[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

diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index a859e82..edb5cac 100644 (file)
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -831,9 +831,10 @@ void LinkerScript::assignOffsets(OutputSection *sec) {
 
   switchTo(sec);
 
+  ctx->lmaOffset = 0;
+
   if (sec->lmaExpr)
     ctx->lmaOffset = sec->lmaExpr().getValue() - dot;
-
   if (MemoryRegion *mr = sec->lmaRegion)
     ctx->lmaOffset = alignTo(mr->curPos, sec->alignment) - dot;
 

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index b924e19..7c2a3fe 100644 (file)
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -2110,12 +2110,11 @@ std::vector<PhdrEntry *> Writer<ELFT>::createPhdrs(Partition &part) {
     // time, we don't want to create a separate load segment for the headers,
     // even if the first output section has an AT or AT> attribute.
     uint64_t newFlags = computeFlags(sec->getPhdrFlags());
-    if (!load ||
-        ((sec->lmaExpr ||
-          (sec->lmaRegion && (sec->lmaRegion != load->firstSec->lmaRegion))) &&
-         load->lastSec != Out::programHeaders) ||
-        sec->memRegion != load->firstSec->memRegion || flags != newFlags ||
-        sec == relroEnd) {
+    bool sameLMARegion =
+        load && !sec->lmaExpr && sec->lmaRegion == load->firstSec->lmaRegion;
+    if (!(load && newFlags == flags && sec != relroEnd &&
+          sec->memRegion == load->firstSec->memRegion &&
+          (sameLMARegion || load->lastSec == Out::programHeaders))) {
       load = addHdr(PT_LOAD, newFlags);
       flags = newFlags;
     }

diff --git a/lld/test/ELF/linkerscript/Inputs/at2.s b/lld/test/ELF/linkerscript/Inputs/at2.s
index 8c6548b..39a8efc 100644 (file)
--- a/lld/test/ELF/linkerscript/Inputs/at2.s
+++ b/lld/test/ELF/linkerscript/Inputs/at2.s
@@ -12,3 +12,6 @@
 
 .section .bar3, "aw"
 .quad 0
+
+.section .bar4, "aw"
+.quad 0

diff --git a/lld/test/ELF/linkerscript/at2.test b/lld/test/ELF/linkerscript/at2.test
index 982e1ca..84c9db9 100644 (file)
--- a/lld/test/ELF/linkerscript/at2.test
+++ b/lld/test/ELF/linkerscript/at2.test
@@ -13,15 +13,24 @@ MEMORY {
 
 SECTIONS {
  .foo1 : { *(.foo1) } > AX AT>FLASH
+## In GNU ld, .foo1's LMA region is propagated to .foo2 because their VMA region
+## is the same and .foo2 does not set an explicit address.
+## lld sets .foo2's LMA region to null.
  .foo2 : { *(.foo2) } > AX
- .bar1 : { *(.bar1) } > AW AT> RAM
+
+ .bar1 : { *(.bar1) } > AW
  .bar2 : { *(.bar2) } > AW AT > RAM
- .bar3 : { *(.bar3) } > AW AT >RAM
+ .bar3 . : { *(.bar3) } > AW
+ .bar4 : { *(.bar4) } > AW AT >RAM
 }
 
 # CHECK:      Type  Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
-# CHECK-NEXT: LOAD  0x001000 0x0000000000002000 0x0000000000006000 0x000010 0x000010 R E 0x1000
-# CHECK-NEXT: LOAD  0x002000 0x0000000000003000 0x0000000000007000 0x000018 0x000018 RW  0x1000
+# CHECK-NEXT: LOAD  0x001000 0x0000000000002000 0x0000000000006000 0x000008 0x000008 R E 0x1000
+# CHECK-NEXT: LOAD  0x001008 0x0000000000002008 0x0000000000002008 0x000008 0x000008 R E 0x1000
+# CHECK-NEXT: LOAD  0x002000 0x0000000000003000 0x0000000000003000 0x000008 0x000008 RW  0x1000
+# CHECK-NEXT: LOAD  0x002008 0x0000000000003008 0x0000000000007000 0x000008 0x000008 RW  0x1000
+# CHECK-NEXT: LOAD  0x002010 0x0000000000003010 0x0000000000003010 0x000008 0x000008 RW  0x1000
+# CHECK-NEXT: LOAD  0x002018 0x0000000000003018 0x0000000000007008 0x000008 0x000008 RW  0x1000
 
 # SECTIONS:      Sections:
 # SECTIONS-NEXT: Idx Name          Size     VMA
@@ -32,3 +41,4 @@ SECTIONS {
 # SECTIONS-NEXT:   4 .bar1         00000008 0000000000003000
 # SECTIONS-NEXT:   5 .bar2         00000008 0000000000003008
 # SECTIONS-NEXT:   6 .bar3         00000008 0000000000003010
+# SECTIONS-NEXT:   7 .bar4         00000008 0000000000003018

diff --git a/lld/test/ELF/linkerscript/at8.test b/lld/test/ELF/linkerscript/at8.test
index 48c0d45..ab0e3c6 100644 (file)
--- a/lld/test/ELF/linkerscript/at8.test
+++ b/lld/test/ELF/linkerscript/at8.test
@@ -11,7 +11,7 @@ MEMORY {
 SECTIONS {
  .text : { *(.text) } > FLASH
  .sec1 : { *(.sec1) } > RAM AT > FLASH
- .sec2 : { *(.sec2) } > RAM
+ .sec2 : { *(.sec2) } > RAM AT > FLASH
  .sec3 : { *(.sec3) } > RAM AT > FLASH
 }
 

diff --git a/lld/test/ELF/linkerscript/lma-align.test b/lld/test/ELF/linkerscript/lma-align.test
index d2f039a..3e0e88d 100644 (file)
--- a/lld/test/ELF/linkerscript/lma-align.test
+++ b/lld/test/ELF/linkerscript/lma-align.test
@@ -9,14 +9,13 @@
 # CHECK-NEXT: .text        PROGBITS 0000000000001000 001000 000001 00  AX  0   0  4
 # CHECK-NEXT: .data.rel.ro PROGBITS 0000000000011000 002000 000001 00  WA  0   0 16
 # CHECK-NEXT: .data        PROGBITS 0000000000011010 002010 000001 00  WA  0   0 16
-# CHECK-NEXT: .bss         NOBITS   0000000000011040 002011 000001 00  WA  0   0 64
+# CHECK-NEXT: .bss         NOBITS   0000000000011040 002040 000001 00  WA  0   0 64
 
 # CHECK:      Type  Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
 # CHECK-NEXT: LOAD  0x001000 0x0000000000001000 0x0000000000001000 0x000001 0x000001 R E 0x1000
 # CHECK-NEXT: LOAD  0x002000 0x0000000000011000 0x0000000000001010 0x000001 0x000001 RW  0x1000
-## FIXME .data and .bss should be placed in different PT_LOAD segments
-## because their LMA regions are different.
-# CHECK-NEXT: LOAD  0x002010 0x0000000000011010 0x0000000000001020 0x000001 0x000031 RW  0x1000
+# CHECK-NEXT: LOAD  0x002010 0x0000000000011010 0x0000000000001020 0x000001 0x000001 RW  0x1000
+# CHECK-NEXT: LOAD  0x002040 0x0000000000011040 0x0000000000011040 0x000000 0x000001 RW  0x1000
 
 MEMORY {
   ROM : ORIGIN = 0x1000, LENGTH = 1K
@@ -28,5 +27,6 @@ SECTIONS {
   .data.rel.ro 0x11000 : { *(.data.rel.ro) } >RAM AT>ROM
   ## ALIGN decides output section alignment.
   .data . : ALIGN(16) { *(.data*) } >RAM AT>ROM
+  ## Start a new PT_LOAD because the LMA region is different from the previous one.
   .bss . : ALIGN(64) { *(.bss*) } >RAM
 }