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[ELF] Refine LMA offset propagation rule in D76995

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If neither AT(lma) nor AT>lma_region is specified,
D76995 keeps `lmaOffset` (LMA - VMA) if the previous section is in the
default LMA region.

This patch additionally checks that the two sections are in the same
memory region.

Add a test case derived from https://bugs.llvm.org/show_bug.cgi?id=45313

  .mdata : AT(0xfb01000) { *(.data); } > TCM
  // It is odd to make .bss inherit lmaOffset, because the two sections
  // are in different memory regions.
  .bss : { *(.bss) } > DDR

With this patch, section VMA/LMA match GNU ld. Note, GNU ld supports
out-of-order (w.r.t sh_offset) sections and places .text and .bss in the
same PT_LOAD. We don't have that behavior.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D81986
This commit is contained in:
Fangrui Song 2020-06-19 09:07:48 -07:00
parent 4171f80d54
commit 8ffb2097cc
3 changed files with 39 additions and 7 deletions
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10
lld/ELF/LinkerScript.cpp
View File

@ -851,7 +851,8 @@ void LinkerScript::assignOffsets(OutputSection *sec) {
if (!(sec->flags & SHF_ALLOC))
dot = 0;
bool prevLMARegionIsDefault = ctx->lmaRegion == nullptr;
const bool sameMemRegion = ctx->memRegion == sec->memRegion;
const bool prevLMARegionIsDefault = ctx->lmaRegion == nullptr;
ctx->memRegion = sec->memRegion;
ctx->lmaRegion = sec->lmaRegion;
if (ctx->memRegion)
@ -872,14 +873,15 @@ void LinkerScript::assignOffsets(OutputSection *sec) {
// ctx->lmaOffset is LMA minus VMA. If LMA is explicitly specified via AT() or
// AT>, recompute ctx->lmaOffset; otherwise, if both previous/current LMA
// region is the default, reuse previous lmaOffset; otherwise, reset lmaOffset
// to 0. This emulates heuristics described in
// region is the default, and the two sections are in the same memory region,
// reuse previous lmaOffset; otherwise, reset lmaOffset to 0. This emulates
// heuristics described in
// https://sourceware.org/binutils/docs/ld/Output-Section-LMA.html
if (sec->lmaExpr)
ctx->lmaOffset = sec->lmaExpr().getValue() - dot;
else if (MemoryRegion *mr = sec->lmaRegion)
ctx->lmaOffset = alignTo(mr->curPos, sec->alignment) - dot;
else if (!prevLMARegionIsDefault)
else if (!sameMemRegion || !prevLMARegionIsDefault)
ctx->lmaOffset = 0;
// Propagate ctx->lmaOffset to the first "non-header" section.

6
lld/docs/ELF/linker_script.rst
View File

@ -71,7 +71,7 @@ The two keywords cannot be specified at the same time.
If neither ``AT(lma)`` nor ``AT>lma_region`` is specified:
- If the previous section is also in the default LMA region, the difference
between the LMA and the VMA is computed to be the same as the previous
difference.
- If the previous section is also in the default LMA region, and the two
section have the same memory regions, the difference between the LMA and the
VMA is computed to be the same as the previous difference.
- Otherwise, the LMA is set to the VMA.

30
lld/test/ELF/linkerscript/lma-offset2.s Normal file
View File

@ -0,0 +1,30 @@
# REQUIRES: x86
## If neither AT(lma) nor AT>lma_region is specified, don't propagate
## lmaOffset if the section and the previous section are in different memory
## regions.
# RUN: echo '.globl _start; _start: ret; \
# RUN: .data; .byte 0; \
# RUN: .bss; .byte 0' | \
# RUN: llvm-mc -filetype=obj -triple=x86_64 - -o %t.o
# RUN: ld.lld -T %s %t.o -o %t
# RUN: llvm-readelf -l %t | FileCheck %s
## GNU ld places .text and .bss in the same RWX PT_LOAD.
# CHECK: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg
# CHECK-NEXT: LOAD 0x001000 0x000000000fb00000 0x000000000fb00000 0x000001 0x000001 R E
# CHECK-NEXT: LOAD 0x002000 0x000000001f400000 0x000000000fb01000 0x000001 0x000001 RW
# CHECK-NEXT: LOAD 0x002001 0x000000000fb00001 0x000000000fb00001 0x000000 0x000001 RW
MEMORY {
DDR : o = 0xfb00000, l = 185M
TCM : o = 0x1f400000, l = 128K
}
SECTIONS {
.text : { *(.text) } > DDR
.mdata : AT(0xfb01000) { *(.data); } > TCM
## .mdata and .bss are in different memory regions. Start a new PT_LOAD for
## .bss, even if .mdata does not set a LMA region.
.bss : { *(.bss) } > DDR
}