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[ELF] Start a new PT_LOAD if LMA region is different

Authored by MaskRay on Feb 8 2020, 10:04 PM.

Description

[ELF] Start a new PT_LOAD if LMA region is different

GNU ld has a counterintuitive lang_propagate_lma_regions rule.

// .foo's LMA region is propagated to .bar because their VMA region is the same,
// and .bar does not have an explicit output section address (addr_tree).
.foo : { *(.foo) } >RAM AT> FLASH
.bar : { *(.bar) } >RAM

// An explicit output section address disables propagation.
.foo : { *(.foo) } >RAM AT> FLASH
.bar . : { *(.bar) } >RAM

In both cases, lld thinks .foo's LMA region is propagated and
places .bar in the same PT_LOAD, so lld diverges from GNU ld w.r.t. the
second case (lma-align.test).

This patch changes Writer<ELFT>::createPhdrs to disable propagation
(start a new PT_LOAD). A user of the first case can make linker scripts
portable by explicitly specifying AT>. By contrast, there was no
workaround for the old behavior.

This change uncovers another LMA related bug in assignOffsets() where
ctx->lmaOffset = 0; was omitted. It caused a spurious "load address
range overlaps" error for at2.test

The new PT_LOAD rule is complex. For convenience, I listed the origins of some subexpressions:

  • rL323449: sec->memRegion == load->firstSec->memRegion; linkerscript/at3.test
  • D43284: load->lastSec == Out::programHeaders (don't start a new PT_LOAD after program headers); linkerscript/at4.test
  • D58892: sec != relroEnd (start a new PT_LOAD after PT_GNU_RELRO)

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D74297

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