[lld] 447aa48 - [ELF] Add REVERSE input section description keyword

[Justin Cady via llvm-commits]

Author: Justin Cady
Date: 2023-03-07T12:44:02-05:00
New Revision: 447aa48b4a02fa9e22fa45b2fb7a85c12df2e6c3

URL: https://github.com/llvm/llvm-project/commit/447aa48b4a02fa9e22fa45b2fb7a85c12df2e6c3
DIFF: https://github.com/llvm/llvm-project/commit/447aa48b4a02fa9e22fa45b2fb7a85c12df2e6c3.diff

LOG: [ELF] Add REVERSE input section description keyword

The `REVERSE` keyword is described here:

https://sourceware.org/bugzilla/show_bug.cgi?id=27565

It complements `SORT` by allowing the order of input sections to be reversed.

This is particularly useful for order-dependent sections such as .init_array,
where `REVERSE` can be used to either detect static initialization order fiasco
issues or as a mechanism to maintain .ctors element order while transitioning to
the modern .init_array. Such a transition is described here:

https://discourse.llvm.org/t/is-it-possible-to-manually-specify-init-array-order/68649

Differential Revision: https://reviews.llvm.org/D145381

Added: 
    

Modified: 
    lld/ELF/Config.h
    lld/ELF/LinkerScript.cpp
    lld/ELF/ScriptParser.cpp
    lld/test/ELF/linkerscript/sort-init.s

Removed: 
    


################################################################################
diff  --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 61faea4e9bce..249fe3c30ece 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -73,7 +73,14 @@ enum class UnresolvedPolicy { ReportError, Warn, Ignore };
 enum class OrphanHandlingPolicy { Place, Warn, Error };
 
 // For --sort-section and linkerscript sorting rules.
-enum class SortSectionPolicy { Default, None, Alignment, Name, Priority };
+enum class SortSectionPolicy {
+  Default,
+  None,
+  Alignment,
+  Name,
+  Priority,
+  Reverse,
+};
 
 // For --target2
 enum class Target2Policy { Abs, Rel, GotRel };

diff  --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 668a79b5edd7..9207751cb7d2 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -457,6 +457,8 @@ static void sortSections(MutableArrayRef<InputSectionBase *> vec,
     return llvm::stable_sort(vec, nameComparator);
   case SortSectionPolicy::Priority:
     return llvm::stable_sort(vec, priorityComparator);
+  case SortSectionPolicy::Reverse:
+    return std::reverse(vec.begin(), vec.end());
   }
 }
 

diff  --git a/lld/ELF/ScriptParser.cpp b/lld/ELF/ScriptParser.cpp
index e83779bcebff..db7f0268cef6 100644
--- a/lld/ELF/ScriptParser.cpp
+++ b/lld/ELF/ScriptParser.cpp
@@ -660,6 +660,7 @@ StringMatcher ScriptParser::readFilePatterns() {
 
 SortSectionPolicy ScriptParser::peekSortKind() {
   return StringSwitch<SortSectionPolicy>(peek())
+      .Case("REVERSE", SortSectionPolicy::Reverse)
       .Cases("SORT", "SORT_BY_NAME", SortSectionPolicy::Name)
       .Case("SORT_BY_ALIGNMENT", SortSectionPolicy::Alignment)
       .Case("SORT_BY_INIT_PRIORITY", SortSectionPolicy::Priority)

diff  --git a/lld/test/ELF/linkerscript/sort-init.s b/lld/test/ELF/linkerscript/sort-init.s
index 0a349fd9e57f..e1f0c83e74e9 100644
--- a/lld/test/ELF/linkerscript/sort-init.s
+++ b/lld/test/ELF/linkerscript/sort-init.s
@@ -9,6 +9,14 @@
 # CHECK:      Hex dump of section '.init_array':
 # CHECK-NEXT: 0x00000001 00010203 04050607
 
+## Test REVERSE can be used to reverse the order of .init_array and .ctors
+
+# RUN: ld.lld -T %t/reverse.lds %t.o -o %t2.out
+# RUN: llvm-readelf -x .init_array %t2.out | FileCheck %s --check-prefix=CHECK2
+
+# CHECK2:      Hex dump of section '.init_array':
+# CHECK2-NEXT: 0x00000001 04030201 00050706
+
 #--- asm
 .globl _start
 _start:
@@ -41,3 +49,11 @@ SECTIONS {
     *(.init_array .ctors)
   }
 }
+
+#--- reverse.lds
+SECTIONS {
+  .init_array : {
+    *(REVERSE(SORT_BY_INIT_PRIORITY(.init_array.* .ctors.*)) SORT_BY_INIT_PRIORITY(foo*))
+    *(REVERSE(.init_array .ctors))
+  }
+}