[PATCH] D79286: [ELF] Move SHF_LINK_ORDER till OutputSection addresses are known

[Peter Smith via Phabricator via llvm-commits]

psmith created this revision.
psmith added reviewers: grimar, MaskRay, ruiu.
Herald added subscribers: kristof.beyls, arichardson, emaste.
Herald added a reviewer: espindola.

Sections L with the SHF_LINK_ORDER flag must be ordered in the same relative order as the Sections they have a link to. When using a linker script an arbitrary expression may be used for the virtual address of the OutputSection. In some cases the virtual address does not monotonically increase as the OutputSection index increases, so if we base the ordering of the SHF_LINK_ORDER sections on the index then we can get the order wrong. We fix this by moving SHF_LINK_ORDER resolution till after we have created OutputSection virtual addresses.

This is a follow up patch from D78422 <https://reviews.llvm.org/D78422> which moved the .ARM.exidx to a similar place.


https://reviews.llvm.org/D79286

Files:
  lld/ELF/Writer.cpp
  lld/test/ELF/linkorder-script.s


Index: lld/test/ELF/linkorder-script.s
===================================================================
--- /dev/null
+++ lld/test/ELF/linkorder-script.s
@@ -0,0 +1,32 @@
+// REQUIRES: x86
+// RUN: llvm-mc --triple=x86_64 -filetype=obj %s -o %t.o
+// RUN: echo "SECTIONS { \
+// RUN:         . = 0x80000000; \
+// RUN:         .linkorder : { *(.linkorder.*) } \
+// RUN:         .text : { *(.text) } \
+// RUN:         .text.1 0x80000200 : AT(0x1000) { *(.text.1) } \
+// RUN:         .text.2 0x80000100 : AT(0x2000) { *(.text.2) } \
+// RUN: } " > %t.script
+// RUN: ld.lld --script %t.script %t.o -o %t
+// RUN: llvm-readobj -x .linkorder  %t | FileCheck %s
+
+/// When a linker script does not have monotonically increasing addresses
+/// the SHF_LINK_ORDER sections should still be in monotonically increasing
+/// order.
+
+// CHECK: Hex dump of section '.linkorder':
+// CHECK-NEXT: 0x80000000 02000000 00000000 01000000 00000000
+
+.section .text.1, "ax", %progbits
+.global _start
+_start:
+.quad 0
+
+.section .text.2, "ax", %progbits
+.quad 0
+
+.section .linkorder.1, "ao", %progbits, .text.1
+.quad 1
+
+.section .linkorder.2, "ao", %progbits, .text.2
+.quad 2
Index: lld/ELF/Writer.cpp
===================================================================
--- lld/ELF/Writer.cpp
+++ lld/ELF/Writer.cpp
@@ -1596,7 +1596,7 @@
   OutputSection *bOut = lb->getParent();
 
   if (aOut != bOut)
-    return aOut->sectionIndex < bOut->sectionIndex;
+    return aOut->addr < bOut->addr;
   return la->outSecOff < lb->outSecOff;
 }
 
@@ -1666,11 +1666,13 @@
   AArch64Err843419Patcher a64p;
   ARMErr657417Patcher a32p;
   script->assignAddresses();
-  // .ARM.exidx does not require precise addresses, but it does require the
-  // relative addresses of OutputSections because linker scripts can assign
-  // Virtual Addresses to OutputSections that are not monotonically increasing.
+  // .ARM.exidx and SHF_LINK_ORDER do not require precise addresses, but they
+  // do require the relative addresses of OutputSections because linker scripts
+  // can assign Virtual Addresses to OutputSections that are not monotonically
+  // increasing.
   for (Partition &part : partitions)
     finalizeSynthetic(part.armExidx);
+  resolveShfLinkOrder();
 
   // Converts call x at GDPLT to call __tls_get_addr
   if (config->emachine == EM_HEXAGON)
@@ -2104,12 +2106,6 @@
   if (!script->hasSectionsCommand && !config->relocatable)
     fixSectionAlignments();
 
-  // SHFLinkOrder processing must be processed after relative section placements are
-  // known but before addresses are allocated.
-  resolveShfLinkOrder();
-  if (errorCount())
-    return;
-
   // This is used to:
   // 1) Create "thunks":
   //    Jump instructions in many ISAs have small displacements, and therefore
@@ -2132,8 +2128,11 @@
   //    sometimes using forward symbol declarations. We want to set the correct
   //    values. They also might change after adding the thunks.
   finalizeAddressDependentContent();
+  if (errorCount())
+    return;
 
-  // finalizeAddressDependentContent may have added local symbols to the static symbol table.
+  // finalizeAddressDependentContent may have added local symbols to the static
+  // symbol table.
   finalizeSynthetic(in.symTab);
   finalizeSynthetic(in.ppc64LongBranchTarget);
 


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