[PATCH] D67164: [ELF] Don't shrink RelrSection

[Fangrui Song via Phabricator via llvm-commits]

MaskRay updated this revision to Diff 218705.
MaskRay marked an inline comment as done.
MaskRay edited the summary of this revision.
MaskRay added a comment.

Add log() as grimar suggested


Repository:
  rLLD LLVM Linker

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D67164/new/

https://reviews.llvm.org/D67164

Files:
  ELF/SyntheticSections.cpp
  test/ELF/pack-dyn-relocs-relr-loop.s


Index: test/ELF/pack-dyn-relocs-relr-loop.s
===================================================================
--- /dev/null
+++ test/ELF/pack-dyn-relocs-relr-loop.s
@@ -0,0 +1,36 @@
+# REQUIRES: aarch64
+# RUN: llvm-mc -filetype=obj -triple=aarch64 %s -o %t.o
+# RUN: ld.lld -pie --pack-dyn-relocs=relr -z max-page-size=4096 --verbose %t.o -o %t 2>&1 | FileCheck %s
+# RUN: llvm-readobj -r %t | FileCheck --check-prefix=RELR %s
+
+## This test makes sure we don't shrink .relr.dyn, otherwise its size may
+## oscillate between 2 words and 3 words.
+
+## The test is very sensitive to the exact section sizes and offsets,
+## make sure .data is located at a page boundary.
+
+# CHECK: .relr.dyn needs 1 padding word(s)
+
+# RELR:      .relr.dyn {
+# RELR-NEXT:   0x2F40 R_AARCH64_RELATIVE - 0x0
+# RELR-NEXT:   0x2F48 R_AARCH64_RELATIVE - 0x0
+# RELR-NEXT:   0x3000 R_AARCH64_RELATIVE - 0x0
+# RELR-NEXT: }
+
+.section .data.rel.ro
+.align 3
+.space 0xce0
+foo:
+## Encoded by the first word of .relr.dyn
+.quad foo
+
+## Encoded by the second word of .relr.dyn
+.quad foo
+
+.section .data
+.align 3
+## If .data is at 0x3000, the relocation will be encoded by the second word.
+## If we shrink .relr.dyn, the end of .dynamic will be at 0x2ff8 and .data
+## will be at 0x3ff8, we will need the third word to encode this relocation,
+## which will cause .relr.dyn to expand again.
+.quad foo
Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -1957,6 +1957,14 @@
     }
   }
 
+  // Don't allow the section to shrink; otherwise the size of the section can
+  // oscillate infinitely. Trailing 1s do not decode to more relocations.
+  if (relrRelocs.size() < oldSize) {
+    log(".relr.dyn needs " + Twine(oldSize - relrRelocs.size()) +
+        " padding word(s)");
+    relrRelocs.resize(oldSize, Elf_Relr(1));
+  }
+
   return relrRelocs.size() != oldSize;
 }
 


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