handle this scenario.
When it processes .rela.text, in the called getOutputSectionName, rel->getOutputSection() is nullptr (input .text doesn't yet have a parent output section), leading to an assertion failure.

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: None (mykouHW)

When the relocation section is placed before the relocated section and the relocated section is not defined in the linker script, an error will occur during the linking process.

Issue Cause:
In the judgment logic, addOrphanSections assumes that the RelocatedSection must be processed before the RelocationSection. Under this assumption, the OutputSection for the RelocatedSection has already been constructed, and the parent relationship associated with the InputSectionBase has been established.

If the RelocationSection is processed before the RelocatedSection, this assumption is violated. As a result, the condition rel->parent evaluates to null, causing add(relIS) to not execute. This skips the registration and construction process of the RelocatedSection, since its createOutputSection and recordSection methods have not yet been called at this point.

However, during the construction and registration of the RelocationSection in the addInputSec function, the RelocatedSection is accessed. Since the RelocatedSection has not been constructed yet, attempting to access it results in a null pointer error.

[MaskRay]

Test case

# REQUIRES: x86
# RUN: rm -rf %t && split-file %s %t && cd %t

# RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o
## In a.ro, .rela.text precedes its relocated section.
# RUN: ld.lld -r a.o -T 1.lds -o a.ro
# RUN: llvm-readelf -S a.ro | FileCheck %s
# CHECK:       .rela.text    RELA
# CHECK-NEXT:  .text         PROGBITS

## Regression test for #156354 , where we added an orphan RELA section before its relocated section.
# RUN: ld.lld -r a.ro -o a1.ro
# RUN: llvm-readelf -S a1.ro | FileCheck %s --check-prefix=CHECK1
# CHECK1:       .text         PROGBITS
# CHECK1-NEXT:  .rela.text    RELA

#--- a.s
.globl foo
foo:
  call foo

#--- 1.lds
SECTIONS {
  .rela.text 0 : { *(.rela.text) }
  .text      0 : { *(.text) }
}

Replacing # RUN: ld.lld -r a.ro -o a1.ro with # RUN: ld.lld --emit-relocs a.ro -o a1 can also reproduce the issue.

The description can be simplified. RelocatedSection is not a variable name, so I'd avoid naming it.

In a.ro, .rela.text precedes its relocated InputSection .text. addOrphanSections doesn't handle this scenario.
When it processes .rela.text, in the called getOutputSectionName, rel->getOutputSection() is nullptr (input .text doesn't yet have a parent output section), leading to an assertion failure.