[lld] 423cb32 - [ELF] Special case --shuffle-sections=-1 to reverse input sections

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2021-03-17T09:32:44-07:00
New Revision: 423cb321dfae8a57f878974ce81730d5ee612f95

URL: https://github.com/llvm/llvm-project/commit/423cb321dfae8a57f878974ce81730d5ee612f95
DIFF: https://github.com/llvm/llvm-project/commit/423cb321dfae8a57f878974ce81730d5ee612f95.diff

LOG: [ELF] Special case --shuffle-sections=-1 to reverse input sections

If the number of sections changes, which is common for re-links after
incremental updates, the section order may change drastically.

Special case -1 to reverse input sections. This is a stable transform.
The section order is more resilient to incremental updates.  Usually the
code issue (e.g. Static Initialization Order Fiasco, assuming pointer
comparison result of two unrelated objects) is due to the relative order
between two problematic input files A and B.  Checking the regular order
and the reversed order is sufficient.

Differential Revision: https://reviews.llvm.org/D98445

Added: 
    

Modified: 
    lld/ELF/Options.td
    lld/ELF/Writer.cpp
    lld/docs/ld.lld.1
    lld/test/ELF/shuffle-sections.s

Removed: 
    


################################################################################
diff  --git a/lld/ELF/Options.td b/lld/ELF/Options.td
index 302afd2a4374..ee4a0610d362 100644
--- a/lld/ELF/Options.td
+++ b/lld/ELF/Options.td
@@ -587,7 +587,8 @@ defm lto_unique_basic_block_section_names: BB<"lto-unique-basic-block-section-na
     "Give unique names to every basic block section for LTO",
     "Do not give unique names to every basic block section for LTO (default)">;
 def shuffle_sections: JJ<"shuffle-sections=">, MetaVarName<"<seed>">,
-  HelpText<"Shuffle input sections using the given seed. If 0, use a random seed">;
+  HelpText<"Shuffle input sections using the given seed. "
+  "If -1, reverse the section order. If 0, use a random seed">;
 def thinlto_cache_dir: JJ<"thinlto-cache-dir=">,
   HelpText<"Path to ThinLTO cached object file directory">;
 defm thinlto_cache_policy: EEq<"thinlto-cache-policy", "Pruning policy for the ThinLTO cache">;

diff  --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 7a505ff8cfa1..f0d4e6e4e685 100644
--- a/lld/ELF/Writer.cpp
+++ b/lld/ELF/Writer.cpp
@@ -1301,8 +1301,15 @@ static void maybeShuffle(DenseMap<const InputSectionBase *, int> &order) {
   for (int &prio : priorities)
     prio = curPrio++;
   uint32_t seed = *config->shuffleSectionSeed;
-  std::mt19937 g(seed ? seed : std::random_device()());
-  llvm::shuffle(priorities.begin(), priorities.end(), g);
+  if (seed == UINT32_MAX) {
+    // If --shuffle-sections=-1, reverse the section order. The section order is
+    // stable even if the number of sections changes. This is useful to catch
+    // issues like static initialization order fiasco reliably.
+    std::reverse(priorities.begin(), priorities.end());
+  } else {
+    std::mt19937 g(seed ? seed : std::random_device()());
+    llvm::shuffle(priorities.begin(), priorities.end(), g);
+  }
   int prioIndex = 0;
   for (InputSectionBase *sec : inputSections) {
     if (order.try_emplace(sec, priorities[prioIndex]).second)

diff  --git a/lld/docs/ld.lld.1 b/lld/docs/ld.lld.1
index 42525df76ada..3c1704c0c5e8 100644
--- a/lld/docs/ld.lld.1
+++ b/lld/docs/ld.lld.1
@@ -487,7 +487,8 @@ Set address of section.
 .It Fl -shared , Fl -Bsharable
 Build a shared object.
 .It Fl -shuffle-sections Ns = Ns Ar seed
-Shuffle input sections using the given seed. If 0, use a random seed.
+Shuffle input sections using the given seed.
+If -1, reverse the section order. If 0, use a random seed.
 .It Fl -soname Ns = Ns Ar value , Fl h Ar value
 Set
 .Dv DT_SONAME

diff  --git a/lld/test/ELF/shuffle-sections.s b/lld/test/ELF/shuffle-sections.s
index bc0f57b98d7a..59b0642d639c 100644
--- a/lld/test/ELF/shuffle-sections.s
+++ b/lld/test/ELF/shuffle-sections.s
@@ -26,6 +26,12 @@
 # SHUFFLE3: Hex dump of section '.text':
 # SHUFFLE3-NEXT: 02cccccc 010403
 
+## As a special case, -1 reverses sections as a stable transform.
+# RUN: ld.lld --shuffle-sections=-1 %t.o -o %t-1.out
+# RUN: llvm-readelf -x .text %t-1.out | FileCheck %s --check-prefix=SHUFFLE-1
+# SHUFFLE-1: Hex dump of section '.text':
+# SHUFFLE-1-NEXT: 040302cc 01
+
 ## .text has an alignment of 4.
 .global _start
 _start: